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Article << Previous     |     Next >>   Contents Vol 52(5)

Nitrogen metabolism and recycling in yaks (Bos grunniens) offered a forage–concentrate diet differing in N concentration

X. S. Guo A B F , Y. Zhang C F , J. W. Zhou C , R. J. Long A C E , G. S. Xin D , B. Qi A , L. M. Ding A B and H. C. Wang C

A International Centre for Tibetan Plateau Ecosystem Management, Lanzhou University, No. 222 Tianshui South Road, Lanzhou, 730000, PR China.
B State Key Laboratory of Pastoral Agricultural Ecosystem, Institute of Arid Agroecology, School of Life Sciences, Lanzhou University, 222 Tianshui South Road, Lanzhou 730000, PR China.
C State Key Laboratory of Pastoral and Agricultural Ecosystems, College of Pastoral Agriculture Science and Technology, Lanzhou University, 768 Jiayuguan West Road, Lanzhou, 730000, PR China.
D College of Life Science, Ningxia University, 489 Henanshan West Road, Xixia District, Yinchuan, 750021, PR China.
E Corresponding author. Email: longrj@lzu.edu.cn
F The first two authors contributed equally to this work.

Animal Production Science 52(5) 287-296 http://dx.doi.org/10.1071/AN11208
Submitted: 28 September 2011  Accepted: 6 January 2012   Published: 20 March 2012

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An experiment was conducted to characterise N use efficiency and quantify urea fluxes in yaks offered four levels of dietary N (1.43%, 1.97%, 2.45% and 2.90% of diet DM) in a 4 × 4 Latin square design. The incremental increase in N intake linearly increased N retention (P = 0.003) and the excretion of urinary N (P < 0.001), but no difference (P > 0.05) in faecal N excretion was observed in growing yaks fed any of the four diets. Microbial N production had quadratic (P < 0.001) responses to dietary N, characterised by the highest microbial N production occurring in the 1.97% N diet (P < 0.05). As the N content of the diet increased, the urinary excretion of urea increased from 13% to 27% of urea entry rate (quadratic, P < 0.001), whereas gastrointestinal entry urea returned to ornithine cycling decreased from 46% to 40% (linear, P < 0.001), and the gastrointestinal entry urea used for anabolism increased from 50% to 56% (linear, P < 0.001). Gastrointestinal entry urea incorporated into bacterial N decreased linearly (P < 0.001) with incremental increase in N intake, and the greatest concentration (23.5%) of bacterial N originating from plasma urea N was in yaks fed the 1.43% N diet. As much as 87% of the urea synthesised in the liver was returned to the gastrointestinal tract when the yaks were fed a diet with 1.43% N (1.1 times the maintenance N level). Moreover, constantly greater urea production than the intake of digestible N, and the gastrointestinal-urea clearance than the kidney-urea clearance were observed, respectively, in the growing yaks, regardless of the level of N intake. These results suggest that yaks might be more efficient at utilising N under harsh environment than are cattle.

Additional keywords: dietary N, protein, urea recycling, yak.


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