IMPACT OF A SUSTAINED-RELEASE IVERMECTIN BOLUS ON WEIGHT-GAIN IN BREEDING AGE HOLSTEIN HEIFERS UNDER COMMERCIAL PASTURE CONDITIONS IN SOUTHERN QUEBEC

This field trial was designed to test the effect of treatment with a s ustained-release