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Determining the optimal digestible isoleucine to lysine ratio in Ross x Ross 708 male broilers from 14 to 28 days of age

Brown, A., J. Lee and K. Wamsley

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.