Estimation of the ideal ratio of standardized ileal digestible threonine to lysine for growing pigs (22-50Â kg) fed low crude protein diets supplemented with crystalline amino acids

Two experiments were conducted to investigate the effects of various standardized ileal digestible (SID) threonine (Thr) to lysine (Lys) ratios on the performance and serum parameters of growing pigs fed low crude protein (CP) diets supplemented with crystalline amino acids (CAA). In Exp. 1, 144 growing pigs (22-50 kg) were randomly allotted to one of three dietary treatments. The treatments included a high CP (186 g/kg) diet with 10.2 g/kg SID Lys or one of two low CP diets (dietary CP reduced by 40 g/kg but supplemented with crystalline Lys, Thr, methionine, tryptophan, isoleucine and valine) providing 9 or 10.2 g/kg SID Lys. Six pens of pigs (three with gilts and three with barrows) were fed each treatment and each pen housed eight pigs. The average daily gain (ADG) of pigs receiving the low CP diet containing 9 g/kg SID Lys was lower (P<0.05) than that of pigs fed diets containing 10.2 g/kg SID Lys with either a high or low CP level confirming that the dietary Lys concentration in this diet was deficient and therefore this SID Lys level was used in all diets fed in Exp. 2. In Exp. 2, 300 growing pigs (22-50 kg) were utilized in a dose-response study. Pigs were randomly allotted to one of five dietary treatments containing 9 g/kg SID Lys. Graded levels of crystalline Thr (0.6, 1.0, 1.5, 1.9 or 2.4 g/kg) were added to the basal diet to produce diets providing SID Thr to Lys ratios of 0.55, 0.60, 0.65, 0.70 or 0.75. Each diet was fed to six pens of pigs (three with barrows and three with gilts) with ten pigs per pen. Serum Thr (linear and quadratic, P<0.05) and glycine were increased (linear, P<0.05) as the dietary SID Thr to Lys ratio increased. Serum urea nitrogen (SUN; linear and quadratic, P<0.05), Lys, methionine and cystine decreased (linear, P<0.05) as the dietary SID Thr to Lys ratio increased. The optimum SID Thr to Lys ratios to maximize ADG and feed conversion ratio (FCR) as well as to minimize SUN levels were 0.68, 0.67 and 0.62 using a linear-breakpoint model and 0.73, 0.70 and 0.65 using a curvilinear-plateau model. Taking an average of these values, we conclude that the optimum SID Thr to Lys ratio for 22-50 kg pigs fed low CP protein diets supplemented with CAA is 0.70 to maximize ADG, 0.68 to optimize FCR and 0.63 to minimize SUN.