Simulated seasonal responses of grazed dairy pastures to nitrogen fertilizer in SE Australia: N loss and recovery

Abstract Evidence from farm level studies indicates that there is potential to improve nitrogen (N) fertilizer efficiency of Australian dairy farms. Increasing N fertilizer application rates to drive pasture dry matter production beyond an agronomic or economical optimum has the potential to result in detrimental environmental outcomes. Our study, using the biophysical whole-farm systems model DairyMod, modelled a range of N fertilizer rates on total N loss for five dairy sites through south-eastern Australia, using 18 years of historical climate. Nitrogen accumulation in plant biomass and soil N accumulation within and below the rootzone were estimated. Total N loss, in the form of volatilization, leaching, runoff and denitrification lost to the environment were also estimated. The reduction in N fertilizer inputs required to achieve 90% of relative yield (Y90), relative to maximum pasture production (Ymax), was > 50% across all sites and seasons. The associated reduction in total N loss when fertilizer was reduced from Ymax to Y90, varied between 34% and 74%, depending on site and season. Nitrogen recovery (proportion of N recovered in biomass relative to N fertilizer applied) exceeded 100% with lower N fertilizer rates (