Scaling from patch to paddock: Are diuretic-based mitigations of leaching and nitrous oxide emissions from urine patches effective at all scales and stocking rates?

Context

Temperate grasslands are important contributors to agricultural production but also to global warming through N₂O emissions and to local water quality issues through leaching of N. These emissions are largely driven by small, concentrated urine patches. Adding diuretic components to the diet of ruminants will reduce the N load in individual urine patches without changing the total N return to the pasture. The effectiveness of lowering the N load has been established at the patch scale. However, diuretics will also increase the number and size of urine patches, and it is not known how much this will offset the reduction in N load at the paddock level.

Objective

We aimed to understand the trade-off between reduced urine patch N load and increased numbers and sizes of urine patches. We further sought to quantify how that trade-off might change with stocking rate.

Methods

We examined this trade-off using a simulation methodology designed for heterogenous urine patches depositions within a representative paddock of a farm. Paddock-level simulations were performed for contracting locations and multiple stocking rates, intensifying the farm by adding more N fertiliser or buying in additional feed, and degrees of diuresis. Supporting patch-level simulations were also done.

Results and conclusions

Patch-scale simulations showed that reducing urine patch load would decrease leaching and N₂O emissions. Paddock-scale simulations suggested that the diuretic would be effective at low stocking rates (by up to 40 %). The effectiveness of the diuretic decreased as stocking rate rose and was largely ineffective above 3 cows /ha if fertiliser was used to intensify the farm. If the farm was intensified through imported feed, the effectiveness still decreased, but the declines were less marked.

The major cause of the decline in effectiveness of the diuretic was the increased urine patch area experiencing short (< 6 months) intervals between successive depositions – this increase was stronger at high stocking rates than lower ones. The effect of adding fertiliser onto ever-larger areas of urine patches was a likely secondary contributor to the decreased effectiveness.

Significance

While these simulations were only at two locations in one country, the pattern of decreased effectiveness with increasing stocking rate is likely to be general. Because the diuretics were most effective when leaching and N₂O emissions were already low, the results suggest that mitigations based solely on diuretics will have limited effectiveness in achieving desired improvements in the environmental performance of grazed pastoral farms.