Calculating a proper stocking density will determine the maximum farm profitability in broiler chickens. Choosing the stocking density implies knowing the impacts on live performance, and its interrelations with the environment and dietary nutrients. Abundant studies about stocking density run under non-commercial conditions might not indicate the best reliable information to know the farm profitability. As a result, few studies are reliable for commercial purposes. Poultry managers should consider the following points for finding the best economic stocking density: (a) stocking density on live performance, (b) stocking density on farm profitability, (c) dietary energy and amino acids, (d) dietary antioxidants, and (e) additives for replacing antibiotics.
a) Stocking density on live performance
The stocking density is expressed better in terms of kilograms of body weight per meter square instead of the numbers of birds per meter square because it considers the age and genetic progress. Broiler chickens at high stocking density reduce the body weight and decrease the feed intake slightly. Therefore, the feed convention, adjusted to similar body weight, is elevated as stocking density increases. Moreover, as compared to low stocking density, birds at high stocking density present less breast meat yield1-4, lower feathering covering5, more skin scratches6-8, and higher footpad dermatitis6-8, and generate more heat output9 and litter moisture6-8. Besides, because crowding birds drink more water10 might impact on litter moisture and footpad dermatitis negatively.
On the other hand, under non-commercial conditions, the stocking density does not affect the mortality statistically. However, a consistent trend indicates that birds with less space died more. Supported this pattern, the broiler chickens in a crowded flock diminish the immunoglobulin affecting the immune system negatively11,12. Even more, broiler chickens under heat stress might die more at increased stocking densities13. Pooling studies1,5-7,11,12,14-26 from 20 to 756 birds per pen and associating the mortality and stocking density (kg/m2) show that birds die by 1.3% more when stocking density augments by 10 kg/m2:
Mortality (%) = 0.47 + 0.126[stocking density, kg/m2]; n = 59; P-value = 0.002
Linear relationships between stocking density and feed conversion, adjusted to similar body weight, reveal that increasing the stocking density by 10 kg/m2 rises the feed conversion by 0.11 hundredths or 11 points. From 49 studies1-8,10-12,14-51 run by JMP, the slopes between stocking density and feed conversion fitted normal distribution: average = 0.011, standard error = 0.0028, lower confidence interval at 95% = 0.0058, upper confidence interval at 95% = 0.0170. From the 56 slopes, 13 slopes only displayed negative values meaning that the feed conversion decreases as stocking density augments. These opposite trends were due to proper trial variation or probably because birds were reared in battery cages by small number of birds per cage29,40, the feeder space was not a limiting factor4,24, or female presented negative slopes as a contrast to ones from males23.
From the above slopes and mortality equation, feed conversion and mortality rates, adjusted to 3% of mortality at 33 kg/m2, elevate as stocking density increases (Figure 1). The bird’s competition to reach the feed might hamper them to eat freely, opting for drinking water more instead of eating. Although in the above studies, birds ate with a minimum of 1.7 cm of circular feeder per bird, thus this feeder space did not influence the feed conversion negatively52. Still, more studies are needed to know the best feeder space with current broiler chickens.
Figure 1. Stocking density on feed conversion and mortality.
Note: Feed conversion = 0.0114x[Kg/m2] + 1.12; Mortality adjusted to 3% at 33 kg/m2 = (0.47 + 0.126x[kg/m2])/(0.47 + 0.126×33)x3.
b) Stocking density on farm profitability
The stocking density varies by regions, countries, and management conditions. Moreover, the European Union Council Directive 2007/43/EC set limits to 33 kg/m2 from June 2010 onwards. In contrast, many European countries produce the highest profitability above 33 kg/m2. For example, farm profitability was the highest at 48 kg/m2 of stocking density53. In this study, the farm profitability was positive above 46 kg/m2 of stocking density without reaching the plateau at 48 kg/m2. Similar to this condition, in other countries, high stocking densities are operating in closed ventilated houses frequently.
A poultry model calculated the farm profitability across different stocking density by the following variables: 2.3 kg of target sell weight, a chicken house with 1000 m2, 4 points of feed conversion equals to 1 day, 14 days of empty house, 1.2 $ per kilo of meat composite price, 0.33 $ /kilo of feed, and the feed cost represents 70% of total cost. With the assumption of 4 points of feed conversion, adjusted to similar body weight (FCadj), equals to 1 day and 14 days of an empty house, the model calculated the cycles per year as follows:
Cycles per year = [27.111x(FCadj) – 5.0651 + 14 days]/365
In Figure 2, the farm profitability in broiler chickens plateaued at 50 kg/m2 following a diminishing return after this density, while the cycles per year decline linearly as stocking density elevates. This stocking density might change when birds died more due to heat stress conditions. Because the model assumes several conditions in this simulation, the poultry manager should calculate the best economic stocking density adjusted to their environment and management conditions without compromise the welfare and mortality. For instance, if poultry farmers sell chickens as carcass processing mainly, the calculation should include the breast meat yield. In this scenario, the model did not include the breast meat variable because the reduction of breast meat by stocking density was not significant with current information, even though producing a negative trend (-0.06 %/ kg/m2).
Figure 2. Farm profitability and cycles per year.
