Whole-carcass residues of the rodenticide difenacoum in anticoagulant-resistant and -susceptible rat strains (Rattus norvegicus).

The present study investigated the whole-carcass residue carried by resistant and susceptible laboratory rat strains following 5, 10, or 20 d of feeding on a diet of 25 mg difenacoum/kg bait. The mean whole-carcass residue of difenacoum was determined by high-performance liquid chromatography to be between 0.52 and 0.74 mg/kg body weight in all three rat strains tested. These values were considerably lower than some comparable data previously reported for other species and second-generation rodenticides as well as from mathematical models. The whole-carcass residue of extractable (i.e., nonrefractory) parent compound carried by highly resistant rats fed for 20 d (0.74 mg/kg body wt) is unlikely to present a significantly increased risk to predators compared to the amount carried by susceptible rats after 5 d of feeding (0.52 mg/kg body wt). However, resistant rats are more likely to be available for predation and to be carrying a whole-carcass residue of anticoagulant throughout the duration of a control program.