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

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 t o: (i) the timing of the onset of grazing, and (ii) the duration of grazing (both estimated from counts of mute swan droppings). Management measures w hich delay the onset of grazing and/or reduce the duration of grazing are t herefore likely to reduce yield loss. Crown Copyright (C) 2001 Published by Elsevier Science Ltd. All rights reserved.