Assessments of N leaching losses from six case study dairy farms using contrasting approaches to cow wintering


  • J.M. Chrystal
  • R.M. Monaghan
  • D. Dalley
  • T. Styles



The expansion of the southern dairy herd in New Zealand has raised a number of concerns about the sustainability of grazing brassica forage crops. Here we provide an assessment of the contribution of these crops to the potential for N losses to water at a wholefarm system level, and compare these with metrics derived for systems that use alternative approaches for wintering cows. The risks of nutrient losses to water from six Monitor Farms that use contrasting approaches to dairy cow wintering were assessed using the Overseer® Nutrient budgets model (Overseer). This modelling assessment was supplemented with detailed information about the management of effluent generated from off-paddock cow wintering facilities such as wintering pads and covered housing. Predictions of N losses from individual farm blocks indicated that both winter- and summer-grazed brassica forage crops have a relatively high potential for N leaching losses. Expressed at a whole-system level (i.e. accounting for the milking platform, winter forage crop and other support land), the winter forage crops accounted for between 11 and 24% of total N leaching losses, despite representing only 4 to 9% of the area. The high N leaching losses predicted for summer-grazed forage crops were attributed to the limited opportunity for N uptake of excreted urinary N by the following new pasture. Another risk identified for some farms was the current practice of applying effluents collected from off-paddock facilities to land during winter. These assessments suggest that off-paddock cow wintering systems can help to minimise N losses from farms to water, although the storage and safe return to land of effluents and manures generated from the housing facilities is essential if this potential benefit is to be realised. Our assessments also suggest that summer crop paddocks have a relatively high potential for N leaching losses, although further research is needed to confirm this. Keywords: dairy cow wintering, Southland, nitrate leaching, grazed brassica forage crops.







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