The effect of winter grazing by barnacle geese on grassland yields on Islay.
The effect of different intensities of winter grazing by barnacle geese, Branta leucopsis, on the yields of agricultural grasslands dominated by Lolium perenne was studied on the island of Islay on the W. coast of Scotland. Exclosure cages were used to compare yields of ungrazed plots with those grazed by geese. The geese caused a large reduction in the grass standing crop in the early spring on all the study fields. Up to 82% of the yield was lost to the geese in the most heavily grazed areas. Some pastures grazed by geese through the winter showed substantial reductions in the yield of silage cut in mid-June. The effect was particularly marked on heavily grazed newly resown fields where as much as 38% of the yield was lost. Other less heavily grazed areas showed no significant loss attributable to the geese. There was a correlation between yield loss and goose grazing intensity, but this explained only 28% of the variance in yield loss in Apr. and only 19% in June: other factors were also responsible for reducing yields below their possible maximum. The standing crop at the end of Apr. and the silage yield in mid-June were correlated with goose grazing intensity, but this relationship varied greatly between years. In 1986 there was a strong negative relationship but in 1987 the trend was in the opposite direction. The difference in weather conditions in the 2 years may have been responsible for this result: spring was colder in 1986 than in 1987.