Due to the continuous genetic improvement of high yielding broiler strains, the reevaluation of essential amino acids is important. Previous research investigating the starter phase utilizing Ross x Ross 708 males determined the optimum ratio for dIle:dLys to be 70%, which is higher than traditionally used ratios (67% dIle:dLys); however, the optimum dIle:dLys ratio for the remaining phases has yet to be reevaluated. Thus, the objective of this study was to evaluate the d14-28 dIle:dLys ratio of Ross x Ross 708 male broilers. A total of 2,400 male chicks were obtained from a commercial hatchery, equally allocated to 96 pens, and fed a common starter diet (70% dIle:dLys) from d0-14. At d14 pen weights were equalized. Experimental diets were created from a common deficient corn and soybean meal-based diet containing a dIle:dLys of 53%, after manufacture half of this diet was retained for the creation of the summit diet (83% dIle:dLys) by crystalline Ile addition. The remaining 5 experimental diets ranged from 58 to 78% dIle:dLys and were obtained by blending different proportions of deficient and summit diets. A practical control diet was formulated to 68% dIle:dLys and batched separately. There were 12 replications/treatment; birds were individually weighed, and feed intake was recorded on d28 to determine average BW, BW gain (BWG), bird uniformity, feed intake/bird, % mortality, and feed conversion ratio (FCR). All % dIle:dLys requirements were estimated using quadratic regression (95% of the asymptote; QR), as well as linear and quadratic broken line models (LBL;QBL). For BW and BWG, the QR model suggests the optimum ratio to be 68% dIle:dLys (P<0.05; R2=0.78, 0.80). When utilizing the LBL model, the ratio was estimated to be 62% (P=0.02; R2=0.88), while the QBL estimated the ratio to be 65% (P=0.02; R2=0.86). Additionally, for FCR, the QR model estimated the ratio to be 68% dIle:dLys (P=0.04; R2=0.80) while the LBL model estimated the ratio to be 67% (P=0.06; R2=0.76) and the QBL model estimated it to be 70% (P=0.07; R2=0.73).
These data are an important step in continued evaluation of the optimum dIle:dLys ratio of Ross x Ross 708 male broilers to optimize performance; future research should continue this work for the remaining grow-out phases.