title: Urea metabolism in beef steers grazing Bermudagrass, Caucasian bluestem, or gamagrass pastures varying in plant morphology, protein content, and protein composition.
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 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.
authors: Animal Science Department, North Carolina State University, Raleigh 27695, USA. Gerald_Huntington@ncsu.edu
authors: G B
authors: GB
authors: Huntington
authors: J C
authors: JC
authors: Burns
authors: S L
authors: SL
authors: Archibeque
mesh_terms: D000818:Animals; D001806:Blood Urea Nitrogen; D001835:Body Weight; D002417:Cattle / Q000378:metabolism*; D031744:Cynodon / Q000737:chemistry / Q000378:metabolism; D004044:Dietary Proteins / Q000032:analysis; D004937:Esophageal Fistula / Q000662:veterinary; D008297:Male; D018515:Plant Leaves / Q000737:chemistry; D018547:Plant Stems / Q000737:chemistry; D006109:Poaceae / Q000737:chemistry / Q000378:metabolism*; D011897:Random Allocation; D014508:Urea / Q000378:metabolism* / Q000652:urine
chemical_list: D004044:Dietary Proteins; D014508:Urea