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Much of the literature cited in this review focused on studies which compared broilers at the same age, rather than the same body weight, which is an important confounding factor, as many of the measures are impacted by body weight itself (Tainika et al., 2023). This is a significant criticism of research comparing fast and slow growth broiler strains as body weight, which influences health and welfare parameters, can be very different at any age between fast- and slow-growing strains. Additionally, many studies also did not account for strain specific management practices, which can affect bird’s responses (i.e. incubation environmental controls, bird stocking density or diet).
A review of the welfare of fast and slow growing broilers chickens was published in late March 2024, which reviewed 63 journal articles (Nicol et al., 2024). The review considered study design and confounding factors including rearing environment and diet. The authors concluded that modern slow growth broiler strains had more positive results for “relevant” welfare parameters. The authors performed a quantitative analysis across comparable studies and found fast-growing strains had higher mortality and poorer contact dermatitis and gait scores. Fast growth broilers were less active, but other impacts on behaviour were inconsistent. It was suggested that growth rate was generally predictive of welfare issues and a possible future management strategy could be to slaughter birds prior to the onset of a decline in welfare.
Please note that the studies included in this document sometimes assessed additional factors beyond growth rate. While the primary focus of this review was on the main effects between fast- and slow- growing broiler strains, appropriate interactions were also included. There were some studies that were excluded from the review for various reasons, and these papers and the reason for exclusion are listed at the end of the document in the excluded literature section.
On d 18 of incubation, there was a tendency for slow growth broilers to have higher egg weight loss, shorter embryo length, and shorter breast length (Vafaeinia et al., 2021).
The breast fiber area was greater for the fast strain and these embryos had a lower capillary number. There was no difference in embryo weight or yolk sac percentage. On day of hatch, chicks from the fast strain had a longer breast and lower capillary number. No differences in chick weight or length or residual yolk sac percentage were observed.
Body weight and gain
Body weight has been assessed in numerous trials, but care must be taken when interpreting the results, as many compare traits at different body weights. When assessed at the same age, fast growth strains had higher body weights (Bokkers and Koene, 2003 (weekly from 1-12 wk); Mikulski et al., 2011 (42 and 65 d); Sarica et al., 2014 (84 d); Yamak et al., 2014 ( 8, 10, 12 wk); Wen et al., 2017b (42 d); Rezaei et al., 2018 (71 d); Sarica et al., 2019 (weekly from 35-84 d); Wilhelmsson et al., 2019 (71 d); Ghayas et al., 2020 (56 d); Özbek et al., 2020 (56 d); Aksoy et al., 2021 (56 d); de Jong et al., 2021 (8, 17, 23, 29, and 38 d); Güz et al., 2021 (weekly from 0-35 d); Akyüz et al., 2022 (weekly from 1-10 wk); and van der Eijk et al., 2022b (weekly from 7-35 d and 38 d)).
Quentin et al. (2003) studied slow, medium, and fast growth broilers, with the medium strain being a crossbreed of the fast and slow strains. They reported that the medium strain chicks were lighter (34.3 g) than slow and fast chicks (40.0 and 43.7 g) at the start of the trial. At slaughter the slow strain broilers (12 wk, 2.9 kg) were heavier than medium (8 wk, 2.6 kg) and fast (6 wk, 2.5 kg) growth broilers (dependant on age of slaughter). However, when looking at growth rate, the fast stain had the highest, followed by the medium and then the slow strain. Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and market body weights were higher for the fast-growing strain (2.9 vs 1.8 and 2.2 kg). The fast growth strain also had higher weight gain, feed intake, and better feed efficiency. Malchow and Schrader (2021) reported that the fast-growing strain at 35 d had heavier body weights than the slow- growing strain at 70 d (2.05 vs 2.24 kg).
Castellini et al. (2016) reared slow-, medium-, and fast-growing broilers to 11 wk and reported the fast-growing strain had the highest feed intake and weight gain. The fast and some of the medium growth strains had the highest live weight and feed efficiency.
Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) on commercial diets or diets low in protein. Market body weights were higher for the medium growth strain, however body weight gain was higher for the fast-growing strain. Additionally, the low protein diet had a negative impact on the fast- and medium- growing broilers, but the slow-growth strain was unaffected.
Tran et al. (2021) fed broilers diets with high, medium, or low lysine from 1- 21 d. The authors reported that slow-growing birds had higher initial body weights, whereas fast- growing birds had higher final weights. When fed a low vs high lysine diet, the impact was larger for the fast growth birds (lower lysine resulted in a larger decrease in body weight, with a similar response noted for body weight gain).
When corrected for body weight, fast strains had higher body weight gain (Dixon, 2020 (2.2 kg); Mohammadigheisar et al., 2021 (2.1 kg); Singh et al., 2021 (2.0-2.2 kg); Torrey et al., 2021 (2.1 and 3.3 kg); Tůmová et al., 2021 (2.0 kg); and van der Eijk et al. 2023 (2.6 kg)). The same was evident when gain was measured at the same age (Han and Baker, 1991 (8-21 d); Wen et al., 2017a (42 d); Ghayas et al., 2020 (28-56 d); Aksoy et al., 2021 (56 d); de Jong et al., 2021 (0-8, 8-17, 17-23, 23-29, and 29-38 d); and Akyüz et al., 2022 (weekly from 1-10 wk)).
Berger et al. (2021) continuously monitored body weight using electronic feeders and radio frequency identification of individual birds from 0-35 d (fast) and 0-82 d (slow). They reported that average daily gain continuously increased with age in the fast- growing strain, however the slow-growing strain showed an increase, followed by a plateau, and then a decrease as they reached market age. The authors also compared a conventional diet to an alternative diet (replaced soybean meal with sunflower meal, fava bean, canola meal, and dried distillers’ grains with solubles) and concluded that both strains adapted well to the alterative diet.
Feed intake and efficiency
Fast-growing broilers consumed more feed at 2.0 kg (Tůmová et al., 2021), 2.1 kg (Torrey et al., 2021) and 2.6 kg (van der Eijk et al., 2023). Torrey et al. (2021) compared fast growth strains to slow growth strains separated into 3 categories (fast-slow, moderate-slow, or slow-slow growing) and reported that at 3.2 kg feed intake was higher in the fast-growing strains compared to the moderate- and slow-slow growing strains but was not different from the fast- slow growing strains. Fast-growing strains also consumed more feed when measured at the same age (Han and Baker, 1991 (8-21 d); Bokkers and Koene, 2003 (weekly from 1-12 wk); Quentin et al., 2003 (0-1, 3-6, 6-8wk); Yamak et al., 2014 ( 8, 10, 12 wk); Wen et al., 2017a (42 d); Rezaei et al., 2018 (71 d); Sarica et al., 2019 (weekly from 35-84 d); Ghayas et al., 2020 (28-56 and 0-56 d); Aksoy et al., 2021 (56 d); de Jong et al., 2021 (0-7, 8-16, 17-22, 23-28, and 29-37 d); Güz et al., 2021 (0-14, 14-35 and 0-35 d); and Akyüz et al., 2022 (weekly from 1-10 wk)). Over different rearing periods, from 0-38 d (fast) and 0-51 d (slow), the fast- growing strain again consumed more feed (van der Eijk et al., 2022b). Singh et al. (2021) reported that when feed intake was compared at the same age (32 d), fast growth birds consumed more feed, however when comparing feed intake for the period taken to reach a target body weight of 2.0-2.2 kg, slow growth birds consumed more feed overall (55 d; 4.03 kg/bird) compared to the fast strain (32 d; 3.04 kg/bird). Once again, many of these differences are a result of heavier weights in the fast growing strains.
Dixon et al. (2020) reported that slow growth birds consumed more feed overall (less starter and grower, but more finisher). Feed was measured when diets were switched (11 and 29 d), at feed withdrawal prior to slaughter (41 and 59 d), and when birds weighed 2.2 kg (35 vs 42 d). At 2.0 kg, slow growth birds (72 d) consumed more feed than the fast strain (38 d) (Doğan et al., 2019). Fanatico et al. (2008) performed two experiments, one with female broilers reared to 63 d (fast) or 91 d (slow) and they found strain had no impact on feed intake. In the second trial, male broilers were reared to 56 d (fast) or 84 d (slow) and the slow growth birds had higher feed intake. This agrees with Fanatico et al. (2005a) who found that overall slow growth broilers reared to 81 d (2.18 kg) and medium growth birds reared to 67 d (2.41 kg), consumed more feed than fast growth birds reared to 53 d (2.48 kg).
Tran et al. (2021) fed broilers diets with high, medium, or low lysine content from 1- 21d. The authors reported feed intake was lower in the fast-growing stain when fed a low vs high lysine diet, but the slow-growing strain was unaffected. The fast growth strain was also reported to have better feed efficiency.
Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) on commercial diets or diets low in protein. The authors stated that fast and medium growth broilers compensated for low protein by increasing feed intake and these birds had better feed efficiency.
When assessed at the same body weight, fast-growing strains were more feed efficient (Doğan et al., 2019 (2.0 kg); Dixon, 2020 (2.2 kg); Mohammadigheisar et al., 2021 (2.1 kg); Torrey et al., 2021 (2.1 and 3.2 kg); Tůmová et al., 2021 (2.0 kg); van der Eijk et al., 2023 (2.6 kg)). These strains were also more feed efficient when measured at the same ages (Han and Baker, 1991 (8-21 d); Yamak et al., 2014 ( 8, 10, 12 wk); Wen et al., 2017a (42 d); Rezaei et al., 2018 (71 d); Sarica et al., 2019 (weekly from 35-84 d); Ghayas et al., 2020 (28-56 d); Weimer et al., 2020 (0-41 and 0-62 d); de Jong et al., 2021 (0-7, 8-16, 17-22, and 23-28 d); Güz et al., 2021 (0-14, 14-35 and 0-35 d); and Akyüz et al., 2022 (weekly from 1-8 wk)). Overall, when the fast-growing strain was reared to 38 d and the slow-growing strain to 51 d, fast growth broilers were more feed efficient (van der Eijk et al., 2022b). This agrees with Fanatico et al. (2005a) who found that fast growth broilers reared to 53 d were more feed efficient than medium growth birds reared to 67 d and slow growth birds reared to 81 d. Fanatico et al. (2008) reared female broilers to 63 d (fast) or 91 d (slow) and male broilers to 56 d (fast) or 84 d (slow) and in both trials fast-growing birds had better feed efficiency.
No difference in feed efficiency was noted from 13-55 d (Nielsen, 2012), from 1-42 d, 22-65, or 1-65 d (Mikulski et al., 2011), or at 9 or 10 wk (Akyüz et al., 2022). Singh et al. (2021) reported that the slow growth strain had better feed efficiency at 7 and 21 d, but that at the target weight of 2.0-2.2 kg, fast growth broilers were more feed efficient.
Feed efficiency showed different responses at different ages between 0-8 wk when slow, medium, and fast growth broilers were fed low, medium, and high dietary concentration diets (Quentin et al., 2003). Generally, fast growth broilers had better feed efficiency at younger ages and poorer efficiency at older ages when fed diets with low and medium dietary concentrations.
Nutrient absorption
Mohammadigheisar et al. (2021) reared one fast strain and 3 slow-medium growing strains to 2.1 kg and reported a higher concentration of lactic acid and lower concentration of isobutyric acid in the ceca digesta of the fast strain and 1 of 3 slow- medium growth strains. No differences in acetic, propionic, or butyric acid were observed. However, Mohammadigheisar et al. (2020) observed no differences in short chain fatty acid profile in ceca digesta when birds were slaughtered at 48 d.
Mohammadigheisar et al. (2021) reared broilers to 2.1 kg and reported no differences between strains for apparent retention of dry matter, neutral detergent fiber, calcium, or phosphorous. The fast strain had higher retention compared to two of the slow strains for crude protein and crude fat. Additionally, the fast-growing strain had higher apparent metabolizable energy corrected for nitrogen than one of the slow strains and the lowest caloric efficiency compared to all slow strains. Singh et al. (2021) reported that birds from the fast growth strain had 1.91% higher apparent metabolizable energy from the same diet. When fed diets low in lysine, energy retention decreased by almost half in the fast growth strain, while there was no effect in the slow growth strain.
