Options for Sustainable Intensification in Dairy Grazed Grassland.

There is increasing pressure on the agricultural sector to reduce its negative impact on the environment through the emission of greenhouse gasses (GHG), and eutrophication of ground and surface waters. In Ireland, the agriculture sector has a strategy Food Wise 2025 to dramatically increase milk production in response to the end of milk quotas. But agricultural expansion has to be coupled with achieving national reductions in GHG emissions, nutrient loss to water and ammonia emissions. In order to develop grassland management strategies that are both environmentally friendly and economically sustainable, it is imperative that individual mitigation measures for N efficiency are evaluated for cost-effectiveness.

A model was developed to evaluate a range of intensification scenarios. The model dynamically links the Moorepark Dairy System Model, with a grass growth module and a number of nitrogen (N) loss modules to simulate the effect of farm management on N efficiency, N losses, production and economic performance. The model includes the option to assess the effect of using nitrification and urease inhibitors. It has been designed in such a way that the emission factors can be adjusted or refined (e.g. soil type, seasonality) based on the latest research data. We applied the model to assess a number of intensification scenarios to increase production and reduce N losses.

The intensification scenarios indicated that grassland yields could be increased from 10.1 to 13.9 T DM ha^-1 yr^-1 resulting in a 59% per hectare increase in milk yield. Fertiliser N inputs ranged from 160 to 250 kg N ha^-1 and resulted in increased N losses from 107 to 185 kg N ha^-1. Ammonia volatilisation was the largest loss pathway ranging from 50 to 90 kg N ha^-1. The use of inhibitors reduced N loss by leaching by 37%, N₂O by 31% and reduced the N foot print of milk from 12.8 to 10.1 kg N t^-1 milk.

Milk production could be increased by 50-60% using a range of management practices. The N foot print of milk can be reduced but total emissions increase compared to baseline. The nitrification and urease inhibitors resulted in the lowest N losses and milk N footprint. 

Acknowledgments: Funding for this work was provided by 1. the Department of Agriculture Food and Marine, Research Stimulus Fund (project RSF11S138) and 2. the New Zealand Government to support the objectives of the Livestock Research Group of the Global Research Alliance on Agricultural Greenhouse Gases (MPI/AgResearch Contract No. 15811). Any view or opinion expressed does not necessarily represent the view of the Global Research Alliance.