Genetic variation among calves in resistance to nematode parasites

Dutch Friesian male calves, which received a dose of 100 000 Cooperia spp. larvae at an age of 3 months, displayed a great variability in reaction, measured by number and length of worms, egg output, serum antibodies and liveweight gain. In an experiment with half-sib groups from randomly chosen sires, only the antibody response was significantly different between sire-groups. In a further experiment with half-sibs from two selected sires, egg output, worm numbers, worm length and antibody response differed. A third trial with half-sib groups of the same two sires was carried out under natural conditions of grazing. It is concluded that, within this breed, genetical variation exists in resistance to Cooperia spp.