Feather cover
Slow growth broilers had better feather cover at 2.2 kg (Dixon, 2020; Abeyesinghe et al., 2021) and 2.5 kg (Dixon, 2020). No difference in feather cover was observed at 3 wk, however at 4, 5, and 6 wk and overall, slow growth birds had better feather cover (Baxter et al., 2021). At 11 wk, the slow-growing strains had the best feather condition (Castellini et al., 2016).
Skin damage
van der Eijk et al. (2023) assessed birds for the presence of scratches/wounds at body weights of 2.0 and 2.3 kg. and found that, at both weights, the slow-growing strain had less damage.
Foot and leg health
Most studies agree that slow-growing broilers have better gait scores, which is an indicator of improved mobility. When corrected for body weight, slow growth broilers had better scores at 2.0 kg (van der Eijk et al., 2023); 2.2 kg (Dixon, 2020; Abeyesinghe et al., 2021; van der Eijk et al., 2022b; Pearce et al., 2023), 2.3 kg (van der Eijk et al., 2023), and 2.5 kg (Dixon, 2020). Additionally, slow growth broilers had better gait scores 2 d prior to slaughter at 2.1 kg (Rayner et al., 2020). However, de Jong et al. (2021) reported no effect of strain at 1.6 and 2.5 kg. Güz et al. (2021) also reported no strain impact when gait score was measured at a similar body weight class (body weight not reported). No strain differences were noted between 2 and 3 wk (Wilhelmsson et al., 2019; Baxter et al., 2021), however at older ages (4-9 wk), slow growth birds were found to have better gait scores (Çavuşoğlu and Petek, 2019; Wilhelmsson et al., 2019; Baxter et al., 2021). Fanatico et al. (2008) reared females to 63 d (fast) or 91 d (slow) and males to 56 d (fast) or 84 d (slow) and in both trials slow-growing broilers had better gait scores. Malchow and Schrader (2021) reported that the slow-growing strain (reared to 10 wk) had better gait scores than the fast-growing strain (reared to 5 wk).
When footpad dermatitis (FPD) was assessed at the same body weight, slow growth broilers had better FPD scores at 2.0 kg (van der Eijk et al., 2023); 2.2 kg (Abeyesinghe et al., 2021; van der Eijk et al. 2022b), 2.3 kg (van der Eijk et al., 2023), and 3.2 kg (Santos et al., 2022b). Rayner et al. (2020) reported the incidence of FPD was better in the slow growth strain 2 d prior to slaughter at 2.1 kg. Santos et al. (2022b) reported a smaller percentage of fast-growing birds with FPD compared to fast-slow and medium- slow growing strains at 2.1 kg, with no difference in severity. Some studies reported no difference in FDP when measured at the same body weight. At 1.6 and 2.5 kg (de Jong et al., 2021) and 2.2 and 2.5 kg (Dixon et al., 2020) no difference was noted. When measured at the same age, FPD was better in slower growing broilers (Williams et al., 2013 (37 d); Castellini et al., 2016 (11 wk); Çavuşoğlu and Petek, 2019 (6, 7, 8 wk); Wilhelmsson et al., 2019 (6 and 9 wk); Ghayas et al., 2021 (unreported age); Akyüz et al., 2022 (10 wk); Abdourhamane and Petek, 2023 (56 d)). However, no differences were observed at 2 wk (Wilhelmsson et al., 2019) or 6 wk (Akyüz et al., 2022). Malchow and Schrader (2021) reported that the slow-growing strain (reared to 10 wk) had better FPD scores than the fast-growing strain (reared to 5 wk). Fanatico et al. (2008) assessed female broilers reared to 63 d (fast) or 91 d (slow) and male broilers reared to 56 d (fast) or 84 d (slow) and found no differences in FPD. Similarly, no differences were observed when fast-growing broilers were reared to 41 d and slow-growing birds were reared to 62 d (Weimer et al., 2020). Baxter et al. (2021) collected processing plant data for 762,079 broilers over 34 flocks and found no difference in average levels of FPD for fast- (39 d, 2.5 kg) or slow- (44 d, 2.3 kg) growing broilers.
Hock-joint dermatitis (HJD) was improved in slow growth broilers, when corrected for body weight, at 2.1 kg (Santos et al., 2022b), 2.2 kg (Dixon, 2020; Abeyesinghe et al., 2021; van der Eijk et al. 2022b), 2.3 kg (van der Eijk et al., 2023), 2.5 kg (Dixon, 2020), and 3.2 kg (Santos et al., 2022b). The incidence of HJD was lower in the slow growth strain 2 d prior to slaughter at 2.1 kg (Rayner et al., 2020). de Jong et al. (2021) measured HJD at 1.6 and 2.5 kg and reported no difference between strains. When hocks were assessed at the same ages the improvement in HJD remained evident in the slow growth strains (Williams et al., 2013 (37 d); Wilhelmsson et al., 2019 (9 wk); Ghayas et al., 2021(unreported age); Akyüz et al., 2022 (6 and 10 wk); Abdourhamane and Petek, 2023 (56 d)). No difference in HJD was noted at 2 or 6 wk (Wilhelmsson et al., 2019) and while not significant, at 8 wk scores of 0 (best) were 83% vs 49% for the slow- and fast-growing strains and scores of 2 (worst) were 0 vs 31% (Çavuşoğlu and Petek, 2019). Malchow and Schrader (2021) reported that the slow-growing strain (reared to 10 wk) had better HJD scores than the fast-growing strain (reared to 5 wk). No differences were noted for female broilers reared to 63 d (fast) or 91 d (slow) or male broilers reared to 56 d (fast) or 84 d (slow) (Fanatico et al., 2008). This agrees with Weimer et al. (2020) who reared fast-growing birds to 41 d and slow-growing birds to 62 d. No difference in the average levels of HJD were reported at the processing plant with a sample size of 762,079 broilers over 34 flocks, where the fast-growing strain was reared to 39 d (2.5 kg) and the slow-growing strain to 44 d (2.3 kg) (Baxter et al., 2021).
Bokkers and Koene (2004) found more slow growth broilers successfully walked the length of a runway between 3-12 wk. Slow growth broilers reached the first 3 zones of the walkway faster, but no difference was noted for the time to reach the end of the walkway. Slow-growing birds walked faster and walking speed decreased more with increasing age in the fast-growing strain. Malchow and Schrader (2021) reported that the slow-growing strain (reared to 10 wk) had better walking ability than the fast-growing strain (reared to 5 wk).
At 2.0-2.2 kg, slow growth birds had a longer latency to lie, indicating better leg strength (Singh et al., 2021). Santos et al. (2022b) measured latency to lie at 2.0 and 3.0 kg and found that, at both weights, the fast growth and slow-slow growth birds were lighter than the fast-slow and medium-slow growth strains. At 2.0 kg the fast and slow-slow growth strains had longer latencies to lie and fewer lying events compared to the fast-slow growth strain, and a lower percentage of birds lying compared to the fast-slow and medium-slow growing strains. At 3.0 kg, the fast-growing strains and fast-slow growing strains had shorter latencies to lie, more lying events per bird, and a tendency to lie down more compared to the slow-slow growth strains. Overall, from 3-6 wk, the slow growth strain had a longer latency to lie (Baxter et al., 2021).
Santos et al. (2022b) performed a group obstacle test at 1.8 and 2.7 kg. At 1.8 kg, fast and slow-slow growth broilers (slowest of the slow growth birds) were lighter than the fast-slow growth strains (fastest of the slow growth birds) and the number of obstacle crossings was greatest for the slow-slow growth strains. At 2.7 kg, the fast-slow growth strain had the heaviest body weights while the slow-slow growth strains were the lightest and the slow-slow growth birds made more crossings than the fast or fast-slow growth strains.
Bone parameters
Vafaeinia et al. (2021) found that on d 18 of incubation, the fast-growing strain had wider tibiae and shanks. On day of hatch, fast growth chicks had heavier tibia, femur, and shank weights, lower tibia ash, and wider femurs and shanks. No difference in tibia length or width or shank length were noted between strains.
At 1.4 and 2.2 kg, fast-growing broilers had lower metatarsal side proximal tibia head thickness, tibia osseous volume, tibia total volume, tibia volume fraction, tibia mineral content, tibia ultimate strength, tibia yield strength, tibia stiffness, and tibia energy to fracture (Güz et al., 2021). At 1.4 kg, fast-growing broilers also had lower femoral side proximal tibia head thickness, while at 2.2 kg, they had reduced tibia weight, proximal tibia length, and tibia mineral density. This agrees with van der Eijk et al. (2022b) who reported that fast-growing broilers at 2.3 kg had lower tibia weight, length, mineral content, and metatarsal head thickness, however they also noted these birds had lower pore volume and higher tibia osseous volume, volume fraction, and elastic modulus. Fast-growing strains had heavier, longer, and wider tibias at 56 d (Ghayas et al., 2021) and lower shank lengths corrected for body weight at 8, 10, and 12 wk (Yamak et al., 2014). No difference in medullary canal diameter was observed and fast growth broilers had a higher robusticity index, which is an indication of bone strength (Ghayas et al., 2021). This agrees with Fanatico et al. (2005a) who reported that fast-growing broilers at 53 d had stronger bones compared to a slow growth strain at 81 d or a medium growth strain at 67 d.
Santos et al. (2022a) reported that, at 2.1 kg, the fast-growing strains had higher tibial breaking strength corrected for body weight, and a smaller relative tibia diameter and longer tibia length than fast-slow and medium-slow growing birds. At 3.2 kg, the fast- growing strains had higher relative tibial breaking strength corrected for body weight compared to the medium-slow growing strain, shorter tibia diameter compared to the slow-slow growth strain, and the shortest tibia length, shortest length:diameter ratio, and lightest relative dry matter and ash weight compared to all slow strains.
However, Mohammadigheisar et al. (2021) reported that fast-growing broilers had lower tibia ash at 2.1 kg compared to 1 of 3 slow-medium growth strains and Mohammadigheisar et al. (2020) observed lower ash in a fast strain compared to 3 of 4 slow-growing strains when reared to 48 d. Fanatico et al. (2008) assessed female broilers reared to 63 d (fast) or 91 d (slow) and males reared to 56 d (fast) or 84 d (slow) and found no strain effect on bone mineral density.
At 1.4 and 2.2 kg there was no strain effect on tibial dyschondroplasia (TD) (Güz et al., 2021). Abdourhamane and Petek (2023) reported no difference between strains for TD at 56 d. Fanatico et al. (2008) assessed female broilers reared to 63 d (fast) or 91 d (slow) and males reared to 56 d (fast) or 84 d (slow) and reported the fast-growing strains had poorer TD scores in both trials. More leg deviations were observed in fast- growing strains at 2.2 kg (Abeyesinghe et al., 2021). Fast growth birds had lower epiphyseal plate abnormalities, but higher varus-valgus scores (overall, right leg, and left leg) at 2.3 kg (van der Eijk et al., 2022b). At 1.4 kg there was no strain effect for varus-valgus angulation, however at 2.2 kg, fast-growing broilers had higher right-side varus-valgus scores with no impact on left-side scores (Güz et al., 2021).
At 6 wk, fast-growing broilers with reduced weight load had increased locomotor activity, reduced torsion and angular deformation of Os tibiotarsus, and increased density and strain strength index of the corticalis (Stojcic and Bessei, 2009). When weight load was increased in slow-growing broilers, locomotor activity was reduced and resting increased, but there were no skeletal impacts. The authors suggested the improvement in bone conditions observed in the fast-growing strain was due to increased activity, whereas the slow-growing broiler bones were more robust, therefore, bone conditions were not impaired by increased load and decreased activity. High weight load in broiler from the fast growth strain along with reduced locomotor activity had negative impacts on bones and joints, resulting in more leg disorders. The bones and joints of slow growth birds are robust enough that an increase in weight load resulting in reduced locomotor activity did not have the same negative impacts observed in the fast-growing strain.
Disease status
No difference between strains in colonization or infection of the liver or spleen after infection with Campylobacer jejuni at 21 d was noted when measured 2, 7, and 16 d post-infection (Williams et al., 2013). Singh et al. (2021) performed an antibody response test at 21 d and measured the response 7 d post-exposure. The fast-growing strain had a higher mean total antibody titer (IgM and IgG) and higher mean IgM titer, indicating a stronger immune response. Castellini et al. (2016) reported lower haemolytic complement assay values and higher lysozyme concentration in the fast- growing strain, which suggests a beneficial impact on immunity, although there was no effect on serum bactericidal activity.
