Realistic nitrogen use efficiency goals in dairy production systems: a review and case study examples

Nitrogen use efficiency (NUE), the ratio between N outputs in products over N inputs, is often used to evaluate N use outcomes of an agricultural system and/or the risk of environmental N losses. In this paper we address the question what NUE goals are realistic for dairy production systems. We use the following definitions of NUE: Crop NUE, defined as the percentage of the total N inputs taken up by crops or pasture; Animal NUE, defined as the percentage of total feed N intake incorporated into milk and meat; and Whole farm NUE, defined as the percentage of total N inputs to the farm that is exported in animal products and/or exported feed. Nitrogen surpluses (i.e. N inputs minus N outputs) are also reviewed. Published values of Crop NUE and N surplus generally ranged between 55-90% and 25-230 kg N/ha/year, respectively, while commonly reported Animal NUE and N surplus values ranged between 15-35% and 110-450 kg N/ha/year. Whole farm NUE and N surplus values ranged between 10-65% and 40-700 kg N/ha/year. In a NZ catchment study, Whole farm NUE was affected more strongly by differences between catchments (e.g. soil and climatic conditions) than by differences in management. In contrast, N surplus values differed both between-catchment and within-catchment and were good indicators of N losses to water. Realistic goals for NUE will therefore 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 be a more useful indicator for this. Regardless of the metric used all metrics are calculated based on estimates of N inputs and N outputs, so it is 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. For systems that import large amounts of purchased feeds, this should include the N inputs required to produce this feed. Any NUE goals targets should be set in the context of other agro-environmental indicators such as losses of phosphorus and faecal organisms to water, carbon footprints, and energy and water use efficiencies.