Control of gastrointestinal parasitism in calves in Sweden over six years using the morantel sustained release bolus.

During the 6 years 1979-1984, the use of the morantel sustained release bolus (MSRB) was monitored in first year grazing calves. Twenty-four to 30 calves each year were allotted to two groups (controls and MSRB-treated) and turned out at the end of May on the same pasture divided into two equal areas. After housing in October, each group was kept in boxes and fed concentrates, whey and hay. The calves were weighed at monthly intervals until the first animals were sent to slaughter. No cases of clinical ostertagiasis Type I were observed in the control calves during the grazing seasons, but in one year (1983) clinical signs occurred 2 weeks after housing. The number of overwintering larvae was influenced by pasture contamination the previous season and climatic conditions during winter and spring. The fecal egg output of the control animals during the grazing season did not reflect the level of pasture contamination at turnout. The build-up of pasture larval contamination during the later part of the grazing season was influenced by the climatic conditions. The fecal egg output of the treated calves was low during the entire grazing season resulting in a significantly reduced pasture contamination. A significantly reduced live-weight gain in the control calves was demonstrated at housing in five of the six years. When heavily infected at housing, the performance of the control calves was still influenced negatively during the fattening period indoors.