Quantifying the whole farm systems impact of nitrogen best practice on dairy farms

Many nitrogen (N) fertilizer recommendations for grazing livestock enterprises are based on cutting experiments, excluding the influence of recycled N in excreta. Grazing experiments are expensive to conduct, and so compromise on variables such as number of N fertilizer rates, replication and number of years of investigation. Biophysical modelling provides an efficient and effective approach to address many of the complexities of field studies. Our study, using the biophysical whole-farm systems model DairyMod, examined the effect of a range of N fertilizer rates on pasture production for five dairy sites through south-eastern Australia over 18 years under both cutting and grazing regimes. The study aims were to highlight the variation in pasture N responses between cutting and grazing experiments and compare results to current best management practice (BMP) guidelines for N fertilizer management. Annual and seasonal maximum and optimum pasture production, defined as 90% of maximum production, N fertilizer rate to achieve optimum pasture production and the slope of the response rate curve between two fertilizer application rates were estimated. For all five sites, at the lower N rates, there was a divergence in annual pasture production between the grazing and cutting management regimes. However, once N was no longer limiting pasture production for the cutting regime, annual pasture production under cutting and grazing converged. For most sites and seasons, current BMPs of applying between 20 and 50 kg N ha-1 post grazing will ensure efficient use of N applied, assuming soil moisture is not first limiting growth. However, this study has refined these recommendations across all sites and seasons. For some seasons and sites, there was high variability in pasture N response rate between years that need to be taken into consideration. At Elliott in Tasmania, an irrigated site, there was merit in increasing N fertilizer rates above the current recommendation above 50 kg N ha-1 post grazing during spring and summer. In contrast, at the rainfed sites of Ellinbank and Terang in Victoria, the recommendation would be to not apply N fertilizer during autumn and only in selected wetter summers.