Impact of a sustained-release ivermectin bolus on weight gain in breeding age Holstein heifers under commercial pasture conditions in southern Québec.

This field trial was designed to test the effect of treatment with a sustained-release ivermectin bolus on average daily and total weight gain in breeding age Holstein heifers under commercial pasture conditions in southern Quebec. One hundred and twelve heifers from 12 herds were randomly assigned at turnout either to treatment with a commercially-available ivermectin bolus or to remain as untreated controls. Ninety-six heifers, 49 treated animals and 47 controls, completed the trial. Animals were weighed at turnout, midseason, and at the end of the grazing season. Fecal samples were collected at each of these times and nematode eggs counted. Nematode egg excretion was relatively low throughout the pasture season, which was abnormally warm and dry until midsummer. Over the entire pasture season, average daily weight gain was higher in treated than in control animals (difference = 0.08 kg/day, P = 0.010). Total weight gain was also higher in treated animals than in control animals (difference = 12.82 kg, P = 0.013). The results of this study suggest that preventive treatment of breeding age, grazing dairy heifers with a sustained-release ivermectin bolus provides a significant weight gain advantage, in situations with moderate utilization of moderately contaminated pastures.