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1 Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
This experiment determined the effects of dietary protein solubility on amount, form, and route of nitrogen loss in lactating Holstein dairy cows, and the ability of the Cornell Net Carbohydrate and Protein System (CNCPS) to accurately predict rumen microbial yield, serum urea N (SUN), milk urea N (MUN), and fecal N. Eighteen multiparous Holstein cows were assigned randomly to one of three dietary treatments that were similar in crude protein (17.7%) content but differed in their content of soluble intake protein (SIP). Dietary contents of SIP, as % of total CP were 30, 36, and 48%. The experimental period was 21 d, and total N balance collections were done during the last 5 d. As dietary content of SIP increased, excretion of urinary N increased quadratically, and it was the primary route of N excretion. Urinary excretion of purine derivatives (PD) responded quadratically as dietary SIP content increased. The CNCPS predicted a quadratic decrease in total metabolizable protein (MP) supply. No effect of dietary content of SIP was detected on MUN and SUN. The CNCPS predicted a quadratic decrease in SUN and MUN as dietary SIP increased. Results from this study indicated that changing the dietary content of SIP altered routes of N excretion in dairy cows, but had no effect on total N balance. The CNCPS did not adequately predict changes in SUN and MUN for cows fed diets varying in SIP.
Key Words: dairy cow • dietary protein solubility • nitrogen balance • purine derivative
Submitted on June 4, 2001
Accepted on November 26, 2001
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