Endectocide use in cattle and fecal residues: environmental effects in Canada

Endectocides, or macrocyclic lactones, are veterinary parasiticides used globally to control nematodes and arthropods affecting livestock. Cattle treated with these products fecally excrete residues that are toxic to dung-inhabiting insects, including species that accelerate dung degradation. Concerns have been raised that use of endectocides may reduce insect diversity and cause the accumulation of undegraded dung on pastures. This article synthesizes the results of studies performed to assess the nontarget effects of endectocide use in Canada. Residues reduce insect activity in dung of treated cattle for weeks to months after application. The duration of effect is influenced by several factors, including insect species and product. For example, in terms of toxicity, doramectin > ivermectin approximately equal to eprinomectin >> moxidectin. Reduced insect activity may retard dung degradation. Within the framework of regional conditions and management practices, endectocide use in Canada is unlikely to pose a significant widespread threat to the environment. Nevertheless, nontarget effects may be of concern to individual cow-calf operators, particularly those treating cattle in the spring. This synthesis, the first assessment of the nontarget effects of endectocide use in Canada, emphasizes the importance of presenting findings within an appropriate context.