For T. colubriformis the development of resistance to ivermectin, as measured by tracer lamb worm burdens, was delayed by treatment with the combination and by leaving 10%of lambs untreated. In addition, the interaction between these factors approached significance (p = 0.052). Similarly, results of the LDAs indicated a slower development of resistance when lambs were treated with the combination and when 10%of lambs were left untreated. For T. circumcincta, results were compromised by the rapid development of resistance, which appears to be the result of low viability in the field of the susceptible isolate used to contaminate the pastures. Although a small delay in the development of resistance to ivermectin was indicated, this was off-set by an increase in the level of resistance to levamisole.
A post-study modelling experiment simulating the conditions of the field study and the starting efficacies for the two nematode species produced equivalent outputs to those measured in the field. Overall, results support the conclusions that use of combination anthelmintics and deliberately increasing 鈥榬efugia鈥?of unselected genotypes will slow the development of anthelmintic resistance. However, as indicated in modelling studies, once resistance to all the constituent actives is well developed (efficacy < 70%) the value of combinations for slowing the development of resistance is largely lost.