PRODUCTION OF DAIRY REPLACEMENT STOCK IN RELATION TO LEVEL OF EXPOSURE TO GASTROINTESTINAL NEMATODE INFECTION IN THE FIRST GRAZING SEASON -2ND-YEAR CALVES AND HEIFERS

In two experiments, groups of calves which had been exposed to differe nt levels and patterns of infection with Ostertagia and Cooperia spp, in a simulated first grazing season (FGS), were followed throughout a natural second grazing season (SGS), Milk yields in the subsequent fir st lactation period were also recorded, The results suggest that altho ugh there had been differences in immune status among groups, which ha d been infected in the FGS, prior to the SGS, weight gain among these groups was not significantly different during the SGS, Apparently, res istance to the pathogenic effects of reinfection had developed more st rongly and at lower levels of exposure to infection than resistance ag ainst establishment of larvae as shown after an experimental challenge , The groups of calves not infected during the FGS did gain less than all other groups during the SGS, Further, infection-induced difference s in weight gain among the infected groups in the FGS appeared to be p ermanent, at least up to the end of the SGS, Finally, first lactation yield was positively correlated with body weight at calving, On averag e, approximately 10 kg less milk was produced for each kg of lower bod y weight at calving. With respect to the implications for preventive c ontrol strategies in the FGS, it is suggested that parasite control sh ould not be applied beyond a level at which weight gain reduction is p revented.