Diurnal variation in urine nitrogen and creatinine concentrations from lactating cows grazing ryegrass-dominant pasture in autumn and late spring–summer

In dairy production systems based on grazed pasture urine patches are the main source of nitrogen (N) losses via leaching or volatilisation. We tested the hypothesis that urine N concentration would vary throughout the day, associated with patterns in feed intake and digestion. Such variation would inform sampling strategy for accurate estimates of daily mean N concentration, and identify scope for feeding management as an option for reducing N leaching. Nine groups of multiparous Friesian and Friesian × Jersey cows in autumn (n=4-6, 220 ± 26 days-in-milk, milked twice daily) and nine groups in late spring-summer (n=6, 228 ± 24 days-in-milk, milked once daily) were sequentially withdrawn from the farm herd at approximately weekly intervals and each group grazed ryegrass pasture for 3 days. Cows were offered a fresh allocation of pasture after each milking (and at the equivalent time when milked once daily), sufficient for them to consume up to 16 kg DM/cow/d. On day 3, urine samples were collected at 11:00 am, 3:00 pm (afternoon milking), 6:00 pm and 7:00 am (the following morning milking), and subsequently analysed for the concentration of N. In autumn, the concentrations were higher (P<0.05) at 6:00 pm (5.8 g N/L) than they were at 3:00 pm or 7:00 am (mean of 4.2 g N/L). In late spring-summer, the concentrations were higher (P<0.05) at 11:00 am (8.0 g N/L) than they were at 3:00 pm, 6:00 pm and 7:00 am (mean of 6.3 g N/L). The high concentrations of N occurred in urine samples collected approximately 3 hours post allocation of fresh feed when cows had grazed actively and consumed the majority of the herbage available. For accurate estimates of daily mean urine N concentration sample collections for should be timed to encompass this variation. Furthermore, feeding management, such as removing cows to stand-off after 2-3 hours grazing, could reduce N losses by reducing the deposition of urine with high N concentration.