Nitrogen performance indicators for dairy production systems

Nitrogen (N) is invaluable for maintaining agricultural production and ensuring food security, but its inefficient use can lead to environmental losses. Nitrogen performance indicators are useful tools to evaluate N use outcomes of an agricultural system and/or the risk of N losses to the environment. This paper reviews N use efficiency (NUE; the ratio between N outputs in products over N inputs) and N surplus (the difference between N inputs and N outputs) indicators for dairy production systems. Using case study examples we also assess realistic goals for these indicators and discuss key issues associated with their use. Published values of Crop NUE and Crop N surplus generally ranged between 55-90% and 25-230 kg N/ha/yr, respectively, while commonly reported Animal NUE and Animal N surplus values ranged between 15-35% and 110-450 kg N/ha/yr. Whole farm NUE and Whole farm N surplus values ranged between 10-65% and 40-700 kg N/ha/yr. In a New Zealand catchment study, Whole farm NUE was more strongly affected by catchment differences in soil and climatic conditions, than by differences in management. In contrast, N surplus differed both between-catchment and within-catchment and was a good indicator of N losses to water. Realistic goals for both NUE and N surplus depend on the agro-climatic context in which a dairy system operates and on the economic and environmental goals the system aims to achieve. Crop and Animal NUE values can be valuable indicators for optimising fertiliser and feed use, and minimizing N losses. However, global or even national Whole-farm NUE values appear to be of limited value if the ultimate goal for setting targets is to reduce the environmental impact of N use; whole-farm level targets based on N surplus would instead be a more useful indicator for this purpose. Regardless of the N performance indicator used, they are all calculated based on estimates of N inputs and N outputs. It is therefore important to agree on which items should be included in the input and output terms, and that all inputs and outputs are measured or adequately estimated to ensure that comparisons between different systems are equitable. Systems that export manure N or import large amounts of purchased feeds may need to adjust calculated N inputs to account for this. Finally, NUE and N surplus targets should also be set in the context of other agro-environmental considerations such as losses of phosphorus and faecal organisms to water, GHG emissions and carbon footprints, and energy and water use efficiencies.