The effects of replacing dried distillers grains with solubles with heat-treated soybean meal in forage-based diets on growth performance, nutrient flow, and digestibility in steers

The objectives of the study were to evaluate increasing replacement of dried distillers grains with solubles (DDGS) with heat-treated soybean meal (TSBM) on growth performance, nutrient flow, and digestibility. Heat-treated soybean meal replaced DDGS in the diet at 0% TSBM/16% DDGS (TSBM0), 4% TSBM/12% DDGS (TSBM4), 8% TSBM/8% DDGS (TSBM8), and 12% TSBM/4% DDGS (TSBM12) of the diet dry matter (DM). Diets were predicted to supply excess metabolizable protein, whereas lysine was predicted to be sufficient only in the TSBM12 treatment. For experiment 1, 70 Angus-based steers (initial BW = 298 ± 16 kg) were utilized in a generalized randomized block design for an 85-d growing study. Steers were housed in 3 pens (23, 23, and 24 steers/pen, respectively) and blocked by weight before being randomly assigned to treatments. In experiment 1, an automated feed intake monitoring system recorded individual intakes. At the end of the 85-d growing period in experiment 1, steers were fed in a single pen, finished on a common diet for 150 d, and transported to a commercial abattoir where carcass characteristics were recorded. For experiment 2, 5 ruminally, duodenally, and ileally fistulated Jersey steers were utilized in a 4 × 5 Youden square design (4 periods and 5 steers) to determine nutrient digestibility and flow. Nutrient flow and digestibility were determined using chromic oxide as an external flow marker. Data were analyzed using the MIXED procedure of SAS. In experiment 1, dietary treatment did not influence (P ≥ 0.14) ending BW, average daily gain, dry matter intake, and gain:feed. However, plasma urea nitrogen increased linearly (P = 0.01) with increasing TSBM on days 28, 56, and 85. Hot carcass weight, longissimus muscle area, backfat thickness, marbling, and calculated yield grade after finishing for experiment 1 were not influenced (P ≥ 0.21) by dietary treatment. In experiment 2, small intestinal digestibility of total and available lysine linearly increased (P < 0.04); whereas, small intestinal digestibility of nitrogen was not affected (P = 0.74) as TSBM replaced DDGS. True ruminal N digestibility linearly increased (P = 0.03) as TSBM replaced DDGS which contradicts rumen degradable protein values reported for TSBM and DDGS. In conclusion, growth performance was not improved when replacing increasing amounts of DDGS with TSBM in growing cattle diets, even when the supply of lysine increased from below to above predicted metabolizable lysine supply.