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 and gaseous emission pathways. The volume and N concentration of urine influences the amount of N in a urine patch. We conducted systematic urine sampling to determine the diurnal variation in concentrations of N and creatinine (a proxy for urine volume), and the N : creatinine ratio, to identify the sampling required for accurately estimating the daily mean concentrations of N and creatinine. Nine groups (n = 6) of multiparous Friesian and Friesian × Jersey cows in autumn (220 ± 26 days-in-milk, milked twice daily) and nine groups (n = 6) in late spring–summer (228 ± 24 days-in-milk, milked once daily) were sequentially withdrawn from the farm herd at approximately weekly intervals and each group was offered a fresh allocation of ryegrass-dominant pasture twice daily after milking for 3 days (including at the equivalent time in the afternoon in late spring–summer when they were milked once daily). For each of the 18 different groups of cows, individual urine samples were collected on Day 3 at 1100 hours, 1500 hours (afternoon milking), 1800 hours and 0700 hours (the following morning milking), and, subsequently, analysed for total N and creatinine concentrations. In autumn, urine-N concentrations were higher (P = 0.0002) at 1800 hours (5.8 g N/L) than they were at 1500 hours or 0700 hours (mean of 4.2 g N/L). In late spring–summer, the concentrations were higher (P < 0.001) at 1100 hours (8.0 g N/L) than they were at 1500 hours, 1800 hours or 0700 hours (mean of 6.3 g N/L). The urine N : creatinine ratio was 214 mol/mol in autumn and 148 mol/mol in late spring–summer, but did not vary among sampling times during the day. The highest concentrations of N were in urine samples collected ~3 h post-allocation of fresh feed when cows had grazed actively and consumed the majority of the herbage available. For accurate estimates of the daily mean urine N concentration, sample collections should be timed to encompass this diurnal variation. For the N : creatinine ratio, which was more stable through the day, the timing of sample collection is less important for estimating a daily mean.