Changes with time after treatment in the concentrations of ivermectin in fresh cow dung and in cow pats aged in the field.

An analytical procedure for detection of ivermectin in plasma samples was modified and used for measuring ivermectin concentrations in dung samples. The residues were quantified by high-performance liquid chromatography and fluorescence detection after extraction, purification, and derivatization into fluorescent reaction products. The concentrations of ivermectin in dung were measured after dosing heifers with 0.2 mg kg-1 by subcutaneous injection or 0.5 mg kg-1 by pour-on. Despite the 2.5 times higher pour-on dose, this formulation did not result in excretion of detectable concentrations of ivermectin over a longer period of time than did the injection treatment. Dung pats with ivermectin residues were exposed on grazing pastures until they were no longer colonised by flies and beetles. In the experiments performed under temperate and tropical conditions, the aging of the pats did not lead to a significant lowering of the concentrations of ivermectin. Flies or beetles colonising the pats some time after deposition may therefore not be exposed to decreased levels of ivermectin.