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Further Observations on Utilization of Urea by Lactating Cows

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

Two experiments measured utilization of urea by lactating cows by intake, milk production, nitrogen balance and related characteristics. In each experiment, 36 cows were in a factorial arrangement of treatments. In Experiment 1, concentrates containing 12, 17, and 22% crude protein equivalent were fed both with and without 1.5% urea (4.2% crude protein). Concentrate was fed at one unit per three units of milk produced and corn silage ad libitum during 2-week pretreatment. Milk production averaged 19.7 kg/day. Fat-corrected milk production as per cent of pretreatment milk production was lower with urea (85 versus 91, P<.05) and higher with increased concentrate protein (82, 90, 92; P<.01). Digestibilities of most nutritive components were lower (P< .01) at the lowest protein but were unaffected by urea feeding. Urinary nitrogen increased and tissue nitrogen retention decreased (P<.05) with urea feeding. Higher protein decreased ruminal acetate (P<.05) and increased propionate (P<.01). Ration urea elevated ruminal propionate and depressed butyrate but had no effect on ruminal ammonia or plasma urea.

In Experiment 2, rations were corn silage plus a 17% crude protein concentrate, without urea (A) and with 1.5% urea (B); corn silage containing .5% urea plus a 12% crude protein concentrate, without (M) and with 1.5% urea (N); and corn silage plus 12% crude protein concentrate, without (X) and with 1.5% urea (Y). Average fat-corrected milk (kg/day) for AB, MN, and XY was 24.3, 23.7 and 20.3. Silage with urea resulted in greater milk production (P<.01). However, the large quantity of nitrogen provided by the urea silage was utilized inefficiently.

¹ Present address: Department of Animal Industries, North Carolina State University, Raleigh.

² Present address: Department of Dairy Science, Michigan State University, East Lansing.