Control of gastrointestinal nematodes in dairy heifers by two strategic treatments with ivermectin.

Dairy replacement heifers given two strategic treatments with ivermectin (0.2 mg/kg) three and eight weeks after turnout to spring pastures in northern USA had mean adjusted weight gains (62.3 kg) that were 62 per cent (P less than 0.02) greater than those of control heifers (38.5 kg) at the time of winter housing. Pasture infectivity was consistently higher in control pastures and there was a sixfold difference between treatment (56 L3/kg) and control (358 L3/kg) pastures by the time of winter housing on October 18. This study confirmed earlier studies in northern USA demonstrating the value of two strategic treatments in achieving significant increases in weight gains and hastening the time of first breeding by one to three months at a saving of $40 to $140 per heifer. It was concluded that this dosing strategy is a satisfactory method of parasite control for dairy replacement heifers in northern USA without undue selection for drug resistance.