Effects of Increasing Ratios of Standardized Ileal Digestible Total Sulfur Amino Acid to Lysine on Growth Performance of 7 to 17 Kg Pigs Under Antibiotics or Antibiotics-Free Regime

The present study aimed to determine the effects of increasing ratios of standardized ileal digestible (SID) total sulfur amino acid (TSAA) to Lys on growth performance of 7 to 17 kg pigs under antibiotics and antibiotics-free regime. A total of 924 weaning piglets were fed common diet without antibiotics from d 0 to 14 post-weaning. Piglets were blocked by BW on d 14 (PIC 337 × 1050, Hendersonville, TN; 7.95 ± 0.50 kg) and sex and randomly allotted to 1 of 6 dietary treatments, with 7 pens per treatment and 22 piglets per pen. Dietary treatments were arranged in 2 × 3 factorial design, with 2 feeding regimes (without or with 1.0% Mecadox 2.5 (50 g Carbodox per ton in the diet); Phibro Animal Health Corp., Ridgefield Park, NJ) and 3 SID TSAA to Lys ratios (51.0, 58.5 and 66.0%). The source of TSAA used in this study was dry calcium salt of D, L-2-hydroxy-4-(methylthio)butanoic acid (84% Met, MHA, Novus International, Inc., St. Charles, MO). The experimental period lasted 21 days. Results showed that there were no interactions between feeding regimes and SID TSAA to Lys ratios in terms of ADG (P = 0.39), ADFI (P = 0.96) and BW on d 35 post-weaning (P = 0.39). Pigs consumed diets supplemented with Mecadox were heavier (16.70 vs. 16.27 kg; P = 0.03) at the end of study compared with pigs on Mecadox-free diets. Increasing ratios of SID TSAA to Lys from 51 to 66% significantly (P = 0.03) improved final BW from 16.13 to 16.78 kg, significantly (P = 0.01) improved ADG from 0.40 to 0.42 kg/d, and tended (P = 0.06) to improve ADFI from 0.62 to 0.65 kg/d. Additionally, there tended (P = 0.07) to be interaction between feeding regimes and SID TSAA to Lys ratios in terms of G:F. As the ratio of SID TSAA to Lys increased from 51 to 66%, G:F of pigs on Mecadox-free diets increased from 0.63 to 0.66 kg/kg, whereas G:F of pigs on Mecadox-diet plateaued at the ratio of 58.5%. In conclusion, optimal SID TSAA to Lys ratio was 58.5% to maximize G:F for 7 to 17 kg pigs under antibiotics feeding regime. However, optimal SID TSAA to Lys ratio was beyond 66.0% in terms of G:F for 7 to 17 kg pig raised under antibiotics-free regime and warrants further investigation.