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

This study aimed to assess the effects and interactions between amino acid (AA) content and starch-to-lipid ratios (S:L) in finisher diets on Ross 308 broilers exposed to cyclic heat stress. Ross 308 mixed sex broilers (21 d of age) were fed isocaloric finisher diets formulated to have 80, 90, 100, 110, 120, or 130% of their estimated AA requirements (AAR) and S:L ratios of either 4:1 or 10:1, resulting in a total of 12 dietary treatments arranged in a 6 × 2 factorial design. Birds were housed at a final estimated stocking density of 31 kg/m2 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.