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This Article Full Text (PDF) All Versions of this Article: jas.2008-1444v1 87/4/1346    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Huntington, G. B. Articles by Burns, J. Search for Related Content PubMed PubMed Citation Articles by Huntington, G. B. Articles by Burns, J.

Urea metabolism in beef steers fed tall fescue, orchardgrass, or gamagrass hays

Gerald B. Huntington^*, Kelly Magee^*, April Matthews^*, Matthew Poore^* and Joseph Burns^*,,

^* Department of Animal Science, North Carolina State University Raleigh NC 27695-7621 , Department of Crop Science, North Carolina State University Raleigh NC 27695-7621 USDA-ARS, North Carolina State University Raleigh NC 27695-7621

Gerald_Huntington{at}ncsu.edu

Abstract

Two experiments were conducted to assess effects of endophyte treatments (Exp. 1), forage species (Exp. 2), 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 50 h of intrajugular infusion of ^15,15N-urea, indicating that valid data on urea metabolism can be obtained from steers fed forages twice daily. After adjustment by covariance for differences in N intake among treatments in Exp. 1, steers 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 those fed endophyte-free or novel endophyte-infected tall fescue. However, urea-N urinary excretion or return to the gut was similar among endophyte treatments when expressed as a proportion of urea-N entry. Urea-N entry and return to the gut in Exp. 2 was similar in steers fed gamagrass or orchardgrass hay after adjustment by covariance for differences in N intake. Less (P < 0.01) urinary excretion, expressed as grams per day or 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 1.76 kg/d of readily fermentable fiber and starch decreased urea entry (P < 0.06) and urinary excretion of urea (P < 0.01). 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 in 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). Closer coordination of ruminal fermentation of carbohydrate and N sources provided greater and more efficient capture of dietary N as tissue protein in forage-fed steers.

Key Words: fescue • gamagrass • orchardgrass • steers • urea metabolism