Effects of Standardized Ileal Digestible Threonine to Lysine Ratio on Growth Performance of PIC Line 337-Sired Grow-Finish Pigs

Abstract Two studies were conducted to estimate the SID Thr:Lys requirement for growth and feed efficiency of PIC 337×1050 pigs from approximately 12- to 70-kg. In Exp. 1, 987 pigs [initial body weight (BW) = 11.8±0.32 kg] were used from 12- to 24-kg. In Exp. 2,875 pigs (initial BW = 43.4±0.58 kg] were used from 43- to 70-kg. Pens of pigs were blocked by BW and randomly allotted to dietary treatments in randomized complete block designs, with 21 to 27 pigs per pen with 8 and 7 replications per treatment (Exp. 1 and 2, respectively). Pens were provided 1 of 5 dietary treatments with increasing SID Thr:Lys ratios at 53, 58, 62, 65, and 68% in each experiment. Two corn-soybean meal-based diets containing 53 and 68% Thr:Lys ratios were blended to achieve target SID Thr:Lys treatment levels in each experiment. In Exp. 1 (12- to 24-kg), ADG and final BW increased (linear, P ≤ 0.006) while ADFI, G:F, Thr intake/d, and Thr intake/kg of gain increased (quadratic, P ≤ 0.001) as Thr:Lys ratio increased (Table). Additionally, Lys intake/d increased (quadratic, P < 0.001) while Lys intake/kg of gain decreased (quadratic, P < 0.001) with increasing Thr:Lys ratio. In Exp. 2 (43- to 70-kg), ADG, G:F, final BW, Thr intake/d and Thr intake/kg of gain also increased (linear, P ≤ 0.036) as dietary Thr:Lys increased. Moreover, Lys intake/kg of gain decreased (linear, P = 0.023) with increasing Thr:Lys. A quadratic polynomial (QP) model predicted a requirement at levels greater than 68% SID Thr:Lys for ADG from 12- to 24-kg, while a QP model suggested that maximum G:F was achieved at 61.8% SID Thr:Lys (Exp. 1). From 43- to 70-kg, QP models suggested maximum ADG and G:F were achieved at levels greater than 68% SID Thr:Lys (Exp. 2). However, similar fitting BLL models predicted no further improvement to G:F and ADG beyond 65 and 67% SID Thr:Lys, respectively. In summary, these results suggest the optimal SID Thr:Lys for 12- to 24-kg pigs is 61.8% for feed efficiency and greater than 68% for ADG. From 43- to 70-kg, the requirement was predicted at or above 65 and 67% SID Thr:Lys for G:F and ADG, respectively.