Urinary nitrogen excretion from cows at different stage of lactation grazing different ryegrass cultivars during spring or autumn.

The effects of season (spring or autumn) and stage of lactation (early or late) on the partitioning of dietary nitrogen (N) between milk and urine were measured in experiments in which cows grazed forages with putative differences in the ratio of water soluble carbohydrates (WSC) to crude protein (CP). Sixty cows (n = 30 for each stage of lactation) grazed one of three forages during spring and autumn. Forage treatments consisted of a ‘standard’ perennial ryegrass (STG) and two ryegrasses with high WSC concentrations: a perennial ‘high sugar’ ryegrass (HSG), and an annual ryegrass (IRG). In spring, grass type had a significant effect on the estimated urinary N excretion. Both the HSG and STG resulted in greater N excretion compared to the IRG, due to the low CP concentration in this forage in one of the grazing periods. In autumn, cows grazing STG excreted less urinary N than both HSG and IRG. In both seasons, cows in early lactation secreted more N into milk but were not different in N excretion. In early lactation cows, the extra N in milk was only a small proportion of the potential N intake arising from higher CP content in autumn grasses.