The effect of total sulfur amino acid levels on growth performance and bone metabolism in pullets under heat stress

A study was conducted to investigate the effects of total sulfur amino acid (TSAA) levels on performance and bone metabolism in pullets under heat stress (HS). Hy-Line W36 day-old pullets (n = 216) were randomly distributed in 3 dietary treatments (70, 85, and 100% of TSAA), with 6 replicates of 12 birds. The treatments were defined as percentages of the TSAA level recommendation (100, 85, and 70%), and 85 and 100% of TSAA were obtained by adding L-Methionine to the basal deficient diet (70% of TSAA). The birds were raised under HS (35°C/7 h/D) from 1 to 18 wk. At 6, 12, and 18 wk, growth performance was measured. At 12 and 18 wk, bone weight, ash, collagenous (ColP), and noncollagenous proteins (NColP), tissue volume (TV), bone mineral content (BMC), and mineral density from total, cortical, and trabecular bones were evaluated. The means were subjected to ANOVA and, when significant (P ≤ 0.05), were compared by Dunnett's test. Regression analyses were performed to evaluate trends of TSAA dose response. Overall, birds fed 70% of TSAA showed poor growth and feed efficiency compared with other groups. Additionally, in at least 1 phase, birds fed 70% of TSAA showed lower bone ash, NColP, total BMC, and TV and higher ColP than the other treatments, whereas the cortical and trabecular TV and BMC were lower than 100% of TSAA (P < 0.04). Quadratic effects of TSAA levels on body weight gain (BWG) were found, and the level for maximum BWG was 95% of the TSAA recommendation (P < 0.03, R(2) > 0.83). In conclusion, the use of a TSAA-deficient diet resulted in poor performance and delayed bone development. Additionally, the use of 100% of TSAA led to better initial structural bone development than 85% of TSAA. Therefore, the TSAA level recommended by the primary breeder guideline was enough to support growth and bone quality under HS, suggesting that HS does not alter TSAA requirement in pullets.