A comparison of the morantel sustained-release bolus with conventional drenching for the control of gastro-intestinal nematodes in grazing calves

A comparison was made of the parasitological and production responses to the prophylactic (summer) and combined prophylactic and therapeutic (summer/autumn) administrations of the morantel intraruminal, sustained-release bolus (MSRB) with responses to conventional drenching in grazing dairy calves over an 8-month period. The MSRB was designed to release morantel tartrate at 150 mg/day for 90 days. In two concurrent trials, groups of calves grazed separate replicated pastures. In Trial 1, the combined summer (December) and autumn (March) administration of the MSRB was compared with a programme of six drenches (oxfendazole at 4.53 mg/kg) at 28-day intervals from December to May. In Trial 2, the summer administration of the MSRB was compared with a programme of three autumn drenches (oxfendazole) at 28-day intervals from March to May. There were no significant between-treatment differences in mean liveweight gain in either trial. In both trials, the low numbers of eggs passed by the MSRB-treated animals during summer (maximum mean 62 eggs per gram of faeces) were sufficiently high to produce a modest autumn larval peak (maximum 1990 larvae/kg herbage). In Trial 2, the summer use of the MSRB resulted in an 83% reduction in peak larval availability compared with that on pastures grazed by untreated calves during summer. Only the 6-drench regimen (Trial 1) prevented the accumulation of appreciable worm burdens of early 4th stage Ostertagia spp.