Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/15/2001
Publication Date: N/A
Citation: Interpretive Summary: Nitrogen level in the forage and the forage itself affected nitrogen digestion and metabolism by steers. There was a greater increase in nitrogen digested as nitrogen level increased for gamagrass than for switchgrass. Also, there was a greater increase in nitrogen digestibility as nitrogen level increased for gamagrass than for switchgrass. The difference between forages in nitrogen fraction was not sufficient to improve dry matter digestibility of gamagrass over switchgrass. The trend for urea, as a percentage of total urinary N, was to decrease with increasing concentrations of nitrogen in gamagrass while the percentage increased in steers fed switchgrass. This is consistent with the presence of anti-nutritional factors in switchgrass that inhibit nitrogen retention. One possible factor is phenolic acid monomers which are detoxified by conjugation with glycine in the liver. Phenolic acid monomers have been found in switchgrass at up to 2.5% of the cell wall and may explain, in part, the overall decrease in urine nitrogen present as urea in steers fed switchgrass. In general, increasing nitrogen fertilization of gamagrass and switchgrass will increase dry matter and nitrogen digestibilities by steers fed those hays. Neither efficiency of nitrogen use nor recycling of urea to the digestive tract, however, was improved with increased nitrogen 0ntake. Hence, once the nitrogen intake exceeds the maintenance requirements of the steers, a greater proportion is excreted as urea in the urine and not captured as a saleabe product, ie, beef.
Technical Abstract: The effects of two forage species and N levels on urea kinetics and whole body N metabolism were evaluated in 4 Angus steers. In a replicated, 4x4 Latin square design, steers were fed gamagrass (Tripsacum dactyloides L.) or switchgrass (Panicum virgatum L.), each of which had 56.2 (LO) or 168.5 (HI) kg of N fertilization per hectare. Diets provided adequate energy for r0.5 kg ADG. Nitrogen balance and urea kinetics were measured from d 22-27 of each period. Urine samples collected during intravenous infusion of bis 15N urea were used to calculate production and recycling of urea N from relative abundance of urea isotopomers. Jugular blood serum was analyzed for serum urea N (SUN). Gamagrass differed from switchgrass (P<0.05) in daily DMI (4,273 vs. 4,185 g), N intake (72 vs.67 g), DM digestibility (61.0 vs. 63.6%), fecal N (30.6 vs. 28.3 g/d), urine urea N (10.5 vs. 8.0 g/d), and percent of urinary N present as urea N (53.5 vs. 40.0%). After adjustment for differences in N intake, fecal N still tended to be greater (P<0.09) for gamagrass than switchgrass. LO differed from HI (P<0.01) in daily N intake (63 vs. 76 g), DM digestibility (61.3 vs. 63.3%), urine N urine N (13.6 vs. 25.9 g/d), and N retained as a percent of N digested (57.3 vs. 43.5%). Compared to switchgrass, gamagrass had greater SUN. N digestibility, and N digested as N level increased (forage x N level interactions P<0.05). As N level increased, N retention increased from 19.5 to 23.5 g/d in gamagrass and decreased from 20.5 to 18.1 g/d in switch grass (interaction, P<0.07). HI was greater than LO (P<0.03) in endogenous production of urea N (44.4 vs. 34.0 g/d), gut entry rate of urea N (31.6 vs 28.2 g/d) and the amount of urea N that reentered the ornithine cycle (9.4 vs. 7.9 g/d).