Milk urea estimates of nitrogen excretion by dairy cows grazing forage species with contrasting chemical and morphological characteristics

White clover has high nutritive value, but is associated with high excretion of surplus dietary nitrogen (N). The objective of this study was to determine if other species of equally high nutritive value could reduce N wastage. Four species of legume, an herb and a brassica, representing contrasting chemical and morphological composition, were compared against ryegrass and white clover controls. Each species was sown in monoculture and milk urea-nitrogen (MUN) measured as a predictor of surplus N excreted when grazed by lactating cows. For each species, two replicate groups of cows grazed ryegrass pasture for three days and baseline MUN measured, followed by grazing the test species for seven days and MUN measured on days six and seven, in autumn and in spring. The low concentrations of MUN when grazing chicory (2.1 mmol/L) or sulla (2.6 mmol/L) were associated with low concentrations of crude protein, and the high concentrations of MUN when grazing lucerne (8.0 mmol/L), red clover (7.7 mmol/L) or white clover (5.7 mmol/L) were associated with high concentrations of crude protein and high rumen degradability. The differences in MUN among species suggest it may be possible to reduce N excretion by grazing alternative species to white clover, but this needs to be confirmed with direct measures of urine and faecal N.