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 15N-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 di...