The objective of this study was to determine the effect of mild coccidial vaccine challenge on standardized ileal amino acid digestibility (SIAAD) in 21- and 42-d old broilers fed a corn-soybean meal-DDGS-poultry by-product meal based diet. The study was designed as a 2 x 2 factorial arrangement of treatments with 2 ages (d 21 and 42) and 2 levels of coccidial vaccine (Coccivac®B) challenge (-/+) with 8 replicate cages of 8 (d 21) or 6 (d 42) birds/cage. Four hundred and forty-eight birds (256 for d 21 and 192 for d 42) were randomized to experimental treatments in a complete randomized design. Half of the birds consisting of 8 replicate cages/treatment were challenged, CHA (d 15 and 36 for birds sampled on d 21 and 42, respectively) with 10x of coccidial vaccine while the remaining half were not challenged (NCH). Standardized ileal amino acid digestibility on d 21 and 42 was determined by correcting apparent ileal amino acid (AA) digestibility values for basal ileal endogenous amino acid (EAA) losses generated within the same study from 21- and 42-d old CHA and NCH birds fed a nitrogen-free diet. Significant interactions (P < 0.05) or tendency for interaction (P < 0.10) between age and treatment for ileal EAA losses were observed for most of the AA except for Cys. Ileal EAA losses for Met, Trp, Arg, and nitrogen (N) were lower (P < 0.05) in 42-d old NCH birds compared to 42-d old CHA birds. There were significant interactions between age and treatment with apparent and standardized ileal DM, N, and AA digestibility values being lower (P < 0.05) in 21-d-old CHA birds. Apparent and standardized ileal DM, N, and AA digestibility for the NCH birds at 21- and 42-d old were not different (P > 0.05). However, coccidial vaccine challenge resulted in a decrease in apparent and standardized digestibility in 21-d old CHA birds. The difference in SIAAD between the NCH and CHA 21-d old birds for Met, Lys, Thr, and Val was 9.7, 10.4, 19.4, and 20.1 percentage units, respectively. Results from this study showed that coccidial CHA significantly influenced ileal AA digestibility on d 21 compared to 42-d old birds.
Key words: broiler, coccidial challenge, endogenous amino acid, standardized digestibility