Microbiota populations
No difference in total count of anaerobic bacteria or counts of Bifidobacterium spp or Lactobacillus spp were observed, however Escherichia coli counts were lower in the slow-growing strain when measured at a body weight of 2.0 kg (Tůmová et al., 2021).
Fear
A higher proportion of broilers from the fast-growing strain were “touched” in a touch test at 2 and 6 wk (indicating less fear, more inactivity, or both according to the authors), with no difference observed at 9 wk (Wilhelmsson et al., 2019). This agrees with Baxter et al. (2021) who reported broilers from the fast-growing strain had a shorter withdrawal distance in an avoidance test overall from 3-6 wk, indicating they were less fearful. Furthermore, fast-growing birds had a shorter avoidance distance and tonic immobility reaction period at 8 wk (Çavuşoğlu and Petek, 2019) and they vocalized less during a walkway test between 3-12 wk (Bokkers and Koene, 2004). No effect of strain was noted for time to peck a novel object, number of birds that pecked the object, or number of birds within 50 cm of the object after 5 minutes (Baxter et al., 2021)
van der Eijk et al. (2022a) performed behavioural tests at 0.4, 1.1, 1.7, and 2.1 kg. They reported that less fast-growing broilers approached a human at 0.4 and 1.7 kg and a novel object at 1.7 kg. Overall, a lower percentage of fast growth birds approached a human and novel object, which the authors suggested indicated they were more fearful (van der Eijk et al., 2022a). This was supported by Ghanyas et al. (2021) who noted that fast-growing broilers had longer tonic immobility latencies (unknown age). Castellini et al. (2016) reported than the fast-growing strain, along with two medium growth strains, showed the least initial interest in an observer within the first 5 minutes of their presence in the pen and had longer tonic immobility latencies. Rayner et al. (2020) found the median percentage of birds within arm’s reach was lower in the fast-growing strains, indicating more fearfulness, at a lower stocking density of 30 kg/m2 compared to the fast- and slow-growing strains at 34 kg/m2 (Rayner et al., 2020). Birds were slaughtered at 2.1 kg and this test occurred 2 d prior to slaughter. van der Eijk et al. (2022a) also performed a free-space test and reported no effect of strain.
Behaviour
Rayner et al. (2020) performed a qualitative behavioural assessment at 14 and 28 d and 3 d prior to slaughter (2.1 kg) and they categorized expressive qualities as: content, flat, active, playful, flighty, stressed, alert, happy, calm, inquisitive, lethargic, comfortable, lively, and relaxed. The authors reported the fast-growing strain were more “flat/stressed” and they also reported the odds of observing any positive behaviour was lower in the fast strains. Bokkers and Koene (2003) assessed behaviour weekly from 1- 12 wk and found that fast-growing broilers tended to perform more behaviours while sitting vs standing. At 22, 36, and 50 d, a smaller number of behavioural changes and position changes within the pen were noted in the fast strain (Nielsen, 2012). Castellini et al. (2016) reported that the fast-growing strain spent the least amount of time outdoors and preferred to stay closer to the house when outdoors.
Fast-growing broilers performed more ingestion behaviours at 0.6 kg (van der Eijk et al., 2022b), 1.7 kg (van der Eijk et al., 2022a), 2.1 kg (van der Eijk et al., 2022a), at an unreported body weight (Güz et al., 2021), and overall, at four target body weights (0.4, 1.1, 1.7, and 2.1 kg) (van der Eijk et al., 2022a). Dawson et al. (2021) assessed behaviour at 2.1 and 3.2 kg, between 2 fast-growing strains and 12 slow-growing strains (categorized as fast-, medium-, and slow-slow growing). At 2.1 kg, the fast-growing strains spent more time feeding than the fast-slow and slow-slow growing birds and no differences in drinking behaviour were noted. At 3.2 kg, there were no differences in the length of time spent feeding or drinking or in bout frequencies or total duration or number of bouts for feeding or drinking (Dawson et al., 2021). When comparing behaviour at similar body weights (+/- 1.1 kg), the fast-growing strain again showed more ingestion behaviours (de Jong et al., 2021). When behaviour was measured weekly from 0-6 wk (Dixon, 2020), 4-8 wk (Ghayas et al., 2021), and 1-12 wk (Bokkers and Koene, 2003) fast-growing birds spent more time feeding and drinking. At 9, 24, and 31 d (de Jong et al., 2021), 8 wk (Abdourhamane and Petek, 2023), and overall (2, 6, 9 wk) (Wallenbeck et al., 2017) the fast-growing strains performed a higher percentage of ingestion behaviours. At 8 and 22 d (Güz et al., 2021) and 22, 36, and 50 d (Nielsen, 2012) a higher proportion of fast-growing broilers were observed feeding. No difference in feeding and drinking behaviours were noted at 29 d (Abeyesinghe et al., 2021; Güz et al., 2021). Bokkers and Koene (2003) assessed behaviour weekly from 1- 12 wk and reported that as fast-growing broilers aged, eating and eating while standing increased, while as slow-growing broilers aged, eating while sitting decreased. Castellini et al. (2016) reported the fast-growing strain and one medium-growing strain spent a higher percentage of time eating at 11 wk.
Fast growth broilers performed fewer locomotive behaviours at 0.6 kg (van der Eijk et al., 2022b),1.9 kg (van der Eijk et al., 2022b), and overall, at four target body weights of 0.4, 1.1, 1.7, and 2.1 kg (van der Eijk et al,2022a). At 2.1 kg, the fast-growing strains spent less time walking than slow-slow growth birds (Dawson et al., 2021). When behaviour was corrected for body weight (weight not reported), the fast growth broilers were observed walking less (Güz et al., 2021) and were reported to be less active (de Jong et al., 2021). At 3.2 kg, there was no difference in the length of time spent walking or for bout frequencies or total duration or number of bouts for walking (Dawson et al., 2021). When behaviour was measured weekly from 4-8 wk (Ghayas et al., 2021) and 1- 12 wk (Bokkers and Koene, 2003) fast-growing broiler walked less. This agrees with Dixon (2020) who noted, from 0-6 wk, fast-growing birds spent less time in locomotion from 0-6 wk, with differences occurring mainly later in life. When behaviour was assessed at the same ages, fast growth broilers walked less at 8, 22, and 29 d (Güz et al., 2021) and were less active at 9, 24, and 31 d (de Jong et al., 2021). Fast-growing strains spent less time in locomotion at 22, 36, and 50 d (Nielsen, 2012) , 8 wk (Abdourhamane and Petek, 2023), and 11 wk (Castellini et al., 2016). Baxter et al. (2021) reported no effect of strain on the average proportion of birds in locomotion from 3-6 wk. The authors also reported no effect on the length of activity bouts, however 7 wk old slow growth birds performed longer bouts than 6 wk old fast growth birds (last week of life for each strain). Bokkers and Koene (2003) assessed behaviour weekly from 1-12 wk and reported walking decreased with age for both fast and slow growing broilers. Fast-growing strains had lower overall activity from 26-31 d compared to the slow- growing strain from 26-49 d (Pearce et al., 2023).
When behavioural assessments were performed at the same unreported body weight, the fast-growing strain performed more sitting (Güz et al., 2021). At 2.1 kg, fast growth birds were more inactive compared to all slow growth strains, but no difference at 3.2 kg was noted (Dawson et al., 2021). No strain difference was observed for inactive behaviours at 0.4, 1.1, 1.7, or 2.1 kg (van der Eijk et al., 2022a). At 3.2 kg, there was no difference in the length of time spent sitting or time off feet or for bout frequencies or total duration or number of bouts for sitting or time off feet (Dawson et al., 2021). de Jong et al. (2021) assessed behaviour at 9, 24, and 31 d and reported that, at all ages, the fast strain spent a higher percentage of time sitting idle, while Güz et al. (2021) reported more fast strain birds were sitting at 22 and 29 d, but not at 8 d (Güz et al., 2021). When behaviour was measured weekly from 0-6 wk (Dixon, 2020), 4-8 wk (Ghayas et al., 2021), and 1-12 wk (Bokkers and Koene, 2003) fast growth birds spent more time sitting, with Dixon (2020) noting that differences occurred mainly later in life. In agreement, Wallenbeck et al. (2017) found that overall (2, 6, and 9 wk), fast growth broilers spent a higher proportion of time sitting. No difference in time spent resting was noted from 1-12 wk (Bokkers and Koene, 2003), 3-6 wk (Baxter et al., 2021), at 11 wk (Castellini et al., 2016), or overall (2, 6, and 9 wk) (Wallenbeck et al., 2017). Bokkers and Koene (2003) found that fast-growing broilers spent less time laying down, but there was no difference in sitting idle from 1-12 wk, while Baxter et al. (2021) reported no effect on the average proportion of birds sitting inactive from 3-6 wk. At 22, 36, and 50 d, fast-growing strains were less active and while a smaller proportion were observed lying down, a higher proportion of observations spent inactive, lying down, and lying close to other birds were observed (Nielsen, 2012). At 29 d, fast-growing birds showed more frequent side-lying, but no difference in sitting inactive (Abeyesinghe et al., 2021). Bokkers and Koene (2003) assessed behaviour weekly from 1-12 wk and reported that as fast-growing broilers aged lying decreased and sitting idle increased in both strains.
Overall, at four target body weights (0.4, 1.1, 1.7, and 2.1 kg) birds from the fast- growing strain spent less time standing (van der Eijk et al,2022a). This was also noted at the same unreported body weight (Güz et al., 2021) and when comparing behaviour at similar body weights (+/- 1.1 kg) (de Jong et al., 2021). At 2.1 and 3.2 kg, the fast- growing strain spent less time standing than the medium- and slow-slow growing strains (Dawson et al., 2021). At 2.1 kg, fast-growing birds had fewer standing bouts than medium- and slow-slow growing birds and there were no differences in bout frequencies or total duration or number of bouts for standing (Dawson et al., 2021). When behaviour was assessed at the same ages, fast growth broilers stood less at 8, 22, and 29 d (Güz et al., 2021) and 8 wk (Abdourhamane and Petek, 2023). At 29 d, fast growth birds showed less frequent and shorter durations of standing inactive (Abeyesinghe et al., 2021). From 0-6 wk (Dixon, 2020), 1-12 wk (Bokkers and Koene, 2003), and overall (2, 6, and 9 wk) (Wallenbeck et al., 2017), fast-growing broilers spent less time standing, with Dixon (2020) noting differences occurred mainly later in life.
Fast-growing broilers performed less comfort behaviours overall, at four target body weights (0.4, 1.1, 1.7, and 2.1 kg) (van der Eijk et al,2022a). However, at an unreported body weight, the fast growth strain performed more comfort behaviours, but less dustbathing (Güz et al., 2021). At 2.1 kg, no strain differences were observed for preening behaviour (Dawson et al., 2021). At 3.2 kg, there was no difference in the length of time spent preening or in bout frequencies or total duration or number of bouts for preening (Dawson et al., 2021). More broilers from the fast growth strain were dustbathing at 8 d, but no difference was noted at 22 or 29 d (Güz et al., 2021). No difference in comfort behaviours were observed at 29 d (Abeyesinghe et al., 2021). At 11 wk, the fast and medium growth strains performed more comfort behaviours than the slow strains (Castellini et al., 2016). Abdourhamane and Petek (2023) reported that fast- growing broilers, at 8 wk, spent more time dustbathing and had a tendency to preen more, but no strain differences for wing flapping were observed. When behaviour was measured weekly from 0-6 wk, fast-growing birds spent less time preening and dustbathing, with differences occurring mainly later in life (Dixon, 2020). From 4-8 wk, birds from the fast strain spent a lower percentage of time dustbathing, wing flapping, and litter scratching (Ghayas et al., 2021). Bokkers and Koene (2003) found that fast- growing broilers spent more time preening and stretching while sitting from 1-12 wk, however no differences in total preening or dustbathing were observed. They also reported that as fast-growing broilers aged preening and preening while standing increased. Baxter et al. (2021) reported no effect of strain on the average proportion of birds preening from 3-6 wk and that too few incidences of dustbathing were observed to include in statistical analyses. Wallenbeck et al. (2017) reported that overall (2, 6, and 9 wk), frequencies of comfort behaviours were very low incidence.
