A New Method of Control of Gastro-Intestinal Parasites in Grazing Calves

Abstract

Control of parasitic gastro-enteritis (PGE) in grazing calves has relied mainly upon strategic therapeutic dosing or modification of pasture management practices, or a combination of both. These procedures, though effective, suffer from the practical disadvantage that the farmer is often unable to provide either the labour or clean pastures at the appropriate time and are consequently not properly applied. An alternative approach, which has been investigated over recent years, is the use of continuous anthelmintic medication during the first part of the grazing season to protect calves from overwintered infection on pasture and prevent the subsequent build-up of disease producing levels of pasture contamination. The practical application of this approach requires the combination of an anthelmintic which is effective at economic dosage levels and a reliable delivery system.

This paper describes the field testing of an intraruminal device (torantel sustained release bolus — MSRB) designed to release morantel tartrate continuously at effective dose levels for a period of about 60 days. Ten studies were conducted on commercial and experimental farms in England, France, Germany and Sweden during the 1978 grazing season to a common experimental design. The morantel sustained release bolus (MSRB) was administered to test calves at turnout in the spring.

The efficacy of the MSRB was assessed principally on comparisons of the weight gains over the grazing season between control treated animals and the incidence of clinical disease. Faecal egg counts provided a further assessment of anti-parasitic activity. Herbage larval counts and tracer calves were also used to measure the efficacy of the MSRB in reducing pasture larval contamination.

The combined results show that the MSRB was consistently effective in controlling PGE on the trial farms where a moderate to severe parasite challenge occurred. Overall, 178 MSRB treated calves showed a mean weight gain advantage over their contemporary controls of 21.2 kg at the end of the grazing season on all sites. The number of acute clinical cases of PGE requiring therapeutic treatment was reduced from 62 to 5. Parasitological data show that this result was achieved by the MSRB preventing the midsummer rise in pasture larval contamination and thus eliminating the detrimental effect of high parasite challenge in the latter part of the grazing season, and the data also show that the MSRB is a complete control mechanism for gastro-intestinal nematodes in calves.