Note: Profit, $/house/year = body weight per year x 1.2 – feed intake per year x 0.33/0.7. Body weight per year = cycles per year x 2.3 x ((100-mortality)/100))x[stocking density]x1000 m2/2.3. Feed intake per year = cycles per year x feed conversion x 2.3 x ((100-mortality)/100))x[stocking density]x1000 m2/2.3.
c) Dietary energy and amino acids
Because the high stocking density yields the best farm profitability, the nutrients working at bird crowding conditions will assure the highest profitability. To illustrate, chickens fed vegetable oils under summer conditions produced better feed conversion in birds with less floor space. In this research, the dietary sunflower oil increased from 0 to 2.5%, and energy rose from 3208 to 3366 kcal/kg, while the dietary amino acids were held constant46. On the other hand, in birds under normal environmental conditions, the interaction between stocking density and dietary energy on live performance was not observed54.
Birds fed more dietary protein enhanced better the body weight gain at 22 kg/m2 than those at 34 kg/m2, 28. In this study, adding protein levels originated from soybean meal and fish oil might generate more bird heat output, and not help in reducing the overall heat from the chicken farm. In contrast, two studies revealed that adding more dietary digestible tryptophan from 0.16% to 0.24% or 0.32% during the grower period resulted in better feed conversion at high stocking density than those at low stocking density (Figure 3). These researches suggest that the tryptophan requirement boosts by 50% or 100% in chickens reared with less floor space17,36, and the synthesis of serotonin from tryptophan might alleviate potential stress factors.
Figure 3. Dietary tryptophan and stocking density on feed conversion.
Note: From Goo et al. 2019b17; dTryp = digestible tryptophan; feed conversion adjusted to similar body weight.
d) Dietary antioxidants
Chickens under crowding conditions deplete their antioxidant defense mechanisms. For instance, crowding flocks produce more free radicals due to reduced glutathione peroxidase activity and increased malondialdehyde levels9. To overcome this oxidative stress, adding 300 mg of alpha-lipoic acid per kg of feed boosted the antioxidant ability and feed conversion at 48 kg/m2 of stocking density12 (Figure 4). How does lipoic acid provoke those benefits? The lipoic acid plays as a coenzyme catalyzing oxidative decarboxylation of pyruvate, alpha-ketoglutarate, and branched-chain alpha-ketoacids, resulting in low redox potential and neutralizing reactive oxygen species55. More studies using chickens under commercial floor conditions and feeding lipoic acid at both low and high stocking density will add more value to this additive for calculating the farm profitability at high stocking density.
Figure 4. Dietary alpha-lipoic acid (ALA) on feed conversion.
Vitamin E is another nutrient with antioxidant properties that might impact positively in chickens with less floor space. For example, in broiler chickens fed more vitamin E from 15 to 115 IU/kg, the feed conversion improved by 14 points at 30 kg/m2,42. Though the 15 IU of vitamin E per kilo is higher than the values from NRC (1994)56, this value is lower than recommended by primary breeding companies (> 50 IU of vitamin E/kg), thereby the extra addition of vitamin E at high stocking density remain as a question mark. In this manuscript, the mortality was not affected by adding vitamin E but the immune systems were fortified, suggesting benefits if chickens are growing at crowding conditions.
e) Additives for replacing antibiotics
In nowadays, the use of additives for replacing antibiotics has expanded in broiler chickens. In particular, chickens fed probiotics show positive results at high stocking density20,45. Similarly, birds fed a combination of probiotic and prebiotic improved the feed conversion by 10 points at 38 kg/m2 of stocking density20. In the latter study, chickens given this feed increased the villus height of the small intestine and diminished stress indicators such as corticosterone and heterophil/lymphocyte ratio. Furthermore, chickens fed this combination, between mannan-oligosaccharides and Bacillus, promoted benefic bacterial in the gastrointestinal tract by increasing the Lactobacillus sp. and reducing Escherichia coli to levels similar to those at low stocking density.
Chickens fed herbal extract boosted the feed conversion by 9 points at 24 kg/m2 of stocking density and by only 5 points at 10 kg/m2,48 (Figure 5). The low stocking densities used in this study are not a reliable indicator for a commercial purpose. Testing more trials using either herbal extract or essential oils might assure more confidence in using this kind of additives at higher stocking density. According to this, overcrowding broiler chickens fed essential oils declined corticosterone levels57. The addition of organic acid and vitamin above requirements strengthens the live performance in overcrowding chickens. As an example, broiler chickens fed butyrate sodium (500 ppm) and niacin (50 ppm) resulted in 15 points better in feed conversion at 48 kg/m2. However, this trial was run in a battery cage and using only one stocking density58, thus testing on the floor, low, and high stocking density will display more alternatives for commercial purposes.
Figure 5. Dietary herbal extract on feed conversion.
- Higher stocking density yields increased feed conversion and mortality in broiler chickens. Further, crowding birds display poor footpad dermatitis, skin scratchers, and feathering covering.
- Fifty kg/m2 generates the maximum farm profitability under this model condition, but lower stocking densities might result in the highest profit at heat stress or specific commercial chicken houses.
- Using vegetable oils under heat stress might ameliorate the negative effect of crowding chicken flocks. Further, the addition of dietary tryptophan over the current requirement provides benefits under high stocking density.
- Dietary antioxidants such as lipoic acid and vitamin E are alternatives for boosting the live performance in chickens with less floor space.
- Organic acids, prebiotics, probiotics, and essential oils demonstrate positive results in crowding chicken flocks.
Conflict of Interest and disclaimer
The author declares that this article was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Every attempt has been made to ensure that the material in this article is accurate, true, correct, and relevant at the time of writing. However, the author accepts no liability for any omissions, damage, loss, or financial consequences of using this article.