Modelling to scale mitigations of leaching and N2O from patch to paddock

Leaching and nitrous oxide emissions from grazed pastures are driven by highly concentrated urine patches deposited onto a small fraction of the grazed area. Previous work, at the scale of a urine patch, has shown that adding diuretic components to the diet of ruminants will reduce the N load in urine patches and reduce emissions of N from the patch. However, diuretics also increase the number and size of urine patches. Conceptually we know that there are both reinforcing and antagonistic forces operating so it is uncertain how effective diuretics will be at the paddock scale. Here, we sought to understand how the effectiveness of a diuretic for emissions reduction varied as the spatial scale increased from 1 m2 (a urine patch) to several hectares (a paddock). To do this we used a validated simulation methodology designed for heterogenous urine patches within a paddock of a farm. Patch-scale simulations showed that reducing urine patch load would decrease both leaching and nitrous oxide emissions. Paddock-scale simulations suggested that the diuretic would be effective at low stocking rates with reductions in emissions of up to 40%. The effectiveness of the diuretic decreased as stocking rate rose and was largely ineffective above 3 cows /ha. The major cause of the decline in effectiveness was the increase in area of the paddock affected by urine patches with short ( 6 months) intervals between successive dispositions – this increase was stronger at high stocking rates than lower ones. These results show the importance of simulation technologies to scale results that can be found by measurement (i.e., a patch) to scales that are meaningful but unmeasurable (a paddock or whole farm).