Kinetics of endogenous urea in lactating goats and cows fed diets varying in their crude protein concentrations

Two balance experiments were carried out using lactating ruminants. In Experiment (Exp.) 1, seven goats per treatment were fed a constant roughage ration plus one of two concentrates, differing in their crude protein concentration. In Exp. 2, which followed a switch-over design, four dairy cows were fed a complete low-protein ration either with or without urea supplementation. According to official recommendations, N intake was either adequate or reduced by about 27 and 18% for goats and cows, respectively. Following a single intravenous dose of N-15-labelled urea, milk, faeces and urine were collected for periods of two weeks in Exp. 1 and of 10 days in Exp. 2. Lowering N intake caused reductions of milk urea concentration in both species but did not significantly affect intake, milk yield or milk protein concentration. In both experiments, lowering of N intake caused substantial reductions in excretion of urea in urine and milk but did not affect excretion of faecal N or urinary non-urea-N. N balances did not deviate significantly from +/- zero in either species or treatment. From comparative total N and N-15 balances, it was calculated that net degradation of urea as well as transfer rates of N from body urea to faecal N, urinary non-urea-N, milk protein N or body protein N were not significantly affected by the level of N intake in either species. The efficiency of utilisation of N originating from net degradation of urea was 66% in the goats and 49% in the cows, respectively, and was not affected by dietary crude protein concentrations.