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Relationship of Dietary Nonprotein Nitrogen to Urea Kinetics in Dairy Cows^1,2,3,

Department of Dairy Science, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691

Switchover experiments were designed to investigate the effect of different levels of urea feeding on urea kinetics and to define more clearly the critical physiological limit of total nonprotein nitrogen (NPN) intake by dairy cows. Changes in rumen fermentation patterns, plasma glutamine levels, urinary creatinine, and nitrogen requirement were assessed. Labeled urea, ¹⁴C-urea, was administered intravenously. Plasma urea concentrations, urea pool sizes and urea entry rates were linearly and positively related to the level of dietary NPN intake. At an entry rate of 20 g/hour and urinary excretion rate of 9 g/hour corresponding to an NPN intake of approximately 175 g/day, excretion rate leveled off, suggesting that the renal ability to concentrate urea had reached a physiological limit. At low nitrogen intake as much as 90% and at the greatest nitrogen intake as little as 59% of urea entering the pool was recycled to the digestive tract. Urea nitrogen did not affect dry matter intake when furnishing as much as 70% of the total dietary nitrogen. Dry matter digestibility was increased by adding urea to the diets. Nitrogen digestibility increased linearly with increasing dietary NPN intake to approximately 73% and then leveled off at an NPN intake of 160 to 170 g/day. Metabolic fecal nitrogen, endogenous urinary nitrogen and apparent nitrogen requirement for maintenance were calculated to average 6.65 g/kilogram dry matter, 15.80 and 80 g/day, respectively.

² Approved for publication as Journal Article No. 47-70 by the Associate Director of the Ohio Agricultural Research and Development Center, Wooster, Ohio 44691.

³ A preliminary report was presented at the American Institute of Nutrition 53rd Annual Meeting Federation of American Societies for Experimental Biology, Atlantic City, N.J. 1969 Federation Proc. 28: 492 (abstr.).

⁴ Present address: Department of Animal Science, Makerere University, P.O. Box 7062, Kampala, Uganda, East Africa.

Manuscript received 24 November 1970.

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