Toxicity of ivermectin residues in aged farmyard manure to terrestrial and freshwater invertebrates

Abstract

Abstract Farmyard manure is often stored, aged and spread on fields as fertiliser; however, dung may be contaminated with residues of veterinary endectocides used to treat livestock for parasites. The persistence of these chemicals during storage and impact on invertebrate biodiversity after spreading are not well understood. This study considered whether residues in aged cattle manure could have impacts on terrestrial and freshwater invertebrate biodiversity and ecosystem function. Fresh cattle dung was spiked with known concentrations of the endectocide ivermectin or an excipient only (control) and aged in the field for 4 months between February and May 2020. Each month rainwater run‐off was collected and used to examine toxicity using the freshwater invertebrate Daphnia magna. In June 2020, manure was spread on mesocosms containing topsoil, and above‐ground insect emergence, soil fauna feeding rate, earthworm abundance and pasture productivity (perennial ryegrass growth), were measured over 10 weeks. Rainwater‐runoff from manure contaminated with ivermectin was highly toxic (60–100% mortality) to D. magna for the entire 4 months of storage. Coleoptera and Diptera emergence was lower from mesocosms spread with ivermectin‐contaminated manure compared with control manure. Pasture productivity was significantly lower (18–20%) in mesocosms spread with ivermectin‐contaminated manure. The results indicate that ivermectin residues aged manure have the potential to retain toxic effects on terrestrial and freshwater invertebrates for at least 4 months of storage, could reduce pasture productivity, and may have pervasive impacts on invertebrate biodiversity in agricultural systems.