The appropriate standardized ileal digestible tryptophan to lysine ratio improves pig performance and regulates hormones and muscular amino acid transporters in late finishing gilts fed low-protein diets1

This study investigated the effects of various standardized ileal digestible (SID) Trp to Lys ratios on the performance and carcass characteristics of late finishing gilts receiving low-CP (9.6%) diets supplemented with crystalline AA. Ninety gilts (89.1 ± 5.1 kg) were used in a dose–response study conducted for 35 d. Crystalline Trp (0, 0.1, 0.2, 0.4, or 0.6 g/kg) was added to a corn–wheat bran basal diet providing SID Trp to Lys ratios of 0.12, 0.15, 0.18, 0.21, or 0.24. Each diet was fed to 6 pens of pigs with 3 gilts per pen. At the end of the experiment, 30 gilts (1 pig per pen) were slaughtered to evaluate carcass traits and meat quality (BW = 121 kg). Increasing the SID Trp to Lys ratio increased ADG (linear and quadratic effect, P < 0.05) and also improved G:F (linear and quadratic effect, P < 0.05). Serum urea nitrogen (SUN) decreased as the SID Trp to Lys ratio increased (linear and quadratic effects, P < 0.05). A quadratic effect of L* light and marbling in the longissimus dorsi was observed as the dietary SID Trp to Lys ratio increased (P < 0.05). Increasing the SID Trp to Lys ratio increased the level of serum GH (quadratic effect, P < 0.05) and also increased the level of serum IGF-1 (linear and quadratic effect, P < 0.05). Increasing the SID Trp to Lys ratio increased the protein abundance of the muscular AA transporter of sodium-coupled neutral amino acid transporter 2 (SNAT2) in the longissimus dorsi muscle (linear and quadratic effect, P < 0.05). The optimum SID Trp to Lys ratios to maximize ADG and G:F as well as to minimize SUN levels were 0.16, 0.17, and 0.16 using a linear-breakpoint model and 0.20, 0.20, and 0.20 using a quadratic model. Tryptophan could influence serum GH and IGF-1 secretion and protein abundance of the muscular AA transporter of SNAT2 in the longissimus dorsi muscle in late finishing gilts fed low-protein diets.