Stocking density for broilers

Review of literature 2013-2020

Karen Schwean-Lardner

Introduction

The Poultry Code of Practice Scientific Committee Report, which preceded the Codes of Practice and provided a review of current literature on select topics, was completed in 2013. One section of this report (Section 5, pages 28-35) focused on the impact of stocking density on meat birds. The scientists who drafted the report were asked to report on the literature, without making recommendations based on that literature. This current report then, is a continuation of the Science Report. Available literature has been summarized within the same categories used in the 2013 Science Report.

To start this document, I would like to include the Conclusions that were published in the Science Report. Again, these were conclusions based on the literature, and were not recommendations. They included:

1. As stocking density increases, poorer measures in biological functioning (e.g. weight gain, injuries, gait scores) are observed on an individual bird The effects are generally linear with no clear cut-offs for the different measures.
2. Environmental conditions (e.g. temperature, relative humidity, air and litter ammonia) are significantly affected by stocking density.
3. Broiler chickens prefer to lie near feeders and drinkers but will increasingly choose to rest away from the feeders as stocking density increases. As stocking density increase, birds are jostled more frequently and resting is disturbed more often.
4. Management has a major impact on bird welfare especially at higher stocking

Please note: the majority of densities are reported as kg/m². Some papers choose to report density as number of birds per m², and if final body weights are included in the paper, these have been converted to kg/m². However other papers include only body weight gain, so the conversion to kg/m² is not possible, and these remain reported as birds/m².

Abbreviations

SD = Stocking density

FCR = Feed conversion ratio

H:L ratio = Heterophil to lymphocyte ratio

Literature from 2013-2020:

Impact of SD on performance

Body weight and gain

The literature published from 2013 to the present focusing on stocking density and performance has continued to demonstrate that increasing stocking density for broilers has negative effects on performance. Because the majority of papers published in this time frame have included performance data in some respects, it is listed below in point form. With respect to market body weight and weight gain, the majority of research projects have determined that decreases are primarily linear in response:

• Son (2013) tested 30-32 vs 42-44 kg/m² and found heaviest weights at the low density;
• Uzum and Toplu (2013) tested broilers at 2 vs 38.4 kg/m²; found heaviest weights at 28.2 kg/m²
• Das and Lacin (2014) tested 12 or 20 broilers/m² and found reduced BW at high density;
• Petek et (2014) tested 15, 19 or 23 broilers/m² (by calculation, approx.. 44, 50 or 56 kg/m²) and found linear reductions in body weight with higher densities;
• Abouelenien et (2016) compared broilers housed at 25, 33, 37 or 44 kg/m² and found higher body weight when reared under 25 vs 44, with the other two intermediate;
• Farhadi et (2016) tested broilers housed at 16, 18, 20 or 22 birds/m² with lowest weight gain for the 22 birds/m² treatment;
• Qaid et (2016) housed broilers at 30, 60, 90 or 120 chicks/m² from 2-14 days, and found reductions in body weight at higher densities even at these earlier ages;
• Vargas-Galicia et (2017) examined broilers housed at 13 or 18 birds/m² with lower performance at high SD;
• Averos and Estevez (2018) studied data from commercial farms in a data meta-analyses, and reported productivity reductions at higher densities;
• Goo et (2019) examined broilers housed at 15.2, 20.2, 25.3 and 30.4 birds/m², and found reductions at higher SD.

Two papers (compared to 10 above) also demonstrated that higher SD reduced body weight but that more moderate SD resulted in performance similar to low densities:

• Alaeldein et (2013) found that 28 and 37 kg/m² outperformed 40 kg/m²;
• Zhao et (2013) examined broilers housed at 12, 16 or 20 birds/m² and found that the lowest weights occurred when birds were housed at high density compared to moderate and low.

To counter the above papers, Lee et al. (2018) tested 7.5 birds, 10 or 12.5 birds/m² and found reduced body weight gain at moderate and high densities compared to low densities.

Only two published works from 2013-current found no difference in performance when broilers were housed at different (Arruda et al., 2015 testing 17, 19 or 21 birds/m²; Najafi et al., 2015, testing low or 10 birds/m² and high density (16 birds/m² from 21-35 days only). This latter study was conducted in cages however, which could explain the differences noted from the previous studies.

Feed intake and feed conversion ratio

Similar to the impact of SD on body weight/gain, increasing SD generally reduces feed intake of broilers. Alaeidein et al. (2013) found reduced feed intake when broilers were housed at 40 kg/m² compared to those at 28 or 37 kg/m², along with a linear reduction in feed conversion ratio (FCR) as density increased. Son (2013) also reported improved FCR at low density (30-32 kg/m²) compared to high (42-44 kg/m²). Uzum and Toplu (2013) reported similar data, and also used a heat stress model in their study. They found the decrease in productivity that occurred under their high density treatment (38.4 kg/m²) was significantly more profound when birds were exposed to heat stress as compared to 28.2 kg/m²), although the decline also occurred under normal condition. Abouelenien et al. (2016) reported lower feed intake when broilers were housed at 37 or 44 kg/m² compared to 25 kg/m², but found no corresponding difference in FCR. Farhadi et al. (2016), comparing 16, 18, 20 or 22 birds/m² found reduced feed intake and efficiency at the highest density. Lee et al. (2018) also found reduced feed intake with medium and higher SD, but no impact on FCR (7.5, 10 or 12.5 birds/m²). Interestingly, only one paper reported data on water intake. Bailie et al. (2018) found that as SD increased from 30 to 32 to 34 to 36 kg/m², water consumption of the broilers decreased.

