A COMPARISON OF EARLY AND MID GRAZING SEASON SUPPRESSIVE ANTHELMINTICTREATMENTS FOR FIRST YEAR GRAZING CALVES AND THEIR EFFECTS ON NATURAL-INFECTION AND EXPERIMENTAL-INFECTION DURING THE 2ND YEAR

A comparison was made of the efficacy and parasitological sequelae ove r 2 years, of continuous and intermittent periods of anthelmintic supp ression applied both early and in the middle of the first grazing seas on of calves. Five groups of 15 calves grazing separate paddocks withi n the same field were allotted to one of the following treatment regim es during their first year at grass: Group 1, untreated controls; Grou p 2, treated with ivermectin injections at 3, 8 and 13 weeks after tur nout; Group 3, treated with ivermectin injections at 10, 15 and 20 wee ks after turnout; Group 4, treated with a morantel slow release intrar uminal bolus at turnout; Group 5, treated with a morantel slow release bolus at 10 weeks after turnout. Five animals from each group were sl aughtered at the end of both grazing seasons. Two months after the end of the second season the remaining five calves were challenged with a n experimental infection of 250 000 third-stage larvae (L(3)) of both Ostertagia ostertagi and Cooperia oncophora. All treatment regimes pro tected the respective calves from parasitic gastroenteritis. Over the 2 year observation period Groups 2 and 4 showed significantly better w eight gain than other groups, and at tile end of the first season, the y were found to harbour significantly fewer O. ostertagi in the early fourth stage of development. During Year 1, Groups 2 and 3 excreted mu ch lower percentages of Ostertagia spp. eggs than other groups. In Yea r 2, Group 2 excreted a higher percentage of Ostertagia spp. eggs alth ough the total egg output was approximately half that of Group 1 durin g the same period. The results showed that the effects of anthelmintic suppression on egg output of different nematode species was affected by the activity of the anthelmintic used.