Urea flux in beef steers: Effects of forage species and nitrogen fertilization

The effects of two forage species and N levels on urea kinetics and whole-b ody N metabolism were evaluated in eight Angus steers (initial BW 217 +/- 1 5 kg). In a replicated, 4 x 4 Latin square design, steers were fed gamagras s (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 0.5 kg ADG. Nitrogen balance and urea kinetics were measured from d 22 to 27 of each period. Urine samples collected duri ng intravenous infusion of bis N-15 urea were used to calculate production and recycling of urea N from relative abundance of urea isotopomers. Jugula r 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 ure a N (10.5 vs 8.0 g/d), and percentage of urinary N present as urea N (53.5 vs 40.0%). After adjustment for differences in N intake, fecal N still tend ed to be greater (P < 0.09) for gamagrass than for switchgrass. The LO diff ered from the HI (P < 0.01) in daily N intake (63 vs 76 g), DM digestibilit y (61.3 vs 63.3%), urine N (13.6 vs 25.9 g/d), and N retained as a;percenta ge of N digested (57.3 vs 43.5%). Compared to switchgrass, gamagrass had gr eater 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 switchgrass (interaction, P < 0.07). The HI group was greater than the LO i ntake group (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 th at re-entered the ornithine cycle (9.4 vs 7.9 g/d). However, the percentage of urea N entering the gastrointestinal tract that was recycled was consta nt among treatments (29.1%), indicating that almost 70% of the urea N that entered the gastrointestinal tract was potentially available for anabolic p urposes of the steers as a component of microbial products that were absorb ed or excreted in the feces. In summary, N levels affected N metabolism of steers more when they were fed gamagrass than when they were fed switchgras s. Although the absolute amounts of N moving through the system changed wit h variations in intake, the proportions remained similar, with a greater ef ficiency of N use at low N intakes.