A similar trend with respect to feed intake was shown by Qaid et al. (2016) when comparing broilers at 30, 60, 90 or 120 chicks/m². Interestingly, however, their data was only collected from 2-14 days of age, suggesting increased SD at young ages can also have negative impacts. This was further supported by Goo et al. (2019), who found linear reductions in feed intake with increasing density from 15.2 to 30.4 birds/m² (4 treatments in total) from 1-28 days. Goo et al. (2019) also found that high SD resulted in increased intestinal permeability, suggesting decreased intenstinal barrier functioning, which would explain the reductions in body weight that occur with the high SD treatment birds.

Some reports also have indicated that interactions can occur between SD and other factors. For example, Sun et al. (2018) tested SD (28.6 vs 39.6 kg/m²) and total calcium level, finding dietary deficiencies resulted in even larger impacts for high SD. This suggests that both can be stressors for birds.

Only two research reports found during the 2013-2020 time have reported no differences in FCR. Petek et al. (2014) compared 3 densities (15, 19 or 23 chicks/m²) and found no difference in FCR. Lee et al. (2018) also found no differences when comparing 7.5, 10 or 12.5 birds/m².

Impact of SD on mortality

Fewer papers report on the effects of SD on mortality in broilers. Reports that were published are sometimes conflicting. Finding significant difference in mortality levels is often difficult, as high variability can easily occur between replications. This may explain some of the inconsistencies.

Some research clearly indicates that mortality increases as SD increases. Son (2013) found higher mortality when broilers were housed at 42-44 kg/m² compared to 30-32 kg/m². Averos and Estevez (2018), who conducted a large analyses of data retrieved from farms in Italy, found a significant increase in mortality with increasing SD.

Other research reported no effect of increasing SD. For example, Farhadi and Hosseini (2016) compared 16, 18 or 22 birds/m², finding no differences in mortality. Tarakdjian et al. (2020) also examined mortality in Italian flocks, and compared broilers reared at 33 vs 39 kg/m². These authors found no relationship between tested SD and mortality levels.

Impact of SD on carcass characteristics

Few papers have focused on the impact of SD on carcass characteristics since 2013. Goo et al. (2019) did include a comparison of some carcass characteristics in their comparison of 15.2, 20.2, 25.3 and 30.5 birds/m² from 1-28 days of age. The authors reported no differences in carcass yield, muscle pH, water holding capacity or meat color with changing SD. Arruda et al. (2015) also reported no relationships between SD and carcass yield. Farhadi et al. (2016) and Farhadi and Hosseini (2016) reported no change in abdominal fat content when SD ranged from 16-22 birds/m², and the latter found no differences in carcass yield either.

Downgrading and scratch levels were reported in two papers. Bailie et al. (2018) found that SD ranging from 30-36 kg/m² did not alter levels of downgrades in their broilers. However, Arruda et al. (2015) found a relationship between increasing SD and higher levels of scratches in their study (17, 19 and 21 birds/m²).

Impact of SD on foot and leg health, general health

The majority of published papers (2013-present) indicate a clear relationship between SD and leg and foot health. Lesions increase with increasing SD. This was reported by Farhadi et al. (2016) and Farhadi and Hosseini (2016), when SD ranging from 16-22 birds/m² increased foot pad lesions. Toghyani et al. (2016) found similar footpad, hockburn and breast lesion scores for birds housed at 12 and 16 birds/m², but that birds housed at a higher density (18 birds/m²) had significantly worse lesions at both 35 and 42 days of age. Karaasian and Nazligul (2018) also reported more hock lesions at higher densities, but found no difference in footpad lesions. Weimer et al. (2020) studied the impact of varying SD on slow-growing breeds of broilers, and found higher hock burns and lower tibial dyschondroplasia levels when birds were housed at 29 kg/m² compared to 37 kg/m².

Vargas-Galicia et al. (2017) did note that proper litter material could result in different impacts of SD on foot pad lesion. In their work, broilers were housed at either 13 or 18 birds/m². Testing various litter materials, they noted that more absorbent litter materials resulted in less detrimental impacts of high SD on foot pad lesions, as well as fewer valgus-varus abnormalities.

Mobility of live birds also impaired with increasing SD. This was noted in work by Averos and Estevez (2018) in their large meta-analysis of data, who also reported better leg health in general under lower SD. Bailie et al. (2018) also found improved gait scores with lower SD (30, 32, 35 and 36 kg/m²), along with higher lesion scores above 30 kg/m².

