Effects of dietary starch-to-lipid ratios and amino acid levels on broilers exposed to cyclic heat stress during the finisher period

in 8 rooms, each containing 12 pens. Rooms were maintained at a temperature of 21°C from d 21 to 27. Starting on d 28, birds were subjected to cyclic heat stress at 31°C for 12 h/d, followed by a cooler temperature of 21°C, with a minimum humidity of 50%. Body weight and residual feed weights were recorded on d 21, 28, and 35, which were used to calculate BW gain (BWG) and feed-to-gain ratios (F:G). Birds fed 80, 100, and 120% AAR were slaughtered on d 35 to determine meat yield. Starch-to-lipid ratios did not affect BWG or F:G ratios from d 21 to 35. Based on the linear (LBL) and quadratic (QBL) broken line models, the estimated breakpoints from d 21 to 35 were 110% and 122% AAR for BWG of 1.474 and 1.475 kg, respectively; whereas estimated breakpoints for LBL and QBL were 114% and 129% AAR respectively, for F:G ratios of 1.525 and 1.515. There was no effect of the dietary treatments on oxidative stress biomarkers (d 31). Highest breast meat yield (% live weight) was observed in birds fed 120% AA. In conclusion, while differing S:L ratios did not affect BWG or F:G ratios, finisher diet (d 21-35) AAR in the range of 114 to 129% improved performance of a population of broilers when exposed to heat stress from d 28-35.