PSIII-2 Optimal ratio of standardized ileal digestible total sulfur amino acid to lysine for 5 to 22 kg pigs under antibiotic-free regime

The present study aimed to determine the optimal ratio of standardized ileal digestible (SID) total sulfur amino acid (TSAA) to Lys for growth performance in 5 to 22 kg pigs under antibiotic-free regime. Experimental diets were formulated to ensure that lysine was the second limiting AA throughout the experiments. A total of 990 weaning piglets (PIC 337 × 1050, Hendersonville, TN; 5.08 ± 0.86 kg), blocked by initial BW and gender, were randomly allotted to 1 of 5 dietary treatments, with 9 pens per treatment and 22 piglets per pen. Dietary treatments consisted of SID TSAA to Lys ratios of 51%, 58%, 65%, 72% and 79%, with 1.31%, 1.28% and 1.20% SID Lys in phases 1, 2 and 3, respectively. The source of TSAA used in this study was the dry calcium salt of D, L-2-hydroxy-4-(methylthio)butanoic acid (84% Met, MHA®, Novus International, Inc., St. Charles, MO). Data was analyzed using general linear mixed models accounting for heterogeneous residual variances. Competing heteroskedastic models included broken-line linear (BLL), broken-line quadratic (BLQ), and quadratic polynomial (QP). For each response, the best-fitting model was selected using Bayesian information criterion. Results showed that increasing ratios of SID TSAA to Lys linearly (P = 0.01) increased ADG during the entire nursery period. Increasing SID TSAA to Lys ratios tended to increase (P = 0.09) overall ADFI in a quadratic manner. The best-fitting model for overall ADG is BLL in which the maximum ADG was estimated at 60% SID TSAA to Lys ratio, with 95% confidence interval (CI) between 55% and 65%. In conclusion, the optimal ratio of SID TSAA to Lys was 60% for maximum ADG. These results indicate that the NRC (2012) recommendation may underestimate the SID TSAA to Lys requirement for nursery pigs raised under antibiotic-free regime.