The effects of the use of the morantel sustained release bolus system on calves grazing a highly contaminated pasture in The Netherlands.

Abstract

The efficacy of the morantel sustained release bolus system for the control of parasitic gastroenteritis in calves was evaluated in the Netherlands. The effect of bolus administration to calves, either on May 1 at the beginning of the summer grazing season or on June 12 after the pasture was mown for silage, was determined in respect of liveweight gain, faecal worm egg output, serum pepsinogen values and worm burdens. In addition, pairs of tracer calves were turned out with each group every four weeks and sacrificed for worm counts in order to monitor pasture larval contamination throughout the grazing period. Compared with nontreated control calves, the worm egg outputs of the bolus-treated calves turned out (Nematodirus helvetianus), while the bolus-treated calves turned out on June 12 had a corresponding reduction of 95 and 100 percent. Over the grazing season the average weight gain of bolus-treated calves turned out on May 1 was 34 kg more than nontreated controls turned out at the same time. Difference in weight gain of the treated and control calves turned out late in the season was 10.5 kg in favour of the treated group. The postmortem worm counts were highest in the control calves turned out early and lowest in the bolus-treated calves turned out late. All tracer calves placed on the paddock grazed by control calves (turned out early) died from parasitic gastroenteritis from July onwards while those from the paddock grazed by the control calves (turned out late) died from parasitic gastroenteritis from September onwards. All the tracer calves placed on the paddocks grazed by bolus-treated calves survived until the intended slaughter date.