Effects of standardized ileal digestible tryptophan: lysine ratio on growth performance of nursery pigs

Two experiments were conducted to estimate the standardized ileal digestible (SID) Trp:Lys ratio requirement for growth performance of nursery pigs. Experimental diets were formulated to ensure that lysine was the second limiting AA throughout the experiments. In Exp. 1 (6 to 10 kg BW), 255 nursery pigs (PIC 327 x 1050, initially 6.3 +/- 0.15 kg, mean +/- SD) arranged in pens of 6 or 7 pigs were blocked by pen weight and assigned to experimental diets (7 pens/diet) consisting of SID Trp: Lys ratios of 14.7%, 16.5%, 18.4%, 20.3%, 22.1%, and 24.0% for 14 d with 1.30% SID Lys. In Exp. 2 (11 to 20 kg BW), 1,088 pigs (PIC 337 x 1050, initially 11.2 kg +/- 1.35 BW, mean +/- SD) arranged in pens of 24 to 27 pigs were blocked by average pig weight and assigned to experimental diets (6 pens/diet) consisting of SID Trp: Lys ratios of 14.5%, 16.5%, 18.0%, 19.5%, 21.0%, 22.5%, and 24.5% for 21 d with 30% dried distillers grains with solubles and 0.97% SID Lys. Each experiment was analyzed using general linear mixed models with 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. In Exp. 1 (6 to 10 kg BW), increasing SID Trp: Lys ratio linearly increased (P < 0.05) ADG and G:F. For ADG, the best-fitting model was a QP in which the maximum ADG was estimated at 23.9% (95% confidence interval [CI]: [< 14.7%, > 24.0%]) SID Trp: Lys ratio. For G: F, the best-fitting model was a BLL in which the maximum G: F was estimated at 20.4% (95% CI: [14.3%, 26.5%]) SID Trp: Lys. In Exp. 2 (11 to 20 kg BW), increasing SID Trp: Lys ratio increased (P < 0.05) ADG and G: F in a quadratic manner. For ADG, the best-fitting model was a QP in which the maximum ADG was estimated at 21.2% (95% CI: [20.5%, 21.9%]) SID Trp: Lys. For G: F, BLL and BLQ models had comparable fit and estimated SID Trp: Lys requirements at 16.6% (95% CI: [16.0%, 17.3%]) and 17.1% (95% CI: [16.6%, 17.7%]), respectively. In conclusion, the estimated SID Trp: Lys requirement in Exp. 1 ranged from 20.4% for maximum G: F to 23.9% for maximum ADG, whereas in Exp. 2 it ranged from 16.6% for maximum G: F to 21.2% for maximum ADG. These results suggest that standard NRC (2012) recommendations may underestimate the SID Trp: Lys requirement for nursery pigs from 11 to 20 kg BW.