Urea metabolism in beef steers grazing bermudagrass, caucasian bluestem, or gamagrass pastures varying in plant morphology, protein content and protein composition

¹ Animal Science Department, North Carolina State University, Raleigh, NC 27695
² Animal Science Department, North Carolina State University, Raleigh, NC 27695; USDA-ARS, North Carolina State University, Raleigh, NC 27695; Crop Science Department, North Carolina State University, Raleigh, NC 27695

^* To whom correspondence should be addressed. E-mail: gerald_huntington{at}ncsu.edu.

	Abstract

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 about 60 and 30 d before sample collection. In 2000 and 2001, 12 steers (250 kg BW) were blocked 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 (SUN) and percentage of urinary N in the form of urea. One steer per pasture (6 steers per year) was infused i.v. with ^15,15N-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. 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 canopy (9.6 vs. 12.0 or 12.3, P = 0.07) and in masticate (9.8 vs. 14.7 or 13.9, P = 0.04). Bermudagrass had less true protein (% of CP) than CBS or GG in canopy (72.9 vs. 83.3 or 83.0, P = 0.07) and in masticate (73.7 vs. 85.8 or 88.0, P = 0.04). Compared to GG and BG, CBS had less SUN (10.1 or 12.2 vs. 2.5 mM, P = 0.01), urea entry rate (353 or 391 vs. 209 mmol N/h, P = 0.07), urinary urea excretion (105 or 95 vs. 18 mmol N/h, P = 0.04), and 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 leaf:stem ratio among the grasses at the herbage mass evaluated.

Key Words: Beef cattle, Urea biosynthesis, Tripsacum dactyloides, Cynodon dactylon, Bothriochloa caucasia