Bone quality also appears to deteriorate at higher SD. Vargas-Galicia et al. (2017) noted that birds housed at 18 birds/m² had poorer tibia strength than broilers housed at 13 birds/m². Similar data was published by Lee et al. (2018) when birds were housed at 12.5 birds/m² compared to 7.5 per m², and by Sun et al. (2018) when birds were housed at 28.6 vs 39.6 kg/m². Tibia ash levels also were lower for broilers housed at 18 birds/m² than for those housed at 12 birds/m². The lower density in this experiment, especially when birds were given perch access (Karaaslan and Nazligul, 2018) significantly increased ash levels. Ash level is used as an estimation of bone quality in birds.

SD also appears to impact broiler feather cover, which is important as feathers are vital for thermoregulation and as a skin protectant. Work by Toghyani et al. (2016) found that broilers housed at 12 birds/m² had better overall feather cover than those housed at either 16 or 18 birds/m².

Impact of SD on environment (humidity, litter quality, air quality)

Increasing SD means more fecal output per unit of space for any facility, so air quality and litter quality are very dependent on management. In studies which show an impact of SD on environmental impacts, the relationship shows higher SD reduces environmental conditions. Wet litter in particular impact incidence and severity of footpad and hock lesions, so this category influences the category discussed earlier.

Vargas-Galicia et al. (2017) found that increasing SD from 13-18 birds/m² increased litter moisture. However, other research found no relationship between SD and litter moisture. For example, Bailie et al. (2018) tested 30, 32, 34 and 36 kg/m² in windowed houses, and found no changes in litter moisture.

Farhadi et al. (2016) and Farhardi and Husseini (2016) reported that increasing SD from 16 to 22 birds/m² had no effect on environmental ammonia levels or litter pH.

Impact of SD on behaviour and natural space preferences

Behavioural expression, which can indicate improved welfare, is affected by SD. De Jong and Goertz (2017) tested broilers housed at 25 and 35 kg/m², finding reduced expression at the higher density, indicating reduced welfare. Enrichment devices were used in their research, and birds housed at the higher density were not able to use those devices to the same extent as those housed at the lower density.

Fear levels of broilers are correlated to stocking density. Toghyani et al. (2016) found increasing levels of tonic immobility, which is a measurement used to estimate fear when SD increases. In their research, birds housed at 12 birds/m² were not as fearful as those housed at 16 or 18 birds/m².

With respect to overall stress, Qaid et al. (2016) reported that increasing SD resulted in increased stress in birds to 14 days of age (testing 30, 60, 90 or 120 chicks/m²). This could be influenced by flock disturbances, which Averos and Estevez (2018) found increases with higher SD. An increased stress with increased SD was supported by work from Lee et al. (2018), who found that heterophil to lymphocyte (H:L) ratio was higher at high (12.5 birds/m²) and medium (10 birds/m²) densities compared to low density (7.5 birds/m²). H:L ratios are a measure of chronic stress in birds.

Revisiting 2013 Science Report Conclusions with literature conclusions (2013-2020)

NFACC 2013 Science Report for Meat Birds Conclusions were:

1. As stocking density increases, poorer measures in biological functioning (e.g. weight gain, injuries, gait scores) are observed on an individual bird The effects are generally linear with no clear cut-offs for the different measures.

The current information primarily supports the 2013 conclusion of reduced biological functioning, including in weight gain, feed intake, scratches, and gait (mobility) scores. In most cases, the literature has discussed this again as occurring in a linear fashion, with no clear demarcation of where these cut-offs occur.

The new information has also provided information on bone quality, and studies have indicated that bone quality deteriorates as SD increases. Feather cover also is poorer as SD increases.

2. Environmental conditions (e.g. temperature, relative humidity, air and litter ammonia) are significantly affected by stocking density.

The information from 2013-2020 has not focused on environmental conditions, with only a few reporting these variables. The results were conflicting. Details from these papers were lacking, in terms of season of year, location and ambient conditions, making conclusions difficult to draw from this limited information. However, it is agreed that lesions (foot, hock and sometimes breast) increase with increasing SD, and this is an effect of wet litter.

3. Broiler chickens prefer to lie near feeders and drinkers but will increasingly choose to rest away from the feeders as stocking density increases. As stocking density increase, birds are jostled more frequently and resting is disturbed more often.

The 2013-2020 information clearly defines that behavioural expression is restricted at higher SD, and that disturbances occur more frequently at higher SD.

The new information has also provided more focus on fear and stress. With respect to those parameters, it is noted that fear and stress levels increase with increasing SD.

4. Management has a major impact on bird welfare especially at higher stocking densities.

This was also confirmed by the paper discussing litter material quality and litter

References

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Abudabos, A.M., E. Samara, E.O.S. Hussein, R.M. Al-Atiyat and A. Al-Haidary. 2013. Influence of stocking density on welfare indices of broilers. Italian Journal of Animal Science 12:e35:213-218.

Abudabos, A.M., E. Samara, E.O.S. Hussein, M.Q. Al-Ghadu and R.M. Al-Atiyat. 2013. Impacts of stocking density on the performance and welfare of broiler chickens. Italian Journal of Animal Science 12:e11:66- 71.

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