Fast-growing broilers performed less foraging at 0.6 kg (van der Eijk et al., 2022b), overall, at four target body weights (0.4, 1.1, 1.7, and 2.1 kg) (van der Eijk et al,2022a), and at an unreported body weight (Güz et al., 2021). When assessed at the same ages, fast-growing broilers foraged less at 22 and 29 d, with no difference at 8 d (Güz et al., 2021). When behaviour was measured weekly from 0-6 wk, fast growth birds spent less time foraging (Dixon, 2020). Overall (2, 6, and 9 wk), no differences in time spent foraging were noted by Wallenbeck et al. (2017). Baxter et al. (2021) reported that too few incidences of foraging were observed from 3-6 wk for statistical analyses.
Broilers from fast-growing strains performed less aggressive behaviours at 0.6 kg (van der Eijk et al., 2022b) and at the same unreported body weight (Güz et al., 2021). When behaviour was assessed at the same ages, fast growth broilers displayed less aggression at 22 and 29 d, with no difference at 8 d (Güz et al., 2021) or 8 wk (Abdourhamane and Petek, 2023). Bokkers and Koene (2003) assessed behaviour weekly from 1-12 wk and found that fast-growing broilers expressed less aggressive behaviours.
When behaviour was measured weekly from 0-6 wk (Dixon, 2020) and 1-12 wk (Bokkers and Koene, 2003), fast-growing birds spent less time perching, with Dixon (2020) noting differences occurred primarily at later ages. Baxter et al. (2021) reported no difference in time spent perching from 3-6 wk, but at slaughter age less fast-growing birds were observed on the perch. At 29 d, fast growth birds perched less (Abeyesinghe et al., 2021) and overall (2, 6, and 9 wk) spent less time perching during the daytime (Wallenbeck et al., 2017). The average number of perching attempts was lower in the fast growth strain and overall, the fast-growing strain was successful 75% of the time compared to 93% for the slow-growing strain (Baxter et al., 2021).
No strain effect on pecking behaviour was observed when behaviour was assessed weekly from 4-8 wk (Ghayas et al., 2021). Baxter et al. (2021) reported no effect of strain on the average proportion of birds sitting pecking from 3-6 wk. Bokkers and Koene (2003) found no differences for total ground pecking from 1-12 wk, although fast- growing broilers ground pecked while standing less. When behaviour was assessed weekly from 4-8 wk (Ghayas et al., 2021) and 1-12 wk (Bokkers and Koene, 2003) birds from the fast-growing strain spent a lower percentage of time litter scratching. At 22, 36, and 50 d (Nielsen, 2012) and 14 and 28 d and 3 d prior to slaughter at 2.1 kg less exploratory behaviour was noted in the fast-growing strain (Rayner et al., 2020).
No strain difference was observed for play behaviour at 0.4, 1.1, 1.7, or 2.1 kg (van der Eijk et al., 2022a). Less play behaviour was observed in the fast-growing strain at 14 and 28 d and 3 d prior to slaughter at 2.1 kg (Rayner et al., 2020) as well as from 3-6 wk (Baxter et al., 2021). Overall (2, 6, and 9 wk), the frequencies of some social behaviours (running, group running, play fighting, aggressive pecking, flying, food running, and severe feather pecking) were very low incidence (Wallenbeck et al., 2017). Baxter et al. (2021) reported no difference in food running (play behaviour) from 3-6 wk between strains.
At 9 wk, a higher percentage of fast-growing broilers were observed panting (Wilhelmsson et al. 2019). No difference in the percentage of birds panting at 2 or 6 wk was noted and no difference in the percentage of birds huddling was observed at any age. Nielsen (2012) concluded that fast-growing broilers use behavioural changes when adapting to warm environments, whereas slow-growing birds use metabolic changes to adapt to cooler ambient temperatures.
When assessed at similar body weights (+/- 1.1 kg), more broilers from the fast-growing strain were observed sitting under the ramp/platform and less were observed sitting on the perch (de Jong et al., 2021). Similarly, when assessing use of enrichments at a similar unreported body weight, fewer fast-growing birds were located on platforms and ramps and perching on barriers, while more were located under platforms and ramps (Güz et al., 2021). van der Eijk et al. (2022b) reported that at 0.6 and 1.9 kg, the percentage of fast growth broilers located on the platform/ramp was less and at 1.9 kg, the percentage under the platform/ramp was higher. These findings are supported by Dawson et al. (2021) who found, at 2.1 kg, a larger proportion of four of twelve slow-growing strains were observed using all enrichments, on top of the platform, and using the area under the platform and ramp but there was no difference in the proportion of birds using the scale. At 3.2 kg, fast growing strains used all enrichments to a lesser extent and therefore, were observed on the litter more. When assessed at the same age (9, 24, and 31 d), slow-growing birds used the total enrichments, ramp/platform, and perch more (de Jong et al., 2021). Güz et al. (2021) reported the percentage of birds on platforms and ramps at d 29 and perching on barriers at d 8, 22, and 29 were higher for slow growth birds, while the percentage of fast growth chickens under platforms and ramps at d 29 was higher. Malchow and Schrader (2021) reported that the slow-growing strain (reared to 10 wk) used the elevated platform more than the fast-growing strain (reared to 5 wk).
van der Eijk et al. (2022a) assessed enrichment use at 0.4, 1.1, 1.7, and 2.1 kg. The authors reported that at 0.4 kg, slow-growing broilers reared at 30kg/m2 sat more on a bale compared to those reared at 24 and 36 kg/m2 and compared to fast-growing broilers at the same density. More slow-growing broilers reared at 42 kg/m2 were on the bale compared to fast-growing broilers at the same density. At 1.1 kg, less fast growth broilers reared at 36 kg/m2 were sitting on the bale compared to those reared at 24 and 30 kg/m2 and compared to slow growth broilers at the same density. Fewer fast-growing broilers reared at 24 kg/m2 were sitting on the bale compared to the slow-growing strain at the same density. At 1.7 and 2.1 kg, more slow-growing broilers reared at 24, 36, and 42 kg/m2 were sitting on the bale compared to the fast-growing strain at the same densities. Overall, a higher percentage of slow-growing broilers were observed on the bale, which agrees with Rayner et al. (2020) who found slow-growing strains occupied more bales at 14 and 28 d and 3 d prior to slaughter at 2.1 kg.
Slow-growing broilers had cleaner feathers when assessed at the same body weights (Dixon, 2020 (2.2 and 2.5 kg); Abeyesinghe et al., 2021 (2.2 kg); van der Eijk et al., 2023 (2.3 kg)). The same was observed when birds were assessed at the same ages (Wilhelmsson et al., 2019 (6 and 9 wk); Ghayas et al., 2021 (unreported age); Çavuşoğlu and Petek, 2019 (6, 7, and 8 wk). Two studies reported no differences in feather cleanliness at 2.0 kg (van der Eijk et al., 2023) or at 2 wk (Wilhelmsson et al., 2019). Malchow and Schrader (2021) reported that the slow-growing strain (reared to 10 wk) had cleaner breast feathers than the fast-growing strain (reared to 5 wk).
Castellini et al. (2016) reported a lower incidence of breast blisters in slow-growing strains at 11 wk.
van der Eijk et al. (2023) reported improved litter quality with slow-growing broilers at target weights of 0.4, 1.1, 1.7, and 2.1 kg. This agrees with Rayner et al. (2020) who assessed litter 2 d prior to slaughter at 2.1 kg. Additionally, litter quality was better in the slow growth treatment at 14, 21, 28, and 35 d (van der Eijk et al., 2022b). Malchow and Schrader (2021) reported that, generally, the slow-growing strain (reared to 10 wk) had better litter quaity than the fast-growing strain (reared to 5 wk). Baxter et al. (2021) reported no difference in litter quality from 3-6 wk, except at 4 wk where an improvement was noted for slow-growing birds. No difference in litter quality at 2 or 6 wk was reported by Wilhelmsson et al. (2019), however at 9 wk litter quality was poorer in the feeding area of the fast-growing strain. Litter moisture was lower for the slow- growing strains at 14 and 28 d, with no difference observed at 42 d (Santos et al., 2022b). No strain effect on litter dry matter was noted at 20, 34, 41, and 48 d (Nielsen, 2012).
Baxter et al. (2021) measured environmental conditions from 3-6 wk and found no differences in dust or ammonia levels. Rayner et al. (2020) reported that ammonia was variable across all strains and did not exceed levels of 20 ppm when measured 2 d prior to slaughter at 2.1 kg.
Weimer et al. (2020) reported that slow-growing broilers had longer bodies (14-27 d and 28-41 d for fast strain and 15-37 d and 38-62 d for slow strain), a narrower pelvis (28-41 d for fast and 38-62 d for slow), longer shanks (1-13 d, 14-27 d, and 28-41 d for fast and 2-14 d, 15-37 d, and 38-62 d for slow), narrower shanks (28-41 d for fast and 38-62 d for slow), longer keels (1-13 d and 28-41 d for fast and 2-14 d and 38-62 d for slow), and tended to have narrower breasts (14-27 d for fast and 15-37 d for slow). There was no effect of strain on breast depth at any measured age. Results from Singh et al. (2021) agree, with slow growth broilers having longer and narrower bodies and breasts and longer shanks when assessed at a target weight of 2.0-2.2 kg.
Huo et al. (2022) compared breast and leg muscle fiber characteristics and glycolytic potential in day old chicks to determine if the parameters were consistent at hatch. No strain differences were observed for muscle fibre diameter or cross-sectional area. However, the fast-growing strain had lower fibre density and higher glycolytic potential, especially in breast muscle. Leg muscle of the fast-growing strain had higher myosin heavy-chain 1 fibres (higher glycogen content) and higher glycolytic potential.
At 2.0 kg, fast- and medium-growing strains had higher numbers of muscle fibers and higher fiber cross-sectional area and diameter (Chodová et al., 2021b). When strains were reared to 2.8 kg, the fast-growing strain had a larger fibre area and lower fibre number per square millimeter and capillary number (Yalcin et al. 2019). When fast growth broilers were slaughtered at 6 wk (2.5 kg) and slow growth birds were slaughtered at 12 wk (2.9 kg), fibre cross sectional area in the breast muscle was greater in fast growth birds (Berri et al., 2005). Chodová et al. (2021a) reported a lower number of muscle fibres with larger cross-section area in the fast-growing strain (35 d) compared to the medium- (56 d) and slow-growing strains (70 d). Weng et al. (2022a) assessed breast muscle characteristics at similar body weights (1.97 kg at 37 d for the fast strain vs 1.77 kg at 101 d for the slow strain) and found the fast-growing strain had smaller muscle fibre diameter and cross-sectional area, the fibres were less closely aligned, and there was no difference in fibre type.
Weng et al. (2022a) observed differences in phosphoropeptide abundance, with 114 phosphopeptides upregulated and 481 downregulated in slow-growing breast meat. Overall, the upregulated phosphopeptides were related to myofibrillar proteins and proteins involved in collagen binding and amino acid transport. The downregulated phosphopeptides were related to protein kinases, glycolytic enzymes, calcium release- proteins, and proteins involved in lipid metabolism, which generally suggests a reduction in glycolysis and metabolism in slow growth birds. Chen et al. (2023) assessed thigh meat in fast-growing broilers at 42 d and slow-growing birds at 120 d and found 55 metabolites (28 upregulated and 27 downregulated) that differed between strains. Fast growth broilers had higher contents for mannose, D-fructose, dopamine, inosine, and 1,4-butanediamine, while arachidonic acid, malic acid, linoleic acid, and benzoic acid, while hypoxanthine contents were lower. Many of the of the dominant expressed metabolites in the fast-growing strain are related to growth, while the differential metabolites in the slow-growing strain were unsaturated fatty acids, which can be related to immune response.
