Estimation of the ideal standardized ileal digestible tryptophan to lysine ratio for growing pigs fed low crude protein diets supplemented with crystalline amino acids

Tryptophan is typically regarded as the second or third limiting amino acid (AA) in low-crude protein (CP), corn-soybean meal based diets fed to growing swine. However, there is limited data available concerning the ideal standardized ileal digestible (SID) Trp to Lys ratio for growing pigs fed low-CP diets supplemented with crystalline AA. Therefore, the aim of this study was to evaluate the Trp requirement for 25-50 kg pigs, expressed as a SID Trp to Lys ratio, using swine housed under commercial conditions. In Experiment 1, 144 growing pigs were used to identify a diet deficient in Lys. The pigs were fed either a high-CP (18.6%) diet with 1.02% SID Lys or one of two low-CP diets (CP content reduced by four percentage units but supplemented with crystalline Lys, Trp, Met, Thr, Ile, and Val) providing 0.9 or 1.02% SID Lys, respectively. Each diet was fed to six pens containing eight pigs (4 barrows and 4 gilts) per pen. The weight gain (688 g/day) of pigs receiving the low-CP diet containing 0.9% SID Lys was lower (P = 0.03) than that of pigs fed diets containing 1.02% SID Lys with a high- (758 g/day) or low-CP (747 g/day) content, confirming that this diet was deficient in Lys and, therefore, 0.9% SID Lys was used in the basal diet fed in Experiment 2. Experiment 2 was a dose-response study involving 300 growing pigs, in which graded levels of crystalline Trp were used to produce five diets providing SID Trp to Lys ratios of 13, 16, 19, 22, or 25%. Each diet was fed to six pens containing ten pigs (five barrows and five gilts) per pen. Serum Ile, Leu, Pile, and Trp concentrations linearly increased (P < 0.05) as the dietary SID Trp to Lys ratio increased. In contrast, serum urea nitrogen (SUN) and Ala concentrations linearly decreased (P < 0.05) as the dietary SID Trp to Lys ratio increased. The SID Trp to Lys ratios that maximized weight gain, feed conversion ratio, and SUN were 19.7, 20.0, and 20.8% using a linear-breakpoint model and 22.6, 23.2, and 23.6% using a curvilinear-plateau model. Taking an average of these six values, we conclude that the optimum SID Trp to Lys ratio is at least 22% for 25-50 kg growing pigs fed low-CP diets supplemented with crystalline AA. (C) 2012 Elsevier B.V. All rights reserved.