Waterfowl grazing on winter wheat: Quantifying yield loss and compensatory growth

Herbivorous waterfowl populations have increasingly adapted to forage on agricultural land and triggered a parallel rise in damage to agricultural crops. Winter wheat (Triticum aestivum) is the most commonly reported crop damaged in northern Europe, and farmers increasingly demand management actions to mitigate economic impacts. Here, we apply two parallel approaches (exclosure experiments in undisturbed fields and yield assessments in fields subject to scaring) to quantify the impact of waterfowl grazing on winter wheat yield, crop height and nutrient content. We demonstrate that waterfowl grazing led to a substantial reduction in crop height during winter and early spring, but also that compensatory growth led to no significant effect on crop height at the time of harvest. In terms of grain yield, the effect of waterfowl grazing varied from non-significant to a 6% loss, and on fields subject to scaring, the negative effect of grazing was only significant when grazing continued into spring (the case for 7.6% of the area sampled). In addition, the exclosure experiments indicated that plots grazed both winter and spring had a protein content 9% lower than ungrazed plots. While these figures express an economic loss to the affected farmers, our findings also indicate that substantial economic impacts from grazing waterfowl on winter wheat was rare in our study area. This may also be the case elsewhere in the temperate region, when most waterfowl abandon crop foraging in due time to allow for compensatory growth in late spring, and as long as the early developmental stages survive the early impacts from grazing. A substantial scaring effort had only a limited effect on grain yield loss when fields were grazed during spring, but seemed to prevent damage on fields that were grazed only during winter. Future decision-making in relation to the waterfowl-agriculture conflict may benefit from studies looking in-depth at the costs and benefits associated with scaring efforts.