The Challenge of the Urine Patch for Managing Nitrogen in Grazed Pasture Systems

Ruminants excrete as much as 70-95% of the nitrogen (N) they consume. The urine patch is the conduit through which much of this N is recycled in grazed pasture systems. This review focuses on three key areas: urine patch characteristics and N cycling processes; implications for N cycling at the farm and paddock-scale; and strategies available to mitigate N losses from the urine patch. The urine patch N loading rate is a key metric for quantifying and modelling fate of N; yet it is a derived value, relying on estimates of urine volume and N concentration, and the urine patch surface area, all of which are variable. Much is known about N cycling processes in the urine patch but further understanding of N2 loss, leaching of dissolved organic N and mineralisation-immobilisation turnover is needed. Typical values (as a percentage of the deposited urinary N) were estimated as: 13% ammonia volatilisation; 2% nitrous oxide emission; 20% nitrate leaching; 41% pasture uptake; 26% gross immobilisation. The relative importance of each process is influenced by urine patch characteristics and environmental factors. Models are an important tool for scaling from the individual urine patch to the paddock and farm scale, though accounting for variability in urine patch characteristics, and spatial and temporal distribution, remains a challenge. Many potential management strategies to decrease N loss from the urine patch are still at the proof of concept stage with few actually deployed on the farm. Further research is required to integrate these into farm management systems.