Ruminal Protein Degradation in Beef Cattle

Five ruminally cannulated steers (initial BW = 375 ± 34 kg; ADG = 1.16 ± 0.13 kg) were used in a 5 x 5 Latin square design experiment to determine the effects of increasing urea level on intake, nutrient digestion, nitrogen balance and ruminal kinetics. Isocaloric (NEm = 2.11 Mcal/kg; NEg = 1.35 Mcal/kg) and isonitrogenous (2.24% N) steam-flaked corn-based diets with urea concentrations of 0.0, 0.8, 1.2, 1.6, or 2.0% (DM basis) were offered ad libitum to steers. Following nine days of diet adaptation total urine and feces were collected for four days. On day 14, ruminal fluid was collected at 0, 3, 6, 9, 12, 15, 18, 21, and 24 h after pulse dosing with Co-EDTA. Dietary urea concentration did not affect (P > 0.50) DM or OM intake. Intake of ADF increased from 0.0 to 0.8% urea, decreased to 1.6% urea, and then increased when 2.0% urea was fed (cubic urea effect, P < 0.01). Increasing dietary urea increased (linear urea effect, P < 0.05) fecal output of OM and ADF. Although total tract digestibility of OM was not affected (P = 0.21) by urea level, total tract ADF digestibility was decreased (linear urea effect, P < 0.01). Dietary urea level did not affect (P > 0.40) fecal N output, total N balance, or N balance as a percent of N intake. Urinary N output responded quadratically (P < 0.05) to increased urea level in the diet. Liquid dilution rate (P = 0.54), ruminal fluid pH (P = 0.13) and total VFA (P = 0.16) were not affected by dietary urea level.