Variation in yield loss of oilseed rape due to grazing by mute swans Cygnus olor

Yield loss due to grazing by mute swans was measured on nine fields of autumn-sown oilseed rape located in five regions of England. Using a paired-plot design, a significant reduction in yield (total dry weight of seed) was observed on three of the fields: 18%, 23% and 24%, respectively. On the first of these fields yield reduction was not simply due to fewer seeds being produced, as the weight of individual seeds was also significantly lower on grazed plots, with a reduction in thousand seed weight of 13%. Between fields (n=9), yield loss was positively correlated with the number of swans per ha, and the number of days of swan grazing per ha. Significant levels of yield loss occurred only on fields grazed by >5 swans per ha. The density of swans grazing per ha can thus be used to predict potentially significant yield loss and provides an indicator of the level of swan grazing activity at which growers should consider deploying measures to reduce grazing.