Field effects of ivermectin and fenbendazole on earthworm populations and the disappearance of dung pats from bolus-treated cattle

Abstract

The disappearance of artificially formed dung pats of grazing cattle treated with sustained-release boluses containing either ivermectin or fenbendazole was compared with the disappearance of pats of untreated cattle through two successive grazing seasons. As earthworms play a major part in dung pat disappearance in northern temperate pastures, possible long-term effects of dung from treated cattle on worm populations were also examined. In a permanent pasture, the earthworm fauna was quantified within 50 m 2 enclosed plots provided with control dung pats, ivermectin pats or fenbendazole pats. In each plot, worms were extracted from the soil at dung-free locations and from 6-week-old dung pats+the underlying soil. Numbers, biomass and species composition of earthworms thus obtained were unaffected by the drug treatments. Whether the treatments had an impact on dung pat disappearance depended on season, weather and local differences between plots at the experimental site. The disappearance of dung with ivermectin was significantly delayed throughout the first grazing season, but this effect was only seen in spring of the following year. The disappearance of dung with fenbendazole did not show any consistent pattern compared to control pats.