Sward patterns created by patch grazing are stable over more than a decade

Under extensive grazing, a mosaic pattern of frequently defoliated short patches and rarely defoliated tall patches is often formed. The agronomic and ecological consequences of this patch-grazing pattern strongly depend on its stability between successive years. We assessed patch structure and temporal stability under three intensities of cattle stocking (moderate, lenient and very lenient) in a cattle grazing experiment established in 2002. Aerial images of the whole area taken in 2005, 2010, 2013 and 2015 were classified into short and tall patches using random forest classification. These were complemented by annual sward height measurements (2007-2017) at 10 permanent plots per paddock, which were classified into sward height classes. The mean proportion (0.72, 0.32, 0.19) and size of short patches decreased with stocking intensity, while size of tall patches increased. Inter-annual stability depended on patch type and stocking intensity and was particularly high for the respective dominant patch type. Of the short patch area in 2015, 0.62, 0.29 and 0.30 were classified as short in all four aerial images under moderate, lenient and very lenient grazing, respectively; the corresponding proportions for tall patches were 0.10, 0.53 and 0.65. Our results imply that short and tall patches experience persistent differences in local grazing intensity over extended periods. The long-term effects of this heterogeneity on soil properties and vegetation composition need to be monitored to assess agronomic sustainability and ecological potential of patch-grazed pastures.