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Effect of frequency and amount of rumen degradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage

T. A. Wickersham^*, E. C. Titgemeyer^*, R. C. Cochran^*, E. E. Wickersham^* and E. S. Moore

^* Department of Animal Sciences and Industry Department of Clinical Sciences, Kansas State University, Manhattan 66506-1600

etitgeme{at}ksu.edu

Abstract

We evaluated the effect of frequency and amount of rumen degradable intake protein (DIP) on urea kinetics in steers consuming prairie hay. Five ruminally and duodenally fistulated steers (366 kg BW) were used in a 5 x 5 Latin square and provided ad libitum access to low-quality prairie hay (4.7% CP). Casein was provided daily in amounts of 61 and 183 mg of N/kg BW (61/d and 183/d) and every third day in amounts of 61, 183, and 549 mg of N/kg BW per supplementation event (61/3d, 183/3d, and 549/3d). Periods were 18 d long with 9 d for adaptation and 9 d for collection. Steers were in metabolism crates for total collection of urine and feces. Jugular infusion of ¹⁵N¹⁵N-urea followed by determination of urinary enrichment of ¹⁵N¹⁵N-urea and ¹⁴N¹⁵N-urea was used to determine urea kinetics. Treatment means were separated to evaluate the effects of increasing DIP supplementation and the effects of frequency at the low (61/d vs. 183/3d) and at the high (183/d vs. 549/3d) amounts of DIP provision. Forage OM and total digestible OM intakes were linearly (P 0.05) increased by increasing DIP provision, but were not affected by frequency of supplementation at either the low or high amounts. Production and gut entry of urea linearly (P 0.006) increased with DIP provision and tended to be greater (P 0.07) for 549/3d than 183/d, but were not different between 61/d and 183/3d. Microbial N flow to the duodenum was linearly (P < 0.001) increased by increasing DIP provision. Additionally, 183/d resulted in greater (P = 0.05) microbial N flow than 549/3d. Incorporation of recycled urea-N into microbial N linearly (P = 0.04) increased with increasing DIP. Microbial incorporation of recycled urea-N was greater for 549/3d than 183/d, with 42 and 23% of microbial N coming from recycled urea-N, respectively. In contrast, there was no difference due to frequency in the incorporation of recycled urea-N by ruminal microbes at the low level of supplementation (i.e., 61/d vs. 183/3d). This study demonstrates that urea recycling plays a substantial role in the N supply to the rumen and to the animal, particularly in steers supplemented infrequently with high levels of protein.

Key Words: cattle • frequency • protein • recycling • urea

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