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Abstract
Two experiments were conducted to determine the effects of supplemental protein degradability and supplementation frequency on intake, digestion, ruminal characteristics, N retention, and nutrient flux across visceral tissues in lambs fed low-quality forage. Lambs were fed a basal diet of crested wheatgrass hay (4.2% CP) for ad libitum consumption and assigned randomly to one of four protein supplements (Exp. 1): isolated soy protein, a source of ruminally degradable protein (RDP; CON), or corn gluten meal (RUP) fed at 50, 100, or 150% of the supplemental N provided by CON (C50, C100, and C150, respectively. In Exp. 2: (1) a high RDP supplement provided daily (RDP-D), (2) a high RDP supplement provided on alternate days (RDP-A), (3) a high RUP supplement provided on alternate days (RUP-A), or (4) a 50:50 mixture of the RDP and RUP supplements provided on alternate days (MIX-A). Forage intake was not affected (P ≥ 0.10) by treatment in either of the experiments. In Exp. 1, total tract nutrient digestibility was greater (P ≤ 0.05) for C100 vs CON. Ruminal ammonia concentrations were greater (P = 0.002) for CON vs C100 lambs and concentrations decreased (P = 0.02) as level of RUP increased. Protein degradability did not affect (P = 0.69) N retention; however, N retention increased (P = 0.001) as level of RUP increased. Net uptake of urea N by the portal-drained viscera (PDV) was greater (P = 0.02) for C100 than CON and increased linearly (P = 0.04) with increasing RUP. In Exp. 2, ruminal digestibility (P ≤ 0.04) was greater and total tract digestibility tended (P ≤ 0.07) to be greater for alternate-day verses daily supplemented lambs. Ruminal NH3 concentrations were greater (P = 0.001) for daily versus alternate-day lambs and concentrations decreased (P = 0.001) as RUP replaced RDP. Although alternate-day supplemented lambs had less (P = 0.004) N retention (% N digested), PDV uptake of urea N was not affected (P ≥ 0.50) by protein degradability or supplementation frequency. Protein degradability and supplementation frequency appear to influence N utilization in ruminants consuming low-quality forage.
