Grazing effects of wintering geese on grassland yield: a long-term study from northwest Germany.
Abstract
Escalating conflicts between grassland farming and wintering geese in northern Germany stimulated a long-term study in order to promote a fair and workable system of compensation of harvest loss. Between 1996 and 2018 standardized experiments were carried out to quantify changes in yield loss and herbage quality. Simultaneously, we weekly monitored the number of geese to relate yield losses to goose numbers and to identify the impact of the different species. Exclosure experiments were established on conventionally managed grasslands. The number of investigated fields differed over the study period (1990s: n = 6, 2000s: n = 14, 2010s: n = 2-18). On each field, we established 12 marked plots (4.5 m2), six with exclosures from early November until the first cut of grass in May and six with access for the geese. In all plots dry biomass and the quality of herbage (contents of energy, crude protein, crude fibre and ash) were determined at first and second harvest. The total goose-dependent yield losses at first harvest increased from 15% in the year 1996/97 to 50% at the end of the 2010s. The increase corresponds with changes in the maximum numbers and the migratory behaviour of the barnacle goose Branta leucopsis. Yield reductions correlated positively with densities of barnacle geese present in April. In contrast, we found no decline in grassland yields with increasing numbers of greater white-fronted geese Anser albifrons. In all periods second harvest was not affected. The combined maximum number of both geese which were counted over approximately 23,000 ha of grasslands increased until 2002/03 but levelled off with numbers around 100,000 birds thereafter. While the maximum wintering population of greater white-fronted geese dropped since 2007/08, the maximum number of barnacle geese increased until mid of 2010s. An increasing proportion of barnacle geese delayed their departure until May. Within each year grazed plots possessed higher energy and crude protein contents than ungrazed controls, suggesting that the geese maximize their potential nutrient intake rate by grazing. Synthesis and applications. The present study reveals a significant increase in goose-related loss of grassland yields which form the basis for a fair and comprehensible system of compensation payments to affected farmers.