Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/21/2007
Publication Date: 5/17/2007
Citation: Huntington, G.B., Burns, J.C., Archibeque, S.L. 2007. Urea metabolism in beef steers grazing bermudagrass, caucasian bluestem, or gamagrass pastures varying in plant morphology, protein content, and protein composition.. Journal of Animal Science. 85:1997-2004. Interpretive Summary: Warm-season grasses vary in the morphology of their canopies giving the grazing animal opportunities to preferentially select and utilize leaf vs. stem tissue differently. The grasses evaluated in this study were gamagrass with a high proportion of leaf, bermudagrass with a low proportion of leaf, and Caucasian bluestem being intermediate in its’proportion of leaf. The differing proportions of the pasture canopy that was leaf had no effect on the animal measurements made, but composition of the leaf did. The nitrogen concentrations among pasture species differed and this altered urea production and nitrogen recycling in the rumen of grazing steers. Energy level was inadequate in the forages to effect better nitrogen utilization as concentrations increased among forages. This was reflected in the rumen of grazing steers as increased urea entry rate and gut urea recycling.
Technical Abstract: Our objective was to evaluate pastures of bermudagrass (Cynodon dactylon, BG), caucasian bluestem (Bothriochloa caucasica, CBS), and gamagrass (Tripsacum dactyloides, GG) 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 randomly assigned to a replicated, randomized complete block design with two pastures of each forage and two steers per pasture. Three other steers with esophageal fistulas were used to collect masticate samples that represent intake preferences. Herbage mass was ' 1,900 kg/ha. After at least 14-d adaptation, urine and blood samples were collected for determination of serum urea N (SUN, mM) and percentage of urinary N in the form of urea (PCTUREA). 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 (UER, mmol N/h), urinary urea excretion (UUE, mmol N/h), gut urea recycling (GUR, mmol N/h), and return of urea-N to the ornithine cycle (ROC). Canopy leaf:stem DM ratio differed (P = 0.05) among BG (0.50), CBS (1.01) and GG (4.00). Caucasian bluestem had less CP (% DM) than GG or BG in canopy (9.6 vs. 12.0 or 12.3, P = 0.10), and masticate (9.8 vs. 14.7 or 13.9, P = 0.05). Bermudagrass had less true protein (% CP) than CBS or GG in canopy (72.9 vs. 83.3 or 83.0, P = 0.10) and masticate (73.7 vs. 85.8 or 88.0, P = 0.05). Compared to GG and BG, CBS had less SUN (10.14 or 12.23 vs. 2.49, P = 0.05), UER (209 vs. 353 or 391, P = 0.05), UUE (18 vs. 105 or 95, P = 0.05), and greater ROC/GUR (0.231 vs. 0.109 or 0.118, P = 0.01). Urea production and recycling in these steers responded more to the N concentration in the grasses than to differences in plant morphology. 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.