A model for nematodiasis in New Zealand lambs: the effect of drenching regime and grazing management on the development of anthelmintic resistance.

A model for nematodiasis in lambs was expanded to incorporate both the contribution of ewes to nematode epidemiology and the genetic parameters required to simulate the development of anthelmintic resistance in the nematode population. The expanded model was used to assess the impact of various drench and grazing management strategies for ewes and lambs on the rate of development of anthelmintic resistance. Three grazing management options, under a range of drenching schedules, were compared: one in which lambs and ewes were rotationally grazed as separate flocks over the same area after weaning (common grazing); a second in which lambs were grazed, after weaning, on areas from which ewes were excluded (separate grazing); and a third in which lambs were moved to "safe" pasture at weaning and again in early autumn (integrated control). Drenching strategies examined under the first 2 grazing options included a 5 lamb-drench "preventive" programme with 0, 1, 2, 3 or 4 additional lamb drenches, and 0 or 1 ewe drench treatment at either tall-docking or mating. Under the third grazing option, lambs were given either 1 or 2 drench treatments at or following each move to safe pasture and ewes 0 or 1 drench treatment at either tail-docking (i.e., 3-4 weeks after lambing) or mating. Model output suggests that drenching ewes prior to any lamb drenching programme is likely to significantly increase selection for drench resistance by pre-selecting the larval challenge to the lambs and, under some grazing systems, by reducing the diluting effect of eggs of susceptible genotypes passed by undrenched ewes. The results highlight the potential importance of undrenched ewes as a refuge for susceptible worm genotypes and indicate that on its own, drenching frequency is likely to be a poor indicator of selection pressure for resistance and thus of limited value in selecting strategies for the management of anthelmintic resistance.