Effect of ruminal vs postruminal administration of degradable protein on utilization of low-quality forage by beef steers

An experiment was designed to determine the effects of ruminal and postrumi nal infusions of ruminally degradable protein (casein) on intake and digest ion of low-quality hay by beef steers. Twelve ruminally fistulated Angus x Hereford steers (initial BW = 563 kg) were blocked by weight and assigned t o one of three treatments: control (C; hay only) or hay plus ruminal (R) or postruminal (P) infusion of 400 g/d of sodium caseinate. The trial consist ed of five periods: 1) 10-d adaptation to the hay diet; 2) 7-d measurement of hay intake (without infusions); 3) 10-d adaptation to protein infusion t reatments (intake measurements continued); 4) 7-d measurement of hay intake and digestibility (infusions continued); and 5) 3-d ruminal sampling perio d (infusions continued). Steers were given ad libitum access to tallgrass-p rairie hay (3.4% CP, 76.6% NDF) throughout the study. Casein was administer ed once daily before feeding, either directly into the rumen or via anchore d infusion lines into the abomasum. Hay intake was increased by supplementa tion (P < 0.01). Ruminal infusion elicited a greater (P = 0.04) increase in hay intake than postruminal infusion. Intake tended (P = 0.11) to be lower in period 4 than in period 2 for control steers but was greater in period 4 than in period 2 (P <less than or equal to> 0.03) for both R and P steers . The increase in intake between periods 2 and 4 was greater for R than for P steers (P = 0.03). Supplementation improved diet OM digestion (P = 0.04) but not NDF digestion (P = 0.18); however, greater relative error for NDF digestion may have limited the ability to elucidate significant treatment e ffects. There were no differences in either OM digestion (P = 0.42) or NDF digestion (P = 0.35) between R and P steers. Plasma urea N at 0 and 3 h aft er feeding on the last day of the experiment was lower (P = 0.05) for C tha n for R and P steers, but no difference (P = 0.48) was evident between R an d P steers. Ruminal ammonia N levels also were increased by supplementation (P < 0.01), with a much larger increase for R than for P steers (P < 0.01) . Total VFA concentrations were not affected (P = 0.21) by treatment, but R steers exhibited lower proportions of acetate and higher proportions of is obutyrate, valerate, and isovalerate than P steers (P < 0.01). In conclusio n, ruminal and postruminal infusion of a degradable protein source improved forage utilization, although the response in forage OM intake and total di gestible OM. intake was greater for ruminal infusion than for postruminal i nfusion.