Stacking nitrogen leaching mitigations in a Canterbury dairy system whilst minimising profitability losses.

The aim of this project was to model combinations (“stacks”) of cost-effective nitrogen (N) leaching mitigations within a dairy system that could reduce N leaching by 40-60%, whilst minimising losses in profitability. A FARMAX and OverseerFM combination was used to model a baseline farm representing a typical Canterbury system, and seven sequentially “stacked” mitigated systems. The mitigations were combined and stacked in the following order based on mechanism(s) of action, practicality, and cost-effectiveness: 1) reduced synthetic N fertiliser input (from 190 to 100 kg N/ha/year); 2) including Italian ryegrass in the pasture sward; 3) including plantain in the pasture sward; 4) earlier calving and drying off (by 10 days); 5) wintering on pasture and baleage; 6) standing cows off-pasture;7) using new-generation nitrification inhibitors. The most cost-effective stack combined mitigations 1 to 5. We estimated that N leaching was reduced by 57% relative to baseline, with an 8% reduction in operating profit. Greenhouse gas emissions were reduced by 8%. The largest single reduction in N leaching was from stack #5, and it coincided with no/little change in milk production pasture eaten and had no capital cost. A careful selection of complementary mitigations could achieve significant reductions in N leaching without compromising greenhouse gas emissions and, to any great extent, profitability.