Nitrogen loss: an emerging issue for the ongoing evolution of New Zealand dairy farming systems

Here we present a case study of two New Zealand dairy farms located in the southern North Island (latitude 40°S) in which we review the evolution of the system over the past 30 years with particular focus on the relation between intensification and N loss to the environment. Over the period evaluated the two case farms lifted per cow production by over 40% and approximately doubled per ha milk production, partly through identification of efficiencies in farm system design and partly through intensification by feed importation. Based on the production data, animal consumption (t DM ha⁻¹ year⁻¹) and N loss to the environment (kg N ha⁻¹ year⁻¹) were modelled for four scenarios representing the two case farms in the early 1980s and in the 2007/2008 production season. For one case farm, the system was modelled for two further scenarios before and after changes aimed at increasing feed conversion efficiency. Increase in N loss to the environment from dairy farm operations arising from intensification between the early 1980s and the 2007/2008 season were largely offset by a change in the system of farm dairy effluent disposal that reduced N loss. Comparing model output for system configurations with low (38%) and high (49%) feed conversion efficiency, production was 893 and 1,115 kg milk solids (MS) ha⁻¹, N loss to the environment was 19 and 20 kg N ha⁻¹ year⁻¹, and ‘environmental efficiency' was 21 and 18 kg N leached tonne⁻¹ MS produced, respectively.