Efficient mitigation of nitrogen leaching in pasture-based dairy systems

Pervasive uncertainty surrounds the selection of cost-effective sets of mitigation options to address nitrogen leaching from grazing systems, particularly as these possess complex nutrient cycles and interdependent ecological processes that govern the plant–animal interface. A non-linear optimisation model is used to explore how the nature of optimal abatement strategies changes with alternative levels of leaching reduction, production intensity, milk price, and the availability of restricted grazing facilities, in the context of pasture-based dairy farms in New Zealand. It is found that costly de-intensification is required if a loafing pad—on which cows can be removed from pasture during periods of high leaching risk—is unavailable. This arises from the fundamental linkage between milk production and nitrogen leaching, driven by energy and protein ingestion present in the same feedstuffs, in these farming systems. Use of a sole mitigation option is only warranted if a loafing pad can be used to decrease urinary nitrogen deposition onto agricultural soils, with its optimal level of use increasing in the degree of reduction required. Model output for farms of different production intensity is conceptually similar. However, the cost of abatement is highest on grass-only farms, as there is less scope for de-intensification and capacity to use imported feed to offset losses in pasture intake associated with the use of a loafing pad.