Determining methionine and cysteine requirements of male and female Ross 708 broilers exposed to heat stress during the finisher period

This study aimed to determine standard ileal digestible methionine and cysteine (dig Met+Cys) requirements of broilers reared under cyclic heat stress conditions. A basal diet deficient in dig Met+Cys was used as a control (0.254 % dig Met, 0.240 % dig Cys), and supplemented with DL-Methionine at 5 graded levels (0.050, 0.090, 0.170, 0.290, and 0.450 %), replacing silicon dioxide for six treatments containing 0.494, 0.541, 0.588, 0.648, 0.738, and 0.837 % dig Met+Cys, was fed from d 20 to 33. Day-old Ross 708 chicks were sex separated and housed in nine rooms (five male, four female), each containing 6 trial pens. From d 27 to 33, the birds were exposed to cyclical heat stress, alternating between a 12-h exposure to 31 °C and a 12-h cool period of 21 °C (minimum humidity 50 %; humidex of 41 °C and 23 °C). Body and feed weights were measured on d 0, 20, 27, and 33. On d 31, 1 bird per pen was euthanized and had a breast and liver sample taken to measure biomarkers for oxidative damage and antioxidant capacity. On d 34, 3 birds per pen were slaughtered and used to measure meat yield. Data were analyzed as a one-way ANOVA using the MIXED procedure in SAS and using an exponential regression model to determine requirements. Increasing dietary dig Met+Cys significantly improved BW gain and feed-to-gain (F:G). The estimated required dig Met+Cys intake to maximize BW gain and F:G for males were 0.976 g/d and 1.116 g/d, respectively, and 1.031 g/d for females for both parameters. No significant effects of dig Met+Cys level on antioxidant activity were observed. Requirements for maximal Pectoralis major yields were approximately 0.808 and 0.726 g/d dig Met+Cys intake for males and females, respectively. Based on F:G response, a daily intake of dig Met+Cys of 1.116 g (males) and 1.031 g (females) for Ross 708 broilers under cyclical heat stress conditions of 31 °C and 21 °C from d 27 to 33 is required, while meeting needs for optimal BWG and meat yield.