Wang et al. (2022b) assessed breast and thigh muscle characteristics in fast-growing broilers at 37 d (2.02 kg) and at 101 d (1.65 kg) in slow-growing birds. They found fast- growing broilers had muscle fibres with less cross-sectional area and that the fibres were less closely aligned with more extracellular space, which agreed with their previous study (Weng et al., 2022a). Fewer oxidative fibres were found in the soleus muscle of fast growth broilers, whereas more oxidative fibres were located in the extensor digitorum longus muscle compared to the gastrocnemius or soleus muscles.
Gene expression of actin alpha 1 (important in the maintenance of muscle contraction (Laing et al., 2009)) was not different in broilers reared to 2.8 kg (Yalcin et al., 2019), however the fast-growing strain had lower expression of vascular endothelial growth factor (important for skeletal muscles to maintain capillarity as well as increase capillarity for greater oxygen metabolism (Tang et al., 2004)), myosin light chain kinase 2 (involved in muscle contraction; expression is greater in fast-twitch muscles (Kamm and Stull, 2001)), and ATPase Ca+2 (involved in transporting calcium ions from the cytoplasm to the sarcoplasmic reticulum, required for muscle relaxation after contraction (Periasamy and Kalyanasundaram, 2007; Lipskaia et al., 2010).
Fast-growing strains exhibited higher creatine kinase when reared to 2.8 kg (Yalcin et al., 2019), at 48 d (Mohammadigheisar et al., 2020), and from 6-10 wk (Mattioli et al., 2017). Creatine kinase activity can be an indicator of muscle damage or myopathy (Sandercock and Mitchell, 2003). However, it may also reflect selection for muscle mass (Szabo and Milistis, 2007). Berri et al. (2007) found positive phenotypic and genetic correlations between muscle fibre area and creatine kinase activity suggesting it could also reflect protein turnover related to muscle growth.
Wen et al. (2017a) assessed the impact of dietary methionine on breast muscle parameters. At 42 d, no difference in insulin-like growth factor-I (IGF-I) was reported. The fast-growing strain had higher mRNA expression of genes that are involved in regulating skeletal muscle development. No difference in the downstream targets of IGF-I were noted in the breast although phosphorylation of one target (involved in cell proliferation and protein synthesis) tended to be higher in birds from the fast-growing strain. The results suggest a different response of strain to dietary methionine, with fast- growing birds being more sensitive to methionine supplementation. Enhanced myogenic gene expression may explain the increased performance and breast muscle yield observed in fast growth broilers. No difference was observed for superoxide dismutase or total antioxidant capacity at 42 d, however the fast-growing strain had lower malondialdehyde content and higher glutathione peroxidase activity in the breast muscle (Wen et al., 2017b).
Zhang et al. (2022) identified 18 metabolites in breast muscle that could act as biomarkers differentiating the commercial fast strain (42 d) from a local slow growing strain (120 d). In addition, the authors found different pathways for the two strains based on the different metabolites identified. The authors concluded these results suggest a strain difference for meat quality-associated metabolism.
Qin et al. (2021) assessed blood glucose weekly from 1-70 d and reported that birds from the fast-growing strain had lower blood glucose at 7 d, but not at any other period. Mattioli et al. (2017) collected blood samples weekly from 6-10 wk and found no effect of strain on reactive oxygen molecular substances (ROMS), vitamin E (a-T3, g-T3, g-T, d- T), vitamin A (retinol), or carotenoids (lutein and zeaxanthin). Fast-growing birds had fewer thiobarbituric acid reactive substances (TBARS), which agrees with Castellini et al. (2016) at 11 wk but differs from Huerta et al. (2023) who reported not difference in TBARS in fast growth (42 d) and slow growth broiler (99 d) . Both strains responded differently to exercise, with slow-growing birds showing a progressive reduction in TBARS and ROMS, whereas the fast-growing birds had a progressive worsening of oxidative and antioxidant status. No difference in plasma proteins, electrolytes, or minerals were noted when broilers were reared to 48 d (Mohammadigheisar et al., 2020). The only observed difference in metabolites was for creatinine, which was higher in broilers from the fast-growing strain which also had higher aspartate transaminase and lactate dehydrogenase. No effect on blood parameters (white blood cell count, red blood cell count, hematocrit, hemoglobin, platelet count, total triiodothyronine (T3), total thyroxin (T4), or thyrotropin-stimulating hormone) were observed in broilers assessed at 81 d (Cömert et al., 2016). At 84 d, fast-growing broilers had higher blood albumin, cholesterol, calcium, and phosphorous and lower globulin and triglyceride (Sarica et al., 2014). No difference in protein or the calcium to phosphorous ratio were noted. At 11wk, Castellini et al. (2016) reported an inconsistent strain impact on heterophil to lymphocyte ratio, which is a measure of stress in poultry. Additionally, fast- and medium- growing strains had higher percentages of heterophils and lower percentages of monocytes and eosinophils, with no difference on basophils or red blood cells, and inconsistent effects on lymphocytes. Fast- and medium-growing strains also had lower haemoglobin and hematocrit values and strain had inconsistent impacts on percentage of platelets.
Fast-growing birds had higher surface body temperature, lower cloacal temperature, higher respiratory rate, and lower heart rate when measured at the same unreported age (Ghayas et al., 2020).
Yield
The fast-growing strain had higher carcass yield at 2.0 kg (Chodová et al., 2021b) and 2.8 kg (Weimer et al., 2022). At 2.1 and 3.2 kg, carcass yield was higher for the fast- growing strains compared to the slowest slow-growing strain (Santos et al., 2021). When yield was assessed at the same age, carcass yield was higher in the fast-growing strains (Grashorn, 2006 (84 d); Yamak et al., 2014 (average from 8, 10, 12 wk); Wen et al., 2017b (42 d); Aksoy et al., 2021 (56 d)). Conversely, Tůmová et al. (2021) reported a higher dressing out percentage for slow-growing broilers at 2.0 kg, while Dixon (2020) slaughtered birds at the same unreported live weight and reported that one of the three fast-growing strains evaluated had the lightest eviscerated carcass weight. No difference in carcass weight was observed at 2.0 kg (Doğan et al., 2019) and 2.6 kg (van der Eijk et al., 2023) or 71 d (Rezaei et al., 2018), 81 d (Cömert et al., 2016), and 84 d (Sarica et al., 2014; 2019). Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) and found the fast-growing strain had the highest dressing out percentage. Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast growth strain had higher carcass yield. Fanatico et al. (2008) performed two experiments, one with female broilers reared to 63 d (fast; 3.4 kg) or 91 d (slow; 2.2 kg) and the second with male broilers reared to 56 d (fast; 2.8 kg) or 84 d (slow; 2.7 kg). In the female trial, where live weights differed by over a kilogram, the fast-growing strain had a higher carcass yield. However, for the male trial, where final body weights were similar, no difference in carcass yield was observed. Fast-growing broilers with outdoor access had higher carcass yields compared to slow or medium growth strains, however, when reared indoors, both the fast- and slow- growing strains had higher carcass yield than the medium-growing strain (Fanatico et al., 2005a).
Breast meat yield was higher in the fast-growing strains at 2.0 kg (Doğan et al., 2019; Chodová et al., 2021b; Tůmová et al., 2021), 2.1 kg (Santos et al., 2021), 2.0-2.2 kg (Singh et al., 2021), 2.6 kg (van der Eijk et al., 2023), 2.8 kg (Weimer et al., 2022), and 3.2 kg (Santos et al., 2021). At 2.1 kg, breast meat yield was higher in the fast-growing strain compared to 2 of the 3 slow-medium growing strains (Mohammadigheisar et al., 2021). When assessed at the same age, fast-growing strains also had higher breast yield (Grashorn et al., 2006 (84 d); Mikulski et al., 2011 (65 d); Sarica et al., 2014( 84 d); Yamak et al., 2014 (for the average from 8, 10, 12 wk); Cömert et al., 2016 (81 d); Wen et al., 2017b (42 d); Rezaei et al., 2018 (71 d); Sarica et al., 2019 (84 d); Aksoy et al., 2021(56 d)). Additionally, breast yield was also higher in fast-growing broilers when measured at different ages and weights. Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast growth strain had higher breast yield. Breast meat yield was higher in a fast-growing strain at 6 wk (2.5 kg) compared to medium (8 wk; 2.6 kg) and slow (12 wk; 2.9 kg) growth strains (Quentin et al., 2003).
Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) and found the fast-growing strain had the highest breast yield. Fanatico et al. (2008) studied female broilers reared to 63 or 91 d and male broilers reared to 56 or 84 d and found breast yield was always higher in the fast-growing strain. Fanatico et al. (2005a) reared birds to 53 or 81 d, with similar live weights of 2.5 and 2.2 kg, and reported higher yield in the fast-growing strain. When fast-growing birds were slaughtered at 6 wk (2.5 kg) and slow-growing birds were slaughtered at 12 wk (2.9 kg), breast yield was again highest in fast-growing broilers (Berri et al., 2005). Zhang et al. (2022) compared breast weight at market age in a local slow growing strain (120 d) to a commercial fast-growing strain (42 d) and the fast-growing strain had significantly larger breasts.
Leg yield was lower in fast-growing strains at 2.0 kg (Doğan et al., 2019), 2.0-2.2 kg (Singh et al., 2021), 2.8 kg (Weimer et al., 2022), and 3.2 kg (Santos et al., 2021). Thigh yield was lower in the fast-growing strains compared to the slowest slow-growing strains, while drumstick yield was lower compared to the medium-slow and slow-slow growing strains at 2.1 kg (Santos et al., 2021). Leg yield was lower for fast-growing birds at 6 wk (2.5 kg) compared to medium (8 wk; 2.6 kg) and slow (12 wk; 2.9 kg) growth broilers (Quentin et al., 2003). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast growth strain had lower leg yield. Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) and found the fast-growing strain had the lowest thigh yield. Fanatico et al. (2005a) reared birds to 53 d (2.5 kg) or 81 d (2.2 kg) and reported fast growth broilers had lower leg yields. At 84 d, thigh yield was lower for fast growth broilers (Grashorn, 2006; Sarica et al. (2019). Fast-growing broilers also had lower leg yield when birds were processed at 81 d (Cömert et al., 2016) and 84 d (Sarica et al., 2014). When the average was calculated from 8, 10, 12 wk, leg yield was lower for fast-growing birds (Yamak et al., 2014). Only one study (65 d) found that a fast strain had higher thigh yield (Mikulski et al., 2011). Fanatico et al. (2008) studied female broilers reared to 63 or 91 d and males reared to 56 or 84 d. Fast-growing females had lower leg yield, but no strain differences were observed for males. Other studies also reported no strain effect on leg yield when birds were processed at the same weight (van der Eijk et al., 2023 (2.6 kg)) and same age (Rezaei et al., 2018 (71 d); Aksoy et al., 2021 (56 d)). When fast growth broilers were slaughtered at 6 wk (2.5 kg) and slow growth birds were slaughtered at 12 wk (2.9 kg), no difference in leg yield was reported (Berri et al., 2005).
Strains selected for fast growth had lower wing yield when assessed at the same body weight (Doğan et al., 2019 (2.0 kg); Santos et al., 2021 (2.1 and 3.2 kg); Singh et al., 2021 (2.0-2.2kg); Weimer et al., 2022 (2.8 kg)van der Eijk et al., 2023 (2.6 kg)). The same is evident when birds were slaughtered at different ages and weights (Fanatico et al., 2005a ( 53 or 81 d, 2.48 and 2.18 kg); 2008 (63 or 91 d, 3.4 and 2.2 kg; 56 or 84 d, 2.8 and 2.7 kg); Huerta et al., 2023 (42 and 99 d, 2.9 vs 1.8 and 2.2 kg)). At 84 d (Sarica et al., 2014) and when the average from 8, 10, and 12 wk was calculated fast-growing birds again had lower wing yield (Yamak et al., 2014).
Fast-growing broilers had lighter frames when birds were processed at the same weight (Doğan et al., 2019 (2.0 kg); Singh et al., 2021 (2.0-2.2kg); Weimer et al., 2022 (2.8 kg)) and at 65 d (Mikulski et al., 2011). The same was found when birds were slaughtered at different ages and weights (Fanatico et al., 2005a (53 or 81 d, 2.5 and 2.2 kg); 2008 (63 or 91 d, 3.4 and 2.2 kg and 56 or 84 d (2.8 and 2.7 kg)).
