Cameron J P Gourley1, Kerry J Stott2 , Sharon R Aarons1, Innocent Rugoho1
1 Agriculture Research, Ellinbank Centre, Department of Economic Development, Jobs, Transport and Resources, Ellinbank, Victoria 3821, AUSTRALIA
2 Agriculture Research, Parkville Centre, Department of Economic Development, Jobs, Transport and Resources, 32 Lincoln Square North, Carlton, Victoria 3053 AUSTRALIA
Nitrogen (N) inputs are critical for productive and profitable grazing-based dairy systems, but inefficient use can contribute to excess N in the broader environment. Whole-farm N balance (WFNB) provides the commonly used recovery metrics: N use efficiency (NUE), milk production N surplus and N surplus/ha; all recognised as environmental performance indicators. We determined annual WFNB for the Australian dairy industry over a 22 year period, and for a diverse range of 16 commercial dairy farms for the 2013/2014 production year. The industry as a whole demonstrated a long-term declining trend in all N recovery metrics, associated with ongoing intensification. Individual farms in a single production year had a wide variation in NUE, productivity N surplus and N surplus/ha, and a poorly defined relationship between NUE and N surplus/ha. At an industry level, the determination of average farm NUE, milk production N surplus and N surplus/ha provides a useful environmental performance indicator but total industry N surplus needs to be adjusted for changes in contributing land area. For individual farms in any production year, we suggest that in addition to quantifying annual N surplus, employing standardised indices that specifically target key N fluxes and utilisation efficiencies at the component level. These will establish more appropriate industry benchmarks for improving N recovery and inform and improve on-farm N management decisions.