Submitted to: Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/8/2008
Publication Date: 7/3/2009
Citation: Huntington, G. B., Magee, K., Matthews, A., Poore, M., & Burns, J. (2009). Urea metabolism in beef steers fed tall fescue, orchardgrass, or gamagrass hays. Journal of Animal Science. 87:1346-1353. Interpretive Summary: The breakdown of forages in the rumen creates a large supply of ammonia. Much of this is absorbed into the bloodstream which is later loss through urinary excretion. In this study interest was in the synchronization of energy and protein in the rumen to improve the capture of dietary N as microbial N and reducing ammonia absorption by the blood. A method to determine N use was first developed by using N15 urea to quantify urea production, urinary excretion, and urea returned to the gut. Different forages were evaluated. In the first study ‘Jesup’ tall fescue with endophyte, without endophyte and with a novel endophyte (MaxQ) was evaluated. In the second study ‘Iuka’ gamagrass and ‘Potomac’ orchardgrass were evaluated. The tall fescue with endophyte had reduced intake of dry matter and N and reduced digestion with less N retained. After adjustment for N intake, Jesup with endophyte had a lower urea entry rate, urea N return to the gut, and urea N entering the ornithine cycle vs. Jesup without endophyte or MaxQ. When expressed as a proportion of the urea entry rate and the return of urea-N to the ornithine cycle as a percentage of urea returned to the gut all were similar indicating metabolism of N through the urea pathway was not an issue in the efficiency of N capture among the tall fescue types. In the second study, when supplement (soybean hull/corn supplement mix/molasses) was fed to steers receiving gamagrass or orchardgrass, N intake was increased. After adjustment for the difference in N intake, supplement was shown to decrease urea entry rate and plasma urea-N and to increase N retained. The two hays responded differently as steers fed orchardgrass responded greater to supplement than those fed gamagrass. Nitrogen use was improved by increasing fermented carbohydrates with more efficient capture of the dietary N as tissue protein in forage-fed steers.
Technical Abstract: Two experiments were conducted to assess effects of endophyte treatments (Exp. 1), forage species, and supplementation (Exp. 2) on urea production, excretion, and recycling in beef steers. Infusion of 15,15N-urea and enrichment of urea in urine samples were used to calculate urea N entry and recycling to the gut. Acceptably stable enrichment of 15N-urea in urine was obtained after 38 h of intrajugular infusion of 15,15N-urea, indicating that useable data on urea metabolism can be obtained from steers fed forages twice daily. After adjustment by covariance for differences in N intake among varieties in Exp. 1, steers when fed endophyte-infected tall fescue had less (P < 0.10) urea N entry, recycling to the gut, and return of recycled urea N to the ornithine cycle than when fed endophyte-free or novel endophyte-infected tall fescue. However, urea N urinary excretion or recycling through the gut was similar among varieties when expressed as a proportion of urea N entry. Urea N entry and recycling through the gut in Exp. 2 was similar in steers fed gamagrass or orchardgrass hay after adjustment by covariance for differences in N intake. Greater (P < 0.07) recycling and less (P < 0.07) urinary excretion of urea N as a proportion of urea N entry with gamagrass than with orchardgrass was associated with faster in vitro NDF-N digestion with gamagrass. Supplementation of gamagrass or orchardgrass with readily fermentable fiber and starch decreased (P < 0.05) urea entry, urinary excretion, and recycling through the gut. Interactions between hay source and supplement reflected a greater response to supplementation for steers fed orchardgrass than for those fed gamagrass. After adjustment for differences among treatments’ N supply, results of both experiments support the concept of improved N use in response to increased carbohydrate fermentability in the rumen, due either to inherent differences in forage fiber, or to supplementation with readily fermentable carbohydrate (starch or fiber). Results of both experiments support the concept of improved N use in response to increased carbohydrate fermentability in the rumen, due either to inherent differences in forage fiber, or to supplementation with readily fermentable carbohydrate (starch or fiber). Closer coordination of ruminal fermentation of carbohydrate and nitrogen sources provided greater and more efficient capture of dietary N as tissue protein in forage-fed steers.