Effects of standardized ileal digestible threonine to lysine ratio on growth performance of nursery, growing, and finishing pigs.

The objective of these experiments was to evaluate the effect of standardized ileal digestible (SID) Thr:Lys ratio on growth performance of late nursery, growing, and finishing pigs. Four experiments were conducted using corn-soybean meal-based diets with pigs allotted to 5 or 6 treatments in randomized complete block designs. In Exp. 1, 987 late nursery pigs (initially 11.8 ± 0.32 kg) were used with 22 to 27 pigs per pen, 8 replications per treatment, and 5 SID Thr:Lys ratios from 53% to 68% of Lys. Increasing SID Thr:Lys ratio increased (linear, P ≤ 0.005) ADG and final BW. Average daily feed intake decreased then increased (quadratic, P < 0.001), therefore increasing then decreasing (quadratic, P < 0.001) G:F. In Exp. 2, 875 pigs (initially 43.4 ± 0.58 kg) were used with 21 to 27 pigs per pen, 7 replications per treatment, and 5 SID Thr:Lys ratios from 53% to 68% of Lys. Increasing SID Thr:Lys ratio increased (linear, P ≤ 0.037) ADG, final BW, and G:F. Dose-response models developed using the data from Exp. 1 and 2 predicted a requirement estimate for ADG beyond the highest dose tested (68% SID Thr:Lys ratio), which led us to test a wider range of SID Thr:Lys ratios in Exp. 3 and 4. In Exp. 3, 684 pigs (initially 37.4 ± 1.36 kg) were used with 9 or 10 pigs per pen, 12 replications per treatment, and 6 SID Thr:Lys ratios from 53% to 78% of Lys. Increasing SID Thr:Lys ratio increased then plateaued (quadratic, P ≤ 0.047) final BW, ADG, G:F and tended (quadratic, P = 0.093) to decrease then increase blood urea N (BUN). In Exp. 4, 662 pigs (initially 96.2 ± 1.04 kg) were used with 8 to 10 pigs per pen, 11 or 12 replications per treatment, and 6 SID Thr:Lys ratios from 53% to 78% of Lys. Increasing SID Thr:Lys ratio tended to increase then decrease (quadratic, P ≤ 0.063) final BW, ADG, and G:F, with the greatest numeric response observed between 63% and 68% SID Thr:Lys ratio. Data from Exp. 3 and 4 were combined and used to fit dose response curves for ADG, G:F, and BUN. A quadratic polynomial (QP) model suggested that ADG was maximized at 68.6% SID Thr:Lys, while a similar fitting broken-line linear (BLL) model suggested no further improvement in ADG beyond 61.1% SID Thr:Lys. A BLL model suggested no further improvement in G:F beyond 59.2% SID Thr:Lys. A QP model suggested that BUN was minimized at 69.2% SID Thr:Lys. These data suggest that SID Thr:Lys requirements are similar for pigs from 37 to 70 and 96 to 120 kg.