Urea metabolism in beef steers grazing Bermudagrass, Caucasian bluestem, or gamagrass pastures varying in plant morphology, protein content, and protein composition.

Pastures of Bermudagrass (Cynodon dactylon, BG), Caucasian bluestem (Bothriochloa caucasica, CBS), and gamagrass (Tripsacum dactyloides, GG) were evaluated from the perspectives of forage composition, selection during grazing, and N metabolism in beef steers. All pastures were fertilized with 78 kg/ha of N approximately 60 and 30 d before sample collection. In 2000 and 2001, 12 steers (250 kg of BW) were blocked based on BW and then assigned randomly to a replicated, randomized complete block design, with 2 pastures of each forage and 2 steers per pasture. Three other steers with esophageal fistulas were used to collect masticate samples to represent intake preferences. Herbage mass was >1,900 kg/ha. After at least 14 d of adaptation, urine and blood samples were collected for determination of serum urea N and percentage of urinary N in the form of urea. One steer per pasture (6 steers per year) was infused i.v. with (15,15)N urea for 50 h before collecting urine for 6 h to measure urea N enrichment, urea entry rate, urinary urea excretion, gut urea recycling, and return of urea N to the ornithine cycle. The canopy leaf:stem DM ratio differed (P = 0.01) among BG (0.50), CBS (1.01), and GG (4.00). Caucasian bluestem had less CP (% of DM) than GG or BG in the canopy (9.6 vs. 12.0 or 12.3, P = 0.07) and in the masticate (9.8 vs. 14.7 or 13.9, P = 0.04). Bermudagrass had less true protein (% of CP) than CBS or GG in the canopy (72.9 vs. 83.3 or 83.0, P = 0.07) and in the masticate (73.7 vs. 85.8 or 88.0, P = 0.04). Compared with GG and BG, CBS had less serum urea N (10.1 or 12.2 vs. 2.5 mM, P = 0.01), urea entry rate (353 or 391 vs. 209 mmol of N/h, P = 0.07), and urinary urea excretion (105 or 95 vs. 18 mmol of N/h, P = 0.04), and a greater return of urea N to the ornithine cycle as a proportion of gut urea recycling (0.109 or 0.118 vs. 0.231, P = 0.02). Urea production and recycling in these steers responded more to the N concentration in the grasses than to differences in plant protein fractions. There was no evidence of improved N capture by the steers due to changes in the leaf:stem ratio among the grasses at the herbage mass evaluated.