Fast-growing broilers had less abdominal fat at 2.0 kg (Doğan et al., 2019; Chodová et al., 2021b; Tůmová et al., 2021), 2.0-2.2 kg (Singh et al., 2021), an unreported weight (Dixon, 2020), and at 65 d (Mikulski et al., 2011). The same results were found when strains were slaughtered at different ages and weights. Fast-growing birds had less abdominal fat when slaughtered at 6 wk (2.5 kg) compared to slow-growing birds at 12 wk (2.9 kg) (Berri et al., 2005), and when fast-growing broilers were slaughtered at 6 wk (2.5 kg) compared to medium (8 wk; 2.6 kg) and slow growth birds (12 wk; 2.9 kg) (Quentin et al., 2003). Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) and found the fast-growing strain had the least abdominal fat. No difference in abdominal fat was noted at 56 d (Aksoy et al., 2021), 81 d (Cömert et al., 2016), 84 d (Grashorn, 2006; Sarica et al., 2014, 2019), or for the average from 8, 10, 12 wk (Yamak et al., 2014).
Myopathies
Fast-growing broilers had a higher proportion of white striping when assessed at an unreported, but statistically similar live weight (Dixon, 2020). At 2.1 kg, fast-growing strains had a higher incidence of white striping compared to medium-slow and slow- slow growing strains and at 3.2 kg compared to all slow strains (Santos et al., 2021). No difference between the fast- and slow-growing strains was noted for severity of white striping at 2.1 kg, however at 3.2 kg, the fast-growing strain had more severe scores compared to all slow growth strains. Average white striping scores at 2.1 kg, were worse for the fast growth strain compared to the medium and slow-slow growth strains, while at 3.2 kg, the fast growth strain had poorer scores than all slow-growing strains.
Two of the three fast-growing strains had a higher proportion of woody breast scores when assessed at an unreported statistically similar live weight (Dixon, 2020). Fast- growing broilers had higher woody breast scores at 2.3 kg (van der Eijk et al., 2022b) and a higher incidence at 2.1 and 3.2 kg (Santos et al., 2021). The severity and average scores of woody breast at 2.1 kg was worse for the fast growth strains compared to the medium and slow-slow growth strains. At 3.2 kg, the fast-growing strains had more severe scores and poorer average scores compared to all slow-growing strains.
Histomorphology
No differences in gut villus height or crypt depth were reported at 2.1 kg (Mohammadigheisar et al., 2021) or at 48 d (Mohammadigheisar et al., 2020). Conversely, Akyüz et al. (2022) assessed histomorphology weekly from 1-10 wk and found villi height was greater for the fast-growing strain at all ages and crypt depth was greater at some ages (1, 4-8 wk). Villus height:crypth depth ratio was larger in fast- growing broilers from 2-5 and 10 wk.
Organ weights
Fast-growing broilers had heavier relative crop and proventriculus weights at 2.0-2.2 kg (Singh et al., 2021).
At 2.0 kg (Doğan et al., 2019) and 2.0-2.2 kg (Singh et al., 2021) fast-growing broilers had lighter relative gizzard weights. Similarly, relative gizzard weight was lighter in the fast strain compared to 1 of the 3 slow-medium growth strains at 2.1 kg (Mohammadigheisar et al., 2021). Mohammadigheisar et al. (2020) found relative gizzard weight was lighter in the fast-growing strain compared to some but not all the slow strains assessed when birds were reared to 48 d. No difference in absolute gizzard weight was noted at 81 d (Cömert et al., 2016).
At 2.0-2.2 kg, fast-growing broilers had lighter duodenums but heavier jejunums and ileums (Singh et al., 2021). No difference in relative small intestine weight was observed at 2.1 kg (Mohammadigheisar et al., 2021) or 2.8 kg (Weimer et al., 2022). Relative small intestine weight was similar between the fast-growing strain and some of the slow- growing strains at 48 d (Mohammadigheisar et al., 2020). Fast-growing broilers had heavier absolute duodenum and ileum-jejunum weights at 81 d, but no difference in absolute large intestine weight was noted (Cömert et al., 2016). At 2.8 kg, fast-growing broilers had lighter but longer large intestines (Weimer et al., 2022).
At 2.0-2.2 kg, fast-growing broilers had lighter relative ceca weights (Singh et al., 2021), while at 2.8 kg, they had lighter and shorter ceca (Weimer et al., 2022). Relative weights for the ceca were lighter in the fast growth strain compared to 2 of the 3 slow-medium growth strains assessed at 2.1 kg (Mohammadigheisar et al., 2021), whereas no difference in absolute ceca weight was noted at 81 d (Cömert et al., 2016).
Heavier relative liver weights were reported for fast-growing broilers at 2.0 kg (Doğan et al., 2019), 2.0-2.2 kg (Singh et al., 2021), and 2.8 kg (Weimer et al., 2022) and heavier absolute weights were reported at 81 d (Cömert et al., 2016). Qin et al. (2021) measured absolute liver weight weekly and observed heavier weights in the fast- growing strain at 28, 49, and 70 d, with no difference at 7 d. Fast-growing males had heavier relative heart weights at 2.0 kg, but no difference was noted for females (Doğan et al., 2019). No difference for heart size relative to body weight at 2.0-2.2 kg (Singh et al., 2021) and 2.8 kg (Weimer et al., 2022) was noted or for absolute heart weight at 81 d (Cömert et al., 2016). No difference in pancreas weight was noted at 2.0-2.2 kg (Singh et al., 2021), however fast-growing broilers had heavier absolute pancreas weights at 81 d (Cömert et al., 2016). No difference in absolute spleen, proventriculus, or bursa weights were observed at 81 d (Cömert et al., 2016).
Nutrient (tissue) composition
Han and Baker (1991) reported no differences in total body protein concentration between a fast- and slow-growing strain when measured at 8 and 21 d. Tran et al. (2021) found that protein content of the carcass decreased with decreasing dietary lysine level and that the decrease was greater in the fast-growing strain. In contrast, the fat content of the carcass increased with decreasing lysine and the increase was greater in the slow-growing strain.
Fast-growing broilers had breast meat with less protein at 2.0 kg (Chodová et al., 2021b) and at 65 d (Mikulski et al., 2011). When fast growth broilers were slaughtered at 6 wk (2.5 kg) and slow growth birds were slaughtered at 12 wk (2.9 kg), breast protein content was also lower in the fast-growing strain (Berri et al., 2005). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast- growing strain had lower crude protein. Fanatico et al. (2007) assessed broilers reared to 63 and 91 d with indoor vs outdoor access and reported that fast-growing broilers had lower protein content. Weng et al. (2022a) compared a local slow growth strain reared to 101 d (1.77 kg) to a commercial fast-growing strain reared to 37 d (1.97 kg) and found lower protein and collagen content in the breast muscle of the fast growth birds. This agrees with Weng et al. (2022b) who reported lower breast protein content in fast- growing broilers at 37 d (2.02kg) compared to slow-growing broilers at 101 d (1.65 kg). Chodová et al. (2021a) reported the fast- (35 d) and medium-growing strains (56 d) had lower crude protein than the slow-growing strain (70 d). No strain difference was noted at 42 d (Wen et al., 2017b), 81 d (Cömert et al., 2016), or 84 d (Sarica et al., 2014). When assessing crude protein content in thigh meat, no strain difference was noted at 81 d (Cömert et al., 2016) or 84 d (Sarica et al., 2014). At 56 d, thigh crude protein was higher in the fast-growing strain (Özbek et al., 2022), whereas Weng et al. (2022b) reported lower thigh protein content in fast-growing broilers at 37 d (2.02kg) compared to slow-growing broilers at 101 d (1.65 kg).
Breast dry matter was lower in the fast-growing strain at 2.0 kg (Chodová et al., 2021b) and at 84 d (Sarica et al., 2014), however no strain difference was noted for breast moisture at 42 d (Wen et al., 2017b). Zhang et al. (2022) reported lower breast dry matter in the fast growth strain at a market age of 42 d compared to a local slow growing strain at 120 d. This agrees with Weng et al. (2022a) who compared slow growth broilers reared to 101 d (1.77 kg) to fast-growing broilers at 37 d (1.97 kg) and found higher moisture content in the breast muscle of the fast growth birds. Weng et al. (2022b) also reported higher moisture content in fast growth broilers at 101 d (1.65 kg) compared to fast growth broilers at 37 d (2.02 kg), as did Huerta et al. (2023) when fast growth broilers were reared to 42 d and slow growth strains to 99 d. Chodová et al. (2021a) reported the fast- (35 d) and medium-growing strains (56 d) had lower dry matter than the slow-growing strain (70 d). When fast-growing broilers were slaughtered at 6 wk (2.5 kg) and slow-growing birds at 12 wk (2.9 kg), breast dry matter was lower in the fast-growing strain (Berri et al., 2005). However, Fanatico et al. (2005b) reported that fast strain birds reared to 53 d with outdoor access, had higher breast meat dry matter than slow growth birds reared to 81 d (indoor or outdoor access), and medium growth strains reared to 67 d (indoor or outdoor access). Fanatico et al. (2007) assessed broilers reared to 63 and 91 d with indoor vs outdoor access and found no difference in breast meat dry matter. Cömert et al. (2016) reported no difference in breast dry matter at 81 d. For thigh moisture, Chen et al. (2023) assessed fast-growing broilers at 42 d and slow-growing birds at 120 d and reported a tendency for fast growth birds to have higher dry matter. At 56 d (Özbek et al., 2022) and 84 d (Sarica et al., 2014) no effect of strain was observed on leg meat moisture content (Özbek et al., 2022). Weng et al. (2022b) also reported no strain impact on moisture content for slow growth broilers reared to 101 d (1.65 kg) compared to fast growth broilers reared to 37 d (2.02 kg). Fast-growing broilers had higher dry matter of drumstick meat at 81 d (Cömert et al., 2016).
No strain difference was noted for breast muscle crude ash 2.0 kg (Chodová et al., 2021b) or at 42 d (Wen et al., 2017b), 81 d (Cömert et al., 2016) or 84 d (Sarica et al., 2014). Chodová et al. (2021a) reported no strain effect on breast ash between the fast- (35 d), medium- (56 d) and slow-growing strains (70 d). Fanatico et al. (2007) assessed broilers reared to 63 and 91 d with indoor vs outdoor access and found no difference in breast ash. Fast-growing birds reared indoors had higher breast ash compared to fast- or slow-growing birds with outdoor access (Fanatico et al., 2005b). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast-growing strain had lower ash. Broilers slaughtered at 56 d (Özbek et al., 2022), 81 d (Cömert et al., 2016) and 84 d (Sarica et al., 2014) showed no strain effects on thigh crude ash.
Fast-growing broilers had breast meat with more breast intramuscular fat at 2.0 kg (Chodová et al., 2021b) and at 65 d (Mikulski et al., 2011). Fanatico et al. (2007) assessed male broilers reared to 56 and 84 d and reported breast meat from fast growth broilers had twice the amount of fat. This agrees with Weng et al. (2022a) who compared slow growth broilers reared to 101 d (1.77 kg) to fast-growing broilers at 37 d (1.97 kg) and found higher intramuscular fat content in the breast muscle of the fast- growing birds. Weng et al. (2022b) found the same result in slow growth broilers reared to 101 d (1.65 kg) compared to fast growth broilers reared to 37 d (2.02 kg). Chodová et al. (2021a) reported the fast- (35 d) and medium-growing strains (56 d) had higher ether extract than the slow-growing strain (70 d). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast-growing strain had higher ether extract. No strain difference was noted for breast fat at 42 d (Wen et al., 2017b), 84 d (Sarica et al., 2014), or when fast growth birds were slaughtered at 53 d, medium growth birds at 67 d, or slow growth birds at 81 d (Fanatico et al., 2005b). This agrees with Berri et al. (2005) who reared fast-growing broilers to 6 wk (2.5 kg) and slow- growing birds to 12 wk (2.9 kg) and found no difference in lipid content of the breast muscle. Zhang et al. (2022) reported lower intramuscular fat in the breast of a fast growth strain at a market age of 42 d compared to a local slow growing strain at 120 d. Broilers slaughtered at 81 d also showed no difference in ether extract in breast muscle, however in the thigh muscle fast-growing broilers had higher ether extract (Cömert et al., 2016). Chen et al. (2023) assessed thigh meat in fast-growing broilers at 42 d and slow-growing birds at 120 d and reported the fast growth birds had higher intramuscular fat. At 56 d, thigh crude fat was lower in fast-growing birds (Özbek et al., 2022). Sarica et al. (2014) reported no strain difference for thigh fat at 84 d.
