Heat stress and corn quality affect amino acid digestibility in broiler chickens

Corn constitutes over 50% of broiler diets and contributes measurable amino acids (AA), but several factors could affect its eventual net AA supply. Heat stress (HS) reduces feed intake and may compromise intestinal integrity and increase endogenous AA losses. A total of 384 one-day-old male Cobb by-product breeder chicks were allocated to 6 treatments (3 diets × 2 environmental temperatures) using a randomized complete block design with 8 replicate cages of 8 birds/cage. All birds received a standard corn-soybean meal-based starter diet until experimental diets were introduced. Experimental diets included a nitrogen-free diet (NFD) and semi-purified diets containing either regular corn or mycotoxin-contaminated corn fines. The NFD was fed for 4 d and the semi-purified diets for 6 d prior to ileal digesta collection on d 22. Data were analyzed using PROC GLM of SAS, with endogenous ileal AA data analyzed using the one-way ANOVA, while apparent ileal amino acid digestibility (AIAAD) and standardized ileal amino acid digestibility (SIAAD) data were analyzed as a 2 × 2 factorial arrangement of treatments. Cyclic HS increased (P < 0.05) endogenous ileal AA losses for nitrogen, and all AAs, with total AA losses increased by 36.5%. Mycotoxin-contaminated corn fines decreased (P < 0.001) AIAAD and SIAAD across all AAs compared to regular corn. There were diet × temperature interactions (P < 0.05) for five AAs (lysine, methionine, valine, aspartate, and glycine), where HS decreased (P < 0.05) AIAAD in birds fed mycotoxin-contaminated corn fines. For SIAAD, a main effect of temperature (P < 0.05) occurred, with HS birds exhibiting higher values than thermoneutral (TN) birds for most AAs (except tryptophan, cysteine, and proline). Nevertheless, the combination of corn fines and HS resulted in the lowest (P < 0.05) SIAAD for arginine, isoleucine, lysine, phenylalanine, valine, aspartic acid, and glycine. Birds under HS fed regular corn had the highest (P < 0.05) SIAAD for these AAs. These results demonstrate that SIAAD coefficients derived under TN conditions are not directly additive under HS.