Enhancing Amino Acid Utilization for Broiler Chickens Under Different Scenarios of Dietary Protein Reduction and Subclinical Coccidiosis

Five experiments were conducted to evaluate responses of broiler chickens fed diets with different dietary amino acid (AA) levels when fed normal and reduced dietary crude protein (RCP) diets and reared under different coccidiosis control programs. In the studies with the use of RCP diets, the relationship between dietary Gly + Ser and the Met:Cys ratio was investigated. Experiment 1 evaluated broilers in 2 coccidiosis control programs (live vaccine or in-feed zoalene) and fed increased dietary AA derived from soybean meal or unbound sources from 0-40 d. Vaccination impaired young (0-26 d) broiler body weight gain (BWG) and feed conversion ratio (FCR) and overall FCR. Vaccination increased small intestinal length and weight and footpad lesions and lowered AA intake. Increasing AA density, regardless of source, improved overall growth performance and processing yields without adverse effects to litter conditions or footpad lesions. Maintaining AA density but increasing only digestible Thr, Val, and Arg levels with feed-grade sources reduced overall feed intake (FI) but did not negatively affect BWG, FCR, or processing yields. Experiments 2 and 3 examined increasing dietary Gly concentrations in RCP diets fed to broilers (0-48 d). In experiment 2, feeding RCP diets (1.68 percentage unit reduction) achieved with feed-grade L-Met, L-Lys, L-Thr, L-Val, and L-Ile did not negatively affect overall broiler performance. The addition of Gly at the highest concentration in RCP diets negatively impacted broiler performance. In experiment 3, broilers that were offed lower CP diets (2.38 percentage unit reduction) achieved with additional feed-grade L-Arg and L-Trp had linear improvements in overall FCR, but not total breast meat yield, with the additions of Gly. Experiments 4 and 5 evaluated the interaction of total (t)Gly + Ser and Met:Cys ratio in diets fed to broilers with or without a live coccidiosis vaccine (0-21 d). In experiment 4, increasing dietary Gly levels in RCP diets (3.64 percentage units) did not influence growth performance of unvaccinated broilers. Broilers did not respond to changes in the dietary Met:Cys ratio, and there were no interactions between Gly concentration and the Met:Cys ratio were observed. In experiment 5, vaccination negatively impacted BWG, FI, and FCR. Broilers fed RCP diets (2.64 percentage unit reduction) had lower broiler BWG and FI and increased FCR than control-fed birds. In both vaccinated and unvaccinated birds, increasing dietary Gly improved FI and BWG, although not to the same level as broilers fed higher CP diets. Additionally, increasing tGly + Ser levels lowered the FCR of unvaccinated but not vaccinated broilers. Changing the Met:Cys ratio did not impact performance or interact with dietary Gly level. Tissue glutathione levels were impacted by vaccination and Met:Cys ratio. In conclusion, medicated or vaccinated broilers benefited from increased dietary essential AA levels, regardless of source. Gly supplementation in moderate (< 3 percentage units), but not larger (> 3 percentage units), CP reductions improved broiler performance. Altering the Met:Cys ratio did not affect bird performance nor interact with changes in tGly + Ser level.