Nitrogen balances for grass-based dairy production systems at different stocking rates

Nitrogen (N) use efficiency is one of the key drivers of environmentally and economically sustainable agricultural production systems. Loss of N to the environment can result in contamination of waters and can contribute to greenhouse gas emissions. As a result there is a requirement for a balance between N input and N output, while minimising surplus N and N loss. An N-balance model was used to assess the N-use efficiency of grass-based dairy production systems with contrasting stocking rates and N-fertiliser application rates. Data from a three-year grazing study, which had three stocking rates (2.0, 2.47, 2.94 LU ha(-1)) and three N-fertiliser application rates per stocking rate, were used to calculate N-use efficiency and N losses. Nitrogen input was primarily in the form of feed, fertiliser and replacement animals. Milk production was the main source of N output. All stocking rate treatments had similar N-use efficiencies per cow (mean 24.7%). The annual farm gate N surplus per ha ranged from 90 to 171 kg N ha(-1), 122.8 to 195 kg N ha(-1) and 160 to 233 kg N ha(-1) for the 2.0, 2.47 and 2.94 LU ha(-1) stocking-rate treatments, respectively. As fertiliser N input increased, N surplus per ha increased and N-use efficiency per ha decreased. Increasing stocking rates and maintaining N fertiliser application rate increased N-use efficiency by an average of 22%.