Estimation of the standardized ileal digestible lysine requirement and the ideal ratio of threonine to lysine for late finishing gilts fed low crude protein diets supplemented with crystalline amino acids

Two experiments were conducted to investigate the effects of various standard Heal digestible (SID) lysine (Lys) levels and SID threonine (Thr) to Lys ratios on the performance and carcass characteristics of finishing gilts receiving low crude protein (CP) diets supplemented with crystalline amino acids (CAA). In Exp. 1,108 gilts (87.8 +/- 5.9 kg) were randomly allotted to one of six diets which consisted of a high CP (135 g/kg) diet with 6.1 g/kg SID Lys or five low CP (100 g/kg) diets providing SID Lys levels of 4.9, 5.5, 6.1, 6.7 and 7.3 g/kg, respectively. Gilts were housed in three pigs per pen with six pens per treatment. At the end of the 28 days experiment, 36 gilts (one pig per pen) with average body weight (BW) of 116 kg were killed to evaluate carcass traits. The SID Lys levels required to maximize average daily gain (ADG) and optimize feed conversion ratio (FCR) as well as to minimize serum urea nitrogen (SUN) levels were 5.7, 5.8 and 6.1 g/kg using a linear-break point model and 6.5, 6.5 and 6.6 g/kg using a quadratic model. The fat-free lean gain tended to increase linearly with the increase in dietary SID Lys levels from 4.9 to 7.3 g/kg when gilts receiving a low CP diet (linear effect, P=0.06). In Exp. 2,90 gilts (90.6 +/- 5.7 kg) were utilized in another dose-response study. Based on the Lys requirement estimated in Exp. 1, dietary treatments were formulated to contain 5.1 g/kg SID Lys to ensure the dietary Lys level was marginally deficient for late finishing gilts. Graded levels of crystalline Thr (0, 0.3, 0.6, 0.9 or 1.2 g/kg) were added to the basal diet providing SID Thr to Lys ratios of 0.54, 0.60, 0.66, 0.72 or 0.78, respectively. Each diet was fed to six pens with three gilts per pen. At the end of Exp. 2,30 gilts (one pig per pen) were slaughtered to evaluate carcass traits (average BW = 118 kg). The optimum SID Thr to Lys ratios to maximize ADG and FCR as well as to minimize SUN levels were 0.61, 0.63 and 0.64 using a linear-break point model and 0.70, 0.75 and 0.74 using a quadratic model. With the exception of L* light (linear effect, P<0.05), no effect on carcass characteristics was observed with increasing dietary SID Thr to Lys ratio. (C) 2015 Published by Elsevier B.V.