Effect of restricted forage intake on ruminal disappearance of bromegrass hay and a blood meal, feather meal, and fish meal supplement.

Two experiments were conducted to determine in situ disappearance of bromegrass hay and a ruminally undegraded protein (RUP) supplement in beef cattle fed restricted amounts of forage. Six Angus crossbred cattle (BW = 589 +/- 44.4 kg; three steers and three heifers) fitted with ruminal cannulas were fed chopped (2.54 cm) bromegrass hay (8.9% CP) at one of three percentages of maintenance intake (30, 55, or 80%; one steer and one heifer per treatment). In both experiments, the cattle were allowed 7 d for diet adaptation followed by 3 d of sample collection. In Exp 1, in situ bags (50 microm pore size) containing 4.1 g of brome-grass hay (OM basis) were inserted into the rumen and subsequently removed at 3, 6, 9, 12, 15, 18, 24, 36, and 48 h after insertion. Nonlinear regression models were used to determine the rapidly solubilized protein Fraction A, the potentially ruminal degradable protein Fraction B, the ruminally undegraded protein Fraction C, and protein degradation rate. Intake level did not affect (P = 0.15 to 0.95) forage protein remaining after in situ incubation or Fractions A, B, and C; however, effective ruminal degradation of hay protein tended to increase quadratically (P = 0.12) as forage intake increased. In Exp 2, 4.2 g (OM basis) of an RUP supplement (6.8% porcine blood meal, 24.5% hydrolyzed feather meal, and 68.7% menhaden fish meal) formulated to provide equal amounts of metabolizable protein across all levels of hay consumption was evaluated in a similar manner as in Exp 1. The undegraded protein fraction of the supplement did not differ (P = 0.16 to 0.74) across treatments at 3, 6, 9, and 18 h; however, increasing forage intake resulted in a linear increase (P < or = 0.06) in undegraded protein remaining at 12, 15, 24, 36, and 48 h. Dietary treatment had no affect (P = 0.30) on protein Fractions A, B, or C; however, protein degradation rate of the supplement decreased linearly (P = 0.03) as forage intake increased. Therefore, effective ruminal degradation of the supplement decreased linearly (P = 0.01) from 50.8 to 40.9% as forage intake increased from 30 to 80% of maintenance. Corresponding estimates of supplement RUP were 49.2, 56.5, and 59.1% for the 30, 55, and 80% of maintenance intake treatments, respectively. Restricting dietary intake can decrease the quantity of dietary protein that escapes ruminal degradation. Tabular estimates of RUP may not be appropriate for formulating diets to balance metabolizable protein in beef cattle consuming limited quantities of forage.