Determining the optimal digestible isoleucine to lysine ratio of Ross 708 pound 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.