Abstract
Sixty cattle (12 first season and 48 second season grazing animals) were allocated to three groups according to age and bodyweight. Each group was divided into "control" and "treated" subgroups. Before turnout, a morantel sustained release bolus (MSRB) was administered to each animal in the "treated" category. The groups were moved from field to field according to the farmer's normal rotational grazing policy. Each field was, however, divided into two equal halves, one of which was reserved for the MSRB treated cattle, while the other was used exclusively for the controls. Severe parasitic gastroenteritis occurred in the first season controls during early September, while milder disease affected the untreated animals in the smaller of the second season groups. No gastrointestinal disease was apparent in the corresponding MSRB treated cattle. A mild outbreak of parasitic bronchitis occurred in the first year controls during October; there was evidence of less sever lungworm infection in the matching MSRB treated animals. The larger second season group showed no signs of parasitic disease.

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