Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2002
Publication Date: 1/18/2003
Citation: Archibeque, S.L., Burns, J.C., Huntington, G.B. 2003. Nitrogen metabolism of beef steers fed endophyte-free tall fescue hay: effects of ruminally protected methionine supplementation.. Journal of Animal Science. Interpretive Summary: Nitrogen use efficiency in ruminants has implications both in terms of efficiency of production and impact of the environment. Nitrogen use may be limited in ruminants due to a lack of amino acids. The feeding of methionine to growing steers was effective in improving N retention and in reducing N loss in urine. This approach provides a positive means of improving the N retention of the animal and removing more N in the animal from the site of forage production. Further, less N is excreted in the more volitale form in urine. This has implications for producers who must operate with established limits of the amount of N that may remain on site.
Technical Abstract: Level of nitrogen (N) intake and ruminally protected methionine supplementation were evaluated in 8 Angus growing steers (initial weight 253 21 kg, final weight 296 21 kg) in a replicated, 4X4 Latin square design. The steers were fed two endophyte-free tall fescue (Festuca arundinacea) hays that contained 2.2 (LO) or 2.8 % (HI) of DM as N and were either supplemented or not with ruminally protected methionine (10 g metabolizable methionine/d). Diets were fed to provide adequate energy for 0.5 kg ADG and sufficient protein for maintenance (LO), or 0.5 kg ADG (HI). Following at least 14 d of adjustment, N balance was measured for 6 d. Isotopic urea was infused (15N, 15N-urea, 0.164 mmol urea N/h) via a jugular catheter for 56 h and urine was collected from 48 to 56 h to measure urea kinetics. Jugular blood was collected during the balance trial and serum was analyzed for serum urea N (SUN). By design, daily N intake was greater (P<0.05) for HI (112 g) than LO (89 g). HI was also greater (P<0.05) than LO in daily DMI (4,217 vs. 4,151 g), fecal N (34.4 vs. 31.1 g), N digested (77.1 vs. 57.7 g), urine N (48.3 vs. 37.5 g), urine urea N excretion (34.6 vs. 24.8 g) and N retained (29.8 vs. 21.1 g). HI also differed (P<0.05) from LO in urine urea N concentration (276 vs. 219 mM), SUN (8.7 vs. 6.7 mM), N digestibility (69.1 vs. 65.9 %), percentage of urinary N present as urea (71.5 vs. 66.7 %, P<0.053), rate of urea N production (59.61 vs. 49.16 g/d), and the percentage of urea N produced that was returned to the ornithine cycle (15.03 vs. 19.21 %). Methionine supplementation decreased daily urine N (44.6 vs. 41.2 g, P=0.10), and increased both the amount of N retained daily (23.4 vs. 27.5 g, P<0.089) and the percentage of N digested that was retained (34.6 vs. 40.4 %, P<0.094). In summary supplemental methionine met a specific dietary limitation by increasing the amount of N being retained within the steers by decreasing the amount of N being lost in the urine.