Comparing tryptophan:lysine ratios in dried distillers grains with solubles-based diets with and without a dried distillers grains with solubles withdrawal strategy on growth, carcass characteristics, and carcass fat iodine value of growing-finishing pigs.

Abstract

A total of 6,240 finishing pigs (DNA 600 × PIC 1050; initially 22.5 ± 1.00kg), divided into 2 groups, were used in a 119 or 120 d study comparing increasing Trp:Lys ratio in diets containing dried distillers grains with solubles (DDGS) or a DDGS withdrawal strategy (removing all DDGS from the last phase before marketing) on growth performance and carcass fat iodine value (IV). Pigs were randomly allotted to 1 of 7 dietary treatments with 30 to 36 pigs per pen and 26 replications per treatment. Diets were fed in 4 phases, approximately 23 to 44, 44 to 71, 71 to 100, and 100kg to market. Diets included a control corn-soy bean meal-based diet (no DDGS) formulated to a 19% standardized ileal digestibility (SID) Trp:Lys ratio, 4 diets with 30% DDGS fed in all four phases and formulated to provide SID Trp:Lys ratios of 16, 19, 22, or 25%, and 2 DDGS withdrawal strategy diets: 19% SID Trp:Lys with 30% DDGS in phase 1 through 3 and then 0% DDGS in phase 4 with either a 19 or 25% Trp:Lys ratio. Overall, body weight (BW), average daily gain (ADG), average daily feed intake (ADFI), and G:F increased (linear, P < 0.05) as SID Trp:Lys ratio increased in diets with 30% DDGS fed in all phases. Simultaneously, hot carcass weight (HCW; quadratic, P = 0.014), carcass yield (quadratic, P = 0.012), and backfat depth (linear, P = 0.040) increased with increasing Trp:Lys ratio. Pigs fed the 19% SID Trp:Lys ratio withdrawal strategy diet had similar ADG and ADFI as those fed the control diet, the 25% Trp:Lys withdrawal diet, or the 30% DDGS diets with 25% Trp:Lys ratio throughout the study. Pigs fed the control diet had decreased (P < 0.05) carcass fat IV compared to pigs fed DDGS diets throughout the study, with pigs fed the two DDGS withdrawal strategy diets intermediate. In summary, increasing the SID Trp:Lys ratio in diets with 30% DDGS resulted in a linear increase in ADG, ADFI, G:F, and BW but did not influence carcass fat IV, with most of the benefit observed as diets increased from 16 to 19% Trp:Lys. Removing DDGS from the diet in the last period reduced carcass fat IV and increased growth rate during the withdrawal period compared to pigs fed 30% DDGS throughout, indicating value in a withdrawal strategy.