Determining the Optimal Digestible Isoleucine to Lysine Ratio of Ross 708 × Ross YP Male Broilers from 28 to 42 Days of Age

Recent research within our laboratory has determined the optimal Ross 708 pound Ross YP male broiler dIle:dLys ratio for the starter and grower phases to be 70 and 68%, respectively. Thus, the current objective was to continue to re-evaluate the optimal dIle:dLys ratio for this broiler, focusing on the finisher phase. On d of hatch, 2,400 chicks were equally allocated to 96 pens and fed a common starter (d 0-14; 70% dIle:dLys) and grower diet (d 14-28; 68% dIle:dLys). Pen weights were equalized at d 28 by block (12/treatment). Experimental finisher diets (d 28-42;18.0% crude protein, 3,200 kcal/kg, 0.97% dLys, 130% dLeu:dLys) were created from a common deficient (54% dIle:dLys) corn and soybean meal-based diet and contained animal protein. Half of the deficient was retained to create the summit (84% dIle:dLys). The remaining 5 diets were blended proportions of the deficient and summit diets (ranging from 59 to 79% dIle:dLys). A practical control diet (PRAC-CON; 69% dIle:dLys) was manufactured separately to verify the blended control diet (BLEND-CON). All dIle:dLys ratios were estimated using quadratic regression (QR; 95% of the asymptote), as well as linear and quadratic broken line models (LBL; QBL). Feeding equivalent to or slightly lower than current breeder dIle:dLys recommendations (69%) improved broiler performance from d 28 to 42. Based on the current data and multiple regression models, the estimated ratios ranged from 62 to 66% for BW and BWG and 63 to 66% for FCR. Further research should evaluate the optimal dIle:dLys ratio for this broiler throughout the remaining withdrawal phase.