Effect of strategic ivermectin treatments on parasitism of calves during their rearing in a permanent summer-camp system and subsequent second seasongrazing on pasture in Lithuania

The present experiment, conducted over two consecutive grazing seasons, was designed to evaluate the effect of strategic ivermectin treatments on para sitism and performance of carves reared in permanent summer-camp systems. P ermanent summer-camp systems are characterised by having small outdoor pens with a hut in which the calves are fed a full diet comprised of fresh cut grass, concentrate, skimmed milk and water ad libitum. Twenty heifer-calves (3-5 months of age) were divided into two comparable groups according to l ive weight. Carves in group A were treated with ivermectin (200 mug/kg) at weeks 3-8-13 after turnout, while group B served as an untreated control gr oup. The herbage larval counts and infectivity level were monitored by anal yzing grass samples and using parasite naive tracer calves in October. The results showed that strategic ivermectin treatments, given to first-season calves at week 3, 8 and 13 after turnout effectively prevented build-up of high numbers of larvae on the grass during the grazing season. The effect o f treatments was clearly reflected in the differences between worm burdens of the tracer carves. During the following grazing season, both groups were grazed in a single herd. In the start of the second grazing season the tre ated calves gained on average 32 kg more than the control animals. The diff erence in weight gains remained until the end of the grazing season. No inc idence of loss producing infections was observed during the second-grazing season.