No strain differences were found in the fatty acid profile of the breast at 65 d (Mikulski et al., 2011). At 81 d, Cömert et al. (2016) found no effect of strain on total fatty acid composition or on individual fatty acids of drumstick meat (except C20:4n-6). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and they reported that the fatty acid profile between strains was generally similar for whole saturated, monounsaturated, and polyunsaturated fatty acids proportions. In the whole saturated fatty acid group, the fast-growing strain has lower stearic acid and other minor acids. For monounsaturated fatty acids, the fast-growing strain had lower palmitoleic acid and higher oleic acid. At 2.0 kg strain had no impact on cholesterol (Chodová et al., 2021b), whereas Chodová et al. (2021a) reported the fast- (35 d) and medium-growing strains (56 d) had higher cholesterol in the breast meat than the slow-growing strain (70 d). Fanatico et al. (2007) assessed broilers reared to 56 and 84 d and, when corrected on a fat basis, found no difference in breast vitamin A content, but vitamin E (a- tocopherol) was higher in the slow growth strain. Castellini et al. (2016) also reported higher a-tocopherol in slow-growing strains at 11 wk.
The overall accreted amino acid profile was similar between strains, except lysine accretion was higher in the fast-growing strain (Tran et al., 2021). The authors noted that the efficiency of protein/amino acids accretion, corrected for percentage of intake, was also higher for fast-growing broilers. Zhang et al. (2022) reported a higher concentration of 16 out of 18 free amino acids in the breast of fast growth broilers at a market age of 42 d compared to a local slow-growing strain at 120 d, while L-anserine concentration was lower, and no difference existed for L-carnosine. Chen et al. (2023) assessed thigh meat in fast-growing broilers at 42 d and slow-growing birds at 120 d and reported taurine, ethanolamine, and anserine were lower in the fast-growing strain.
Colour of meat
At 56 d, Özbek et al. (2020) reported no effect of strain on C* (chroma) or ho (hue) in breast or thigh meat.
At 2.8 kg, breast meat L* values (lightness) were lower in fast-growing broilers (Weimer et al., 2022), whereas, at 2.0-2.2 kg, they were higher (Singh et al., 2021). At 2.0 kg, the fast- and medium-growing strains had lower L* values for breast muscle compared to the slow growth strain (Chodová et al., 2021b). L* values were higher for fast growth birds (6 wk; 2.5 kg) compared to medium (8 wk; 2.6 kg) and slow growth strains (12 wk; 2.9 kg) (Quentin et al., 2003). This agrees with Berri et al. (2005) when fast-growing broilers were slaughtered at 6 wk (2.5 kg) and slow-growing birds were slaughtered at 12 wk (2.9 kg). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast-growing strain had higher L* values. Weng et al. (2022b) reported higher L* values for fast growth broilers at 37 d (2.02 kg) compared to slow growth broilers at 101 d (1.65 kg). No difference in L* values were reported at 42 d (Wen et al., 2017b), 56 d (Özbek et al., 2020), 65 d (Mikulski et al., 2011), or 84 d (Grashorn, 2006; Sarica et al., 2014, 2019). This agrees with Weng et al. (2022a) who assessed colour in fast-growing broilers at 37 d (1.97 kg) and slow-growing birds at 101 d (1.77 kg). Fanatico et al. (2007) compared broilers at 63 and 91 d (females) and 56 and 84 d (males) and found no difference in L* values. Fanatico et al. (2005b) reported that when reared indoors, no difference in L* values were noted, whereas when birds had outdoor access, fast-growing birds had lower values. At 2.0 kg, the medium-growing strain had lower L* values for breast skin compared to fast and slow growth strains (Chodová et al., 2021b).
Weimer et al. (2022) found no difference in a* values (redness) of the breast at 2.8 kg, however at 2.0-2.2 kg, Singh et al. (2021) reported lower a* values in the fast-growing strain. Weng et al. (2022a) compared values at similar body weights (1.97 vs 1.77 kg) and also reported lower a* values in the fast growth strain. At 2.0 kg, the fast- and medium-growing strains had lower a* values for breast muscle compared to the slow growth strain (Chodová et al., 2021b). No differences in a* values in breast muscle were observed at 42 d (Wen et al., 2017b), 65 d (Mikulski et al., 2011), or 84 (Sarica et al., 2019). Weng et al. (2022b) reported lower a* values for fast strain broilers at 37 d (2.02 kg) compared to slow growth broilers at 101 d (1.65 kg). At 56 d a* values of breast meat were lower in the fast growth strain (Özbek et al., 2020), which agrees with Quentin et al. (2003) who compared slow growth birds (12 wk; 2.9 kg) to medium (8 wk; 2.6 kg) and fast growth broilers (6 wk; 2.5 kg). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast-growing strain had lower a* values that one slow growth strain, but was not different from the other. Other studies report higher a* values in breast meat from fast growing strains (Grashorn, 2006 (84 d); Fanatico et al., 2007 (56 and 63 d vs 84 and 91 d); Sarica et al., 2014 (84 d)). Similarly, fast growth birds reared to 53 d and 6 wk had higher a* values than slow growth birds reared to 81 d and 12 wk and medium growth strains reared to 67 d and 8 wk (Berri et al., 2005; Fanatico et al., 2005b). At 2.0 kg, the fast-growing strain had higher a* values for breast skin compared to slow- and medium-growing strains (Chodová et al., 2021b).
Higher breast b* values (yellowness) for fast growth broilers were reported at 2.0-2.2 kg (Singh et al., 2021), 2.8 kg (Weimer et al., 2022), at 84 d (Sarica et al., 2014, 2019), and at 6 wk compared to a medium strain at 8 wk and a slow growth strain at 12 wk (Quentin et al., 2003). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast-growing strain had higher b* values. In contrast, Mikulski et al. (2011) reported lower b* values in fast growth broilers at 65 d. This agrees with Fanatico et al. (2007) who compared fast and slow growth broilers at 63 and 91 d (females) as well as at 56 and 84 d (males). No differences in b* values in were observed at 42 d (Wen et al., 2017b), 56 d (Özbek et al., 2020), or 84 d (Grashorn, 2006). No differences were reported when fast growth broilers were slaughtered at 6 wk and slow growth birds were slaughtered at 12 wk (Berri et al., 2005). Fanatico et al. (2005b) reared slow (81 d), medium (67 d), and fast (53 d) growth strains with indoor and outdoor access and found no differences when birds were reared indoors. When reared with outdoor access, fast growth birds had lower b* values than medium or slow growth strains. Weng et al. (2022a) compared values at similar body weights (1.97 kg at 37 d for the fast strain vs 1.77 kg at 101 d for the slow strain) and reported lower b* values in the fast growth strain. Weng et al. (2022b) reported lower b* values for fast strain broilers at 101 d (1.65 kg) compared to fast growth broilers at 37 d (2.02 kg).
For thigh meat, L* values were lower for the fast-growing strain at 2.8 kg (Weimer et al., 2022), however at 2.0 kg no difference was noted (Doğan et al., 2019). Similarity, no difference in L* was noted at 56 d (Özbek et al., 2020) or 84 d (Sarica et al., 2014, 2019) or when fast growth broilers were slaughtered at 6 wk and slow growth birds were slaughtered at 12 wk (Berri et al., 2005). Lower a* values were reported in the fast- growing strain at 2.8 kg (Weimer et al., 2022) and 84 d (Sarica et al., 2014, 2019), however higher a* values were reported in fast growth strains slaughtered at 6 wk compared to slow growth birds at 12 wk (Berri et al., 2005). No effect of strain was observed at 2.0 kg (Doğan et al., 2019) or at 56 d Özbek et al. (2020). No difference in b* values were reported at 56 d (Özbek et al., 2020) or when birds were reared to 2.0 kg (Doğan et al., 2019) and 2.8 kg (Weimer et al., 2022). Fast growing strains had higher b* values at 84 d (Sarica et al., 2014, 2019) and when slaughtered at 6 wk compared to slow growth birds at 12 wk (Berri et al., 2005). Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) on commercial diets or diets low in protein. They found that the slow-growing strain fed the low protein det had the highest a* and b* values, whereas the slow-growing strain fed the commercial diet, had the lowest values L* values were lower in that strain. For thigh skin, fast growth birds had higher a* values, but no difference in L* or b* values (Doğan et al., 2019). Fanatico et al. (2007) compared fast and slow growth broilers at 63 and 91 d (females) as well as at 56 and 84 d (males) and found fast growth broilers had lower thigh skin L* and b* values and higher a* values.
pH (can impact meat quality: either too high or too low can result in abnormality in meat)
Breast pH was higher in the fast-growing strain at 2.0-2.2 kg (Singh et al., 2021) and 2.8 kg (Weimer et al., 2022). At 2.0 kg, fast-growing females had higher breast pH, but no difference was noted for males (Doğan et al., 2019). At 2.0 kg, the fast- and medium- growing strains had higher pH values for breast muscle compared to the slow growth strain (Chodová et al., 2021b). At 56 d (Özbek et al., 2020) and 84 d (Sarica et al., 2014) pH was higher in the breast for fast growth broilers. Breast muscle pH was unaffected at 42 d (Wen et al., 2017b), 81 d (Cömert et al., 2016), and 84 d (Sarica et al., 2019). The pH of breast meat was higher in the medium (8 wk; 2.6 kg) and fast (6 wk; 2.5 kg) growth strains compared to the slow strain (12 wk; 2.9 kg) (Quentin et al., 2003; Berri et al., 2005). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast-growing strain had higher pH. Zhang et al. (2022) compared breast pH of a local slow growing strain (120 d) to a fast strain (42 d) and the fast strain had higher pH after 15 minutes and tended to have higher pH after 24 h. Fanatico et al. (2007) assessed female broilers reared to 63 and 91 d with indoor vs outdoor access and noted all pH values were within normal range, however fast growth broilers had a higher pH of breast meat. However, Weng et al. (2022a) measured pH at similar body weights (1.97 kg at 37 d for the fast strain vs 1.77 kg at 101 d for the slow strain) and found lower pH in the fast growth strain. This disagrees with Weng et al. (2022b) who reported higher breast pH for fast-growing broilers at 37 d (2.02 kg) compared to slow growth broilers at 101 d (1.65 kg).
At 2.0 kg, no difference in thigh pH was noted for males or females, but when an average was calculated between sexes, fast growing strains had higher pH (Doğan et al., 2019). Thigh muscle pH was unaffected at 56 d (Özbek et al., 2020), 81 d (Cömert et al., 2016) and 84 d (Sarica et al., 2019). At 84 d, fast growth broilers had lower thigh pH (Sarica et al., 2014). When fast growth broilers were slaughtered at 6 wk (2.5 kg) and slow growth birds were slaughtered at 12 wk (2.9 kg), pH was higher in the thigh meat of the fast-growing strain (Berri et al., 2005). Chen et al. (2023) assessed thigh pH in fast-growing broilers at 42 d and slow-growing birds at 120 d and reported the fast- growing birds had lower pH after 15 minutes, but no difference existed after 24 h. Chodová et al. (2021a) noted higher pH in the medium growth strain (56 d) compared to the slow-growing strain (70 d).
Water holding capacity (measure of meat quality: indicates how much moisture is held on cooking)
No difference in breast meat water holding capacity was noted at 2.0 kg (Doğan et al., 2019) or 84 d (Sarica et al.,2014). At 2.0-2.2 kg, drip loss was lower for the fast growth strain 24 h, 3 d, 6 d, and 10 d after processing (Singh et al., 2021), however at 2.0 kg no difference in drip loss was noted (Chodová et al., 2021b). Lower drip loss for breast meat in fast-growing broilers was reported at 42 d (Wen et al., 2017b) and when fast growth broilers were slaughtered at 6 wk (2.5 kg) and slow growth birds were slaughtered at 12 wk (2.9 kg) (Berri et al., 2005). No difference in breast meat drip loss was reported between slow (12 wk; 2.9 kg), medium (8 wk; 2.6 kg), and fast (6 wk; 2.5 kg) growth strains (Quentin et al., 2003). When given outdoor access, fast growth birds reared to 53 d had lower breast meat drip loss compared to the slow strain reared to 81 d, but no differences were noted when birds were reared indoors (Fanatico et al., 2005b). Fanatico et al. (2007) reported lower drip loss in fast-growing birds reared to 63 d compared to slow-growing broilers reared to 91 d. Zhang et al. (2022) reported higher drip loss in the fast growth strain at a market age of 42 d compared to a local slow growing strain at 120 d.
