Spatially and temporally variable urinary N loads deposited by lactating cows on a grazing system dairy farm

Feed nitrogen (N) intakes in Australian grazing systems average 545 g cow^-1 day^-1, indicating that urinary N is likely to be the dominant form excreted. Grazing animals spend disproportionate amounts of time in places on dairy farms where N accumulation is likely to occur. We attached to grazing cows sensors that measure urine volume and N concentration, as well as global positioning systems sensors used to monitor the times the cows spent in different places on a farm and the location of urination events. The cows were monitored for up to 72 h in each of two seasons. More urination events and greater urine volumes per event were recorded in spring 2014 (3.1 L) compared with winter 2015 (1.4 L), most likely influenced by environmental conditions and the greater spring rainfall observed. Mean (range) N concentration (0.71%; 0.02 to 1.52%) and N load (12.8 g cow^-1 event^-1; 0.3 to 64.5 g cow^-1 event^-1) did not differ over the two monitoring periods. However, mean (range) daily N load was greater in spring (277 g cow^-1 day^-1; 200 to 346 g cow^-1 day^-1) than in winter (90 g cow^-1 day^-1; 44 to 116 g cow^-1 day^-1) due to the influence of urine volume. Relatively greater time was spent in paddocks overnight (13.3 h) than in paddocks between morning and evening milking (6.4 h), compared with the mean numbers of urinations in these places (6.4 and 3.8 respectively). The mean N load deposited overnight in paddocks (89.6 g cow^-1) was more than twice that deposited in paddocks during the day (43.8 g cow^-1), due to the greater N load per event overnight, and was more closely linked to the relative difference in time spent in paddocks than in the number of urination events. These data suggest that routinely holding cows in the same paddocks overnight will lead to high urinary N depositions, increasing the potential for N losses from these places. Further research using this technology is required to acquire farm and environment specific urinary data to improve N management.