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Feed costs and nitrogen excretion may be reduced by replacing a portion of the dietary crude protein (CP) in swine diets with crystalline amino acids (CAA). Pigs fed low CP diets (CP level reduced by 30–40g/kg) supplemented with CAA have been shown to achieve the same performance as those fed normal CP levels, while the amount of protein ingredients used and feed costs were both reduced. The aim of this study was 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 CP diets supplemented with CAA.
In Exp. 1, 144 growing pigs (22–50kg) were randomly allotted to one of three dietary treatments. The treatments included a high CP (186g/kg) diet with 10.2 g/kg SID Lys or one of two low CP diets (dietary CP reduced by 40g/kg but supplemented with crystalline Lys, Thr, methionine, tryptophan, isoleucine and valine) providing 9 or 10.2 g/kg SID Lys. Six pens with eight pigs were fed each treatment. 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 10.2g/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 (22–50kg) were utilized in a dose–response study. Pigs were randomly allotted to one of five dietary treatments containing 9g/kg SID Lys. Graded levels of crystalline Thr (0.6, 1.0, 1.5, 1.9 or 2.4g/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 with ten pigs. Serum Thr 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. Taking an average of these values, we conclude that the optimum SID Thr to Lys ratio for 22–50kg 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. Further research should be conducted to determine whether or not the optimum SID Thr to Lys ratio is affected by dietary CP level.
G J Zhang, C Y Xie, P A Thacker, J K Htoo, S Y Qiao (2013). Estimation of the ideal ratio of standardized ileal digestible threonine to lysine for growing pigs (22–50kg) fed low crude protein diets supplemented with crystalline amino acids. Animal Feed Science and Technology, 180; 83–91. http://dx.doi.org/10.1016/j.anifeedsci.2013.01.006