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Effect of rumen-degradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage¹

T. A. Wickersham^*,2, E. C. Titgemeyer^*,3, R. C. Cochran^*, E. E. Wickersham^* and D. P. Gnad

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

³ Corresponding author: etitgeme{at}ksu.edu

We evaluated the effect of increasing amounts of rumen-degradable intake protein (DIP) on urea kinetics in steers consuming prairie hay. Ruminally and duodenally fistulated steers (278 kg of BW) were used in a 4 x 4 Latin square and provided ad libitum access to low-quality prairie hay (4.9% CP). The DIP was provided as casein dosed ruminally once daily in amounts of 0, 59, 118, and 177 mg of N/kg of BW daily. Periods were 13 d long, with 7 d for adaptation and 6 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. Forage and N intake increased (linear, P < 0.001) with increasing DIP. Retention of N was negative (–2.7 g/d) for steers receiving no DIP and increased linearly (P < 0.001; 11.7, 23.0, and 35.2 g/d for 59, 118, and 177 mg of N/kg of BW daily) with DIP. Urea synthesis was 19.9, 24.8, 42.9, and 50.9 g of urea-N/d for 0, 59, 118, and 177 mg of N/kg of BW daily (linear, P = 0.004). Entry of urea into the gut was 98.9, 98.8, 98.6, and 95.9% of production for 0, 59, 118, and 177 mg of N/kg of BW daily, respectively (quadratic, P = 0.003). The amount of urea-N entering the gastrointestinal tract was greatest for 177 mg of N/kg of BW daily (48.6 g of urea-N/d) and decreased (linear, P = 0.005) to 42.4, 24.5, and 19.8 g of urea-N/d for 118, 59, and 0 mg of N/kg of BW daily. Microbial incorporation of recycled urea-N increased linearly (P = 0.02) from 12.3 g of N/d for 0 mg of N/kg of BW daily to 28.9 g of N/d for 177 mg of N/kg of BW daily. Provision of DIP produced the desired and previously observed increase in forage intake while also increasing N retention. The large percentage of urea synthesis that was recycled to the gut (95.9% even when steers received the greatest amount of DIP) points to the remarkable ability of cattle to conserve N when fed a low-protein diet.

Key Words: cattle • protein • recycling • urea

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				G. B. Huntington, K. Magee, A. Matthews, M. Poore, and J. Burns Urea metabolism in beef steers fed tall fescue, orchardgrass, or gamagrass hays J Anim Sci, April 1, 2009; 87(4): 1346 - 1353. [Abstract] [Full Text] [PDF]

				T. A. Wickersham, E. C. Titgemeyer, R. C. Cochran, E. E. Wickersham, and E. S. Moore 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 J Anim Sci, November 1, 2008; 86(11): 3089 - 3099. [Abstract] [Full Text] [PDF]