Evaluation of Protein Supplementation on Protein Digestibility in Beef Cattle Fed a Low to Medium Quality Forage Diet

Four ruminally and duodenally fistulated steers (469 ± 37 kg initial BW) were housed individually and arranged in a 4 x 4 Latin square to evaluate protein digestibility in steers fed a low to medium quality forage in response to protein supplementation. Steers were fed a chopped grass hay ad libitum (8.3% CP, 64.9% NDF) twice daily, plus 1) high-fat dried corn distillers grains (HDG, 10.8% fat, 0.8% of BW), 2) low-fat dried corn distillers grains (LDG, 5.7% fat, 0.7% of BW), 3) cotton seed meal (CSM, 3.0% fat, 0.4% of BW), or no supplement (CON). Supplements were formulated to provide isonitrogenous intake and to increase post-ruminal ARG supply. Each 21 d experimental period had 16 d of adaptation and 5 d collection period. From d 12 to 20, 8 g of chromic oxide was dosed ruminally twice daily at h 0 and h 12 via gelatin capsule for use as a digesta flow marker. Grab duodenal and fecal samples were collected over 3 d in a manner that allowed for digesta to be sampled every other h in a 24-h period. Protein supplementation resulted in greater (P ≤ 0.0027) total OM and CP intake compared to CON. Intake of NDF was not different (P = 0.43) among treatments, whereas ADF intake was greater (P ≤ 0.0001) for CSM vs CON, LDG and HDG. Fat intake was greatest (P = 0.001) for HDG and lowest for CON and CSM and LDG were intermediate. Greater (P ≤ 0.014) OM and CP duodenal flow rate was observed with protein supplementation vs. CON whereas differences were not (P ≥ 0.14) observed for NDF and ADF. Duodenal flow rate was greater (P = 0.005) for TAA with protein supplemention vs CON and ARG duodenal flow rate was greatest (P = 0.008) with CSM, lowest for CON and intermediate for HDG and LDG. Apparent total tract digestibility for OM and EE was greater (P ≤ 0.03) with HDG, LDG, and CSM compared to CON and NDF and ADF apparent total tract digestibility tended (P ≤ 0.07) to be greater with protein supplements than CON. Apparent total tract digestibility of CP, TAA, and ARG was lower (P ≤ 0.02) in CON than protein supplemented treatments. Post-ruminal supply of TAA and ARG was greater with vs without protein supplementation. Overall fiber and protein digestibility improved with protein supplementation in low to medium quality forage diets.