Does cattle and sheep grazing under best management significantly elevate sediment losses? Evidence from the North Wyke Farm Platform, UK

Purpose: Intensive livestock grazing has been associated with an increased risk of soil erosion and concomitant negative impacts on the ecological status of watercourses. Whilst various mitigation options are promoted for reducing livestock impacts, there is a paucity of data on the relationship between stocking rates and quantified sediment losses. This evidence gap means there is uncertainty regarding the cost–benefit of policy preferred best management. Methods: Sediment yields from 15 hydrologically-isolated field scale catchments on a heavily instrumented ruminant livestock farm in the south west UK were investigated over ~26 months spread across six years. Sediment yields were compared to cattle and sheep stocking rates on long-term, winter (November–April) and monthly time scales. The impacts of livestock on soil vegetation cover and bulk density were also examined. Cattle were tracked using GPS collars to determine how grazing related to soil damage. Results: No observable impact of livestock stocking rates of 0.15 – 1.00 UK livestock units (LU) ha-1 for sheep and 0 - 0.77 LU ha-1 for cattle on sediment yields was observed at any of the three timescales. Cattle preferentially spent time close to specific fences where soils were visually damaged. However, there was no indication that livestock have a significant effect on soil bulk density on a field-scale. Livestock were housed indoors during winters when most rainfall occurs and best management practices were used which when combined with low erodibility clayey soils likely limited sediment losses. Conclusion: A combination of clayey soils and soil trampling in only a small proportion of the field areas lead to little impact from grazing livestock. Within similar landscapes with best practice livestock grazing management, additional targeted measures to reduce erosion are unlikely to yield a significant cost-benefit.