Estimated standardized ileal digestible (SID) Ile:Lys ratio requirement for ADG of nursery pigs, Exp. 1. QP: BIC = 558.3; Estimated 64.7% SID Ile:Lys that maximized ADG; [95% CI: (51, > 65%)]; 99% of Max ADG at 57.0% SID Ile:Lys. BLL: BIC = 556.6; Estimated breakpoint: 52.0% SID Ile:Lys; [95% CI:(51.96, 52.04)]. BLQ: BIC = 557.9; Estimated breakpoint: 52.0% SID Ile:Lys; [95% CI:(51.97, 52.03)]. A total of 280 nursery pigs (PIC 327 × 1,050, initially 6.7 ± 0.08 kg BW) were used in a 26-d growth trial with 5 pigs per pen and 8 pens per treatment. Pigs were weaned at approximately 21 d, fed a common starter diet for 6 d post-weaning, then placed on experimental diets. Experimental diets were fed from d 0 to 12 and a common diet was fed from d 12 to 26. Quadratic polynomial (QP), broken-line linear (BLL), and broken-line quadratic (BLQ) models were fit for the experimental period to estimate SID Ile:Lys. Bayesian Information Criterion (BIC) was used to determine the best fitting models; a lower value indicates a better fit to the data.