At 2.0 kg (Doğan et al., 2019) and 2.8 kg (Weimer et al., 2022), no difference was noted for cook loss in breast meat. At 2.0 kg, the fast- and slow-growing strains had lower cooking loss for breast muscle compared to the medium growth strain (Chodová et al., 2021b). Wen et al. (2017b) reported higher cook loss for breast meat in the fast growth strain at 42 d which agrees with Fanatico et al. (2007) who reported higher cook loss in fast growth birds reared to 63 d compared to slow growth broilers reared to 91 d. Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and the fast-growing strain had higher cook loss. Conversely, fast growth birds reared to 53 d had lower cook loss in the breast meat than slow growth birds reared to 81 d (Fanatico et al., 2005b). When broilers were fed both a low nutrient and conventional diet, fast- growing birds (56 d) had lower cook loss compared to slow growth broilers (84 d) (Fanatico et al., 2007). Zhang et al. (2022) reported higher cook loss in the fast-growing strain at a market age of 42 d compared to a local slow growing strain at 120 d.
At 2.0 kg, the fast- and slow-growing strains had lower thaw loss for breast muscle compared to the medium growth strain (Chodová et al., 2021b). Fanatico et al. (2007) reported higher thaw loss in breast meat from fast-growing broilers reared to 63 d compared to slow growth broilers reared to 91 d. When broilers were fed a low nutrient diet, fast growth birds (56 d) had higher breast thaw loss compared to slow growth broilers (84 d), whereas, when fed a conventional diet, thaw loss was lower for the fast strain (Fanatico et al., 2007).
Grilling loss in breast meat was greater in fast-growing strains at 84 d (Grashorn, 2006). Fanatico et al. (2007) found lower total moisture loss in breast meat from fast growth broilers reared to 63 d, compared to slow growth broilers reared to 91 d. White cured- cooked meat from fast growth broilers had a more humid aspect, a less dry texture, and poorer slice cohesiveness (Berri et al., 2005).
At 2.0 kg, no difference in thigh water holding capacity was noted for males or females, but when an average was calculated between sexes, fast growing strains had higher water holding capacity (Doğan et al., 2019). At 56 d, fast growth birds had lower water holding capacity in thigh meat (Özbek et al., 2022), however Mikulski et al. (2011) reported fast growth birds tended to have better water-holding capacity at 65 d. No difference in water holding capacity was noted at 84 d (Sarica et al., 2014). At 2.0 kg no difference was noted for cook loss (Doğan et al., 2019). Weimer et al. (2022) observed no difference in thigh cook loss when birds were reared at 29 kg/m2, but fast growth birds had lower loss when reared at 37 kg/m2. When fast-growing broilers were slaughtered at 6 wk (2.5 kg) and slow growth birds were slaughtered at 12 wk (2.9 kg) thigh drip loss was higher in the fast growth strain (Berri et al., 2005). Chen et al. (2023) assessed thigh meat quality in fast-growing broilers at 42 d and slow-growing birds at 120 d and reported the fast-growing strain had higher drip and cook loss.
Sensory properties
At 65 d, no impact of strain was observed for taste, aroma, or juiciness of breast meat (Mikulski et al., 2011). At 84 d, texture values for breast meat were higher for the fast- growing strains (Grashorn, 2006) and no difference in breast or thigh hardness, springiness, gumminess, chewiness, or resilience were observed (Sarica et al., 2014).
At 2.8 kg, fast-growing broilers had lower shear force in both the breast and thigh meat (Weimer et al., 2022). This agrees with Chodová et al. (2021b) who measured shear force for breast muscle at 2.0 kg and Weng et al. (2022a) at similar body weights (1.97 vs 1.77 kg. Weng et al. (2022b) observed the same impact for the breast and thigh in fast growth broilers at 37 d (2.02 kg) and slow strain broilers at 101 d (1.65 kg). Huerta et al. (2023) reared fast growth broilers to 42 d and slow growth strains to 99 d and found no difference in shear force for breast muscle. At 56 d, shear force was higher in the breast of fast-growing broilers, but unaffected in the thigh strain (Özbek et al., 2020). Zhang et al. (2022) compared shear force for breast muscle at market age in a local slow growing strain (120 d) to a fast strain (42 d) and it was higher for the fast strain. Chen et al. (2023) assessed the same strains at the same ages as Zhang et al. (2022) for thigh meat and reported higher thigh shear force in the fast-growing strain.
At 65 d, no impact of strain was observed on tenderness of the breast (Mikulski et al., 2011). Fanatico et al. (2007) assessed female broilers reared to 63 and 91 d with indoor vs outdoor access and male broilers reared to 56 and 84 d fed conventional or low nutrient diets. In both studies, meat from fast growth broilers was less tender. Fanatico et al. (2005b) found no difference in tenderness (force) between fast- and slow-growing strains reared indoors to 53 or 81 d, however when given outdoor access, fast growth birds had lower values. For tenderness (energy), no differences were observed between strains reared indoors or outdoors, but fast growth birds reared outdoors had a lower value than any strain reared indoors.
Higher mortality was observed in fast-growing broilers reared to 2.1 kg (Rayner et al., 2020) and 2.6 kg (van der Eijk et al.,2023) or to 2.5 vs 2.3 kg (Baxter et al., 2021). The same was noted in a fast growth strain (6 wk; 2.5 kg) compared to medium (8 wk; 2.6 kg) and slow (12 wk; 2.9 kg) growth strains (Quentin et al., 2003). Strains selected for fast growth also had higher mortality independent of body weight (Fanatico et al., 2008 (84 and 91 d vs 56 and 63 d); Mikulski et al., 2011 (65 d); Castellini et al., 2016 (11 wk); Abeyesinghe et al., 2021 (0-42 vs 0-56 d); van der Eijk et al., 2022b (0-38 vs 0-51 d). Dixon et al. (2020) tested one slow strain (60 d) and three fast strains (42 d) and reported that two of the fast strains had higher overall mortality.
Chodová et al. (2021a) reared fast- (35 d), medium- (56 d), and slow-growing broilers (70 d) on commercial diets or diets low in protein and the authors reported that the low protein det reduced mortality in the fast-growing strain, increased mortality in the medium-growing strain, and no mortality was observed in the slow-growing strain.
Other studies differ in results. No difference was noted for mortality between fast- and slow-growing strains (Yamak et al., 2014 ( 8, 10, 12 wk); Wen et al., 2017b (0-42 d); Ghayas et al., 2020 (0-56 d); Weimer et al., 2020 (0-41 and 0-62 d); de Jong et al., 2021 (0-38 and 0-53 d); Güz et al., 2021 (0-38 and 0-49 d); Torrey et al., 2021 (0-48 and 0-62 d). Fanatico et al. (2005a) reported no difference in mortality between fast- (56 d), medium- (67 d), and slow-growing strains (81 d). When birds were reared to 56 d, no difference in mortality was observed during the spring, however mortality was higher in the fast-growing strain during the summer (Aksoy et al., 2021).
No difference was observed for the number of culls due to leg issues for fast-growing broilers reared to approximately 39 d (2.5 kg) compared to slow-growing broilers reared to approximately 44 d (2.3 kg) (Baxter et al.,2021). Rezaei et al. (2018) and Wilhelmsson et al. (2019) found that culling due to leg issues was higher in the fast- growing strain when reared to 71 d. Dixon et al. (2020) tested one slow strain (60 d) and three fast strains (42 d) and found two of the fast strains had higher culls due to lameness. When broilers were reared to 12 wk, more heart abnormalities, tendon degeneration, scoliosis, and rotated tibias were observed in the fast-growing strain, while slow-growing strain had more deviated breast bones (Bokkers and Koene, 2003).
When birds were slaughtered at 2.1 kg, no difference in dead on arrival (DOA) numbers were observed, however the fast-growing strain had a higher percentage of pre- processing culls and 9.6 times more rejections (Rayner et al., 2020).
Baxter et al. (2021) reported that fast-growing broilers had more condemnations due to perihepatitis and ascites when reared to approximately 39 d (2.5 kg) compared to slow- growing broilers reared to approximately 44 d (2.3 kg). More slow-growing birds were classified as runts and no difference was noted for number of DOAs.
Forseth et al. (2023) sampled 63,209,415 broilers processed over a 5-year period. Fast- growing broilers had a higher prevalence of condemnations and more condemnations due to ascites, discolouration, hepatitis, small size, and skin lesions. Fractures were the only cause of condemnation more prevalent in the slow-growing strain.
Research is limited into the economic and environmental costs of utilizing slow growth broilers in commercial production. Tran et al. (2021) concluded that conventional broiler production using fast-growing strains result in a smaller environmental footprint due to more efficient conversion of feed protein into animal protein. In 2020, Chicken Farmers of Canada estimated the environmental impact of switching 1/3 of Canadian broiler production to slow growth based on a study by Elanco Animal Health in 2016. The model estimated that the switch would require 668,509,777 extra liters of water/year, 212,527,411 extra kilograms of feed/year, 103,776 acres of additional land required for feed, and an additional 393,242,406 kg of manure/year would be produced. Chan et al. (2022) used modelling to identify the land use required for fast vs slow growth production and reported that fast- growing broilers required less direct (88 km2 vs 224-289 kg/m2) and indirect (75,488 vs 90,437-98,521 km2) land use. They calculated that chick numbers would need to increase from 98 million/year to 142- 183 million/year.
The 2020 Chicken Farmers of Canada model estimated that switching 1/3 of Canadian broiler production to slow growth would result in an increase in production costs of $390,794,112, which would be shared between the farmer, processor, retailer, and consumer.
Lusk et al. (2019) calculated the cost and market outcomes for three scenarios: (i) feeding flocks for a fixed number of days (42 or 49 d), (ii) feeding flocks until they reach a target weight (2.7 kg), or (iii) feeding flocks until net partial returns were maximized.
Results accounted for reduced stocking density being used in slow growth flocks. They used Ranger Classic and Ranger Gold breeds for slow growth data and Ross 308 and Cobb 500 for fast growth data. The tables included below illustrate some of the results. The authors reported that conversion to slow growth would increase production costs in the U.S by about 12% if producers target a specific weight, 14% to optimize partial returns, and 25% if targeting a fixed number of days on feed. These cost estimates did not include costs for new barns which would be required to produce the same volume of chicken from slow-growing strains. The annual costs of an industry wide switch to slow growth production would be $450 million for consumers and $3.1 billion for producers and consumer willingness to pay would need to increase 10.8% (if fed to a constant weight) or 12.6 % (to optimize net returns) to offset producer losses. When corrected for body weight (Table 2) or for optimal days on feed corrected for stocking density (Table 4), the partial returns are higher for the fast-growing strains.
Generally, fast-growing broilers had better performance, higher carcass and breast yield, and, when corrected for body weight, higher incidence of mortality. Fast growth broilers had stronger bones, but more leg issues. In terms of behaviour, these birds performed more ingestion behaviours, but less locomotor, standing, foraging, aggressive, and perching behaviours. A higher incidence and severity of breast muscle myopathies and more condemnations at the processing plant were also noted in these strains, but meat yield is increased in conventional fast-growth broilers. Breast meat had increased pH and lower drip loss.
Once again, in general, slow-growing broilers had better feather cover, which resulted in less skin damage. They also had cleaner feathers and better litter quality (management dependant). Slower growing broilers had improved gait scores, which is an indicator of better mobility, and they used enrichments more. These strains had increased leg, wing, and frame yields and increased abdominal fat.
Inconsistent results were noted for footpad and hock-joint dermatitis, fear response, and comfort behaviours. No clear impacts on breast and thigh muscle colour and breast cook and thaw loss were identified.
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