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Determining the optimal digestible isoleucine to lysine ratio of Ross 708 x Ross YP male broilers from 42 to 56 days of age

Dennehy, D., J. Millican, J. Lee, R.Adhikari and K. Wamsley

The objective of this study was to reevaluate the optimal dIle:dLys ratio during the withdrawal phase (d 42-56). Ross 708 x Ross YP male chicks were provided common starter, grower and finisher diets (70, 69, and 69% dIle:dLys, respectively) formulated using breeder recommendations, as well as optimal dIle:dLys ratios established from our previous research. On d 42, pen weights were equalized by block (12 reps/trt), and each pen contained 25 birds (0.24 m2/bird); average BW was 3.206 kg. Experimental diets were created from a common deficient diet (54% dIle:dLys; 130% dLeu:dLys; 82% dVal:dLys). Crystalline Ile was added to the deficient diet to create the summit diet (84% dIle:dLys); intermediate levels of dIle:dLys (59-79% dIle:dLys) were obtained by blending these diets. A practical control diet (69% dIle:dLys; 146% dLeu:dLys; 78% dVal:dLys) was separately formulated and batched to verify blending techniques. This study was analyzed as a randomized complete block design using Proc GLM; significance was set at P≤0.05; LS means and calculated dIle:dLys ratios were utilized for regression analyses. Birds were individually weighed on d 55 to measure CV of BW and BW, d 42-55 BW gain, feed intake, and FCR. On d 56, 5 birds/pen of average weight (±100g) were processed, and yields were calculated. Metrics not significant included CV of BW, d 55 BW, and d 42-55 feed intake (P>0.05). Numerical and significant differences were observed for d 42-55 BWG and FCR (P=0.0517 and P<0.0001, respectively); FCR improved by 13 points as dIle:dLys increased from 54 to 84% (2.046 vs 1.921, respectively). Carcass and drumstick yields were significant, whereas feeding 54% dIle:dLys increased yields (P<0.05). Preplanned contrasts between the control diets demonstrated no differences for all metrics (P>0.05), verifying blending. Optimal dIle:dLys ratios were estimated using quadratic regression (QR; 95% of vertex), as well as linear and quadratic broken line models (LBL; QBL). The QR model estimated the ratio for BW at 72% (P=0.0028, R2=0.95) and BWG at 71% (P=0.0264, R2=0.84). For LBL, the ratio was estimated to be 65% for both BW (P=0.0002, R2=0.99) and BWG (P=0.0075; R2=0.91); QBL estimated the ratio at 70% for both BW (P=0.0002; R2=0.99) and BWG (P=0.0123; R2=0.89). For FCR, the QR model estimated the ratio to be 77% (P=0.0125; R2=0.89), while LBL and QBL estimated it at 70% (P=0.0171; R2=0.87) and 78% (P=0.0118; R2=0.90), respectively.

Overall, manipulating the dIle:dLys ratio had the largest impact on FCR, with a minor effects on BWG and processing. These data may be impacted by early nutritional strategies and other BCAA ratios, therefore future research should consider these factors when estimating optimal dIle:dLys.