Timing and intensity of goose grazing: Implications for grass height and first harvest

Grazing birds like geese can have a big impact on agricultural land, potentially causing damage to crops. Their exploitation of agricultural land varies within and between seasons, and these temporal grazing patterns can be influenced by management. Furthermore, management practices using scaring and accommodation aim to influence local grazing pressures. To achieve efficient goose management we need a good understanding of the effects of timing and intensity of grazing on grass development and subsequent yield loss. We performed an exclosure study on twelve fields in Friesland (Fryslân), the Netherlands to study the effect of grazing on grass development and potential yield loss. Grazing was prevented either from November or from early April until farmers anticipated their first harvest. Every two weeks we measured grass height and calculated growth in exclosures and in ungrazed plots, and performed dropping counts. The results show that the difference in grass height between grazed and ungrazed plots increases with grazing pressure, but less so for higher grazing pressures. We also find that both winter and spring grazing can result in a larger difference in grass height between grazed and ungrazed plots, and grazing into the growing season delays the start of grass growth. This appears due to the relationship between grass height and growth rate. Because the optimal height for growth increases over the season, spring grazing has a much larger impact on grassland yields than winter grazing. Overall these results show that the amount of yield loss depends on different aspects of grazing, most prominently the recovery time, duration and grazing intensity. We hypothesize that barnacle geese may select fields with denser swards and may stimulate sward density by frequent grazing throughout winter and early spring as well as across consecutive years. Future studies should look into the effect of harvest delays and sward density on the size and quality of yield across the season and especially at the last cut. Management with scaring and accommodation may be able to reduce overall yield losses, but the effect may depend on the timing and location of scaring.