Mute swan grazing on winter crops: estimation of yield loss in oilseed rape and wheat

Abstract

Yield loss due to grazing by mute swans was measured on two fields each of autumn-sown oilseed rape and winter wheat, located in three regions of the UK. Using a paired plot design, a 33.7% reduction in yield (total dry weight of seed) and a 12.1% reduction in the thousand-seed weight was measured in the rape field subjected to the highest grazing intensity. No significant yield loss was measured in the other three fields. Yield loss was related to: (i) the timing of the onset of grazing, and (ii) the duration of grazing (both estimated from counts of mute swan droppings). Management measures which delay the onset of grazing and/or reduce the duration of grazing are therefore likely to reduce yield loss.