Effects of standardized ileal digestible tryptophan:lysine in diets containing 30% dried distillers grains with solubles on finishing pig performance and carcass traits.

Two experiments were conducted to determine the effects of standardized ileal digestible (SID) Trp:Lys in grow-finish swine diets containing 30% dried distillers grains with solubles (DDGS). Within each experiment, crystalline Lys and Trp replaced soybean meal to alter the dietary SID Trp:Lys concentration but maintain minimum ratios of other AA to Lys. In Exp. 1, 638 pigs (36.3 kg initial BW) were used in a 105-d trial (6 pens per treatment). Pens of pigs were randomly allotted to 1 of 4 dietary treatments with SID Trp:Lys of 14.0, 15.0, 16.5, and 18.0%. From d 0 to 42, as Trp:Lys increased, ADG increased (quadratic; P = 0.05) and ADFI tended to increase (quadratic; P = 0.07) with no changes in G:F. Both ADG and ADFI were maximized at Trp:Lys of 16.5%. From d 42 to 105, increasing SID Trp:Lys increased (linear; P < 0.001) ADG and ADFI. Unlike data from d 0 to 42, the response was linear through the greatest SID Trp:Lys of 18.0%. Overall (d 0 to 105), increasing SID Trp:Lys increased (linear; P < 0.001) final BW, ADG, ADFI, and HCW. In Exp. 2, 1,214 pigs (66.3 kg initial BW) were used in a 73-d finishing trial (9 pens per treatment). Pens of pigs were randomly allotted to 1 of 5 dietary treatments with SID Trp:Lys of 15.0, 16.5, 18.0, and 19.5 and the 15.0% Trp:Lys diet with l-Trp added to achieve 18.0% SID Trp:Lys. Overall (d 0 to 73), ADG, ADFI, G:F, and final BW improved (linear; P < 0.03) as dietary SID Trp:Lys increased through 19.5%. No differences were found in growth performance between the 2 diets containing 18.0% SID Trp:Lys. For carcass traits, increasing SID Trp:Lys resulted in increased HCW (linear; P = 0.01) and a tendency toward a decreased (quadratic; P = 0.09) backfat depth and fat-free lean index (FFLI), with pigs fed diets containing 16.5 and 18.0% SID Trp:Lys having increased FFLI and lower backfat depth compared with pigs fed 15.0 and 19.5% SID Trp:Lys. Pigs fed the diet with added crystalline Trp tended to have increased (P = 0.08) backfat depth and decreased FFLI (P = 0.10) compared with pigs fed the same SID Trp:Lys without crystalline Trp. The results indicated that the optimal SID Trp:Lys was 16.5% from 36.3 to 72.6 kg but at least 19.5% from 72.6 to 120.2 kg in corn-soybean meal diets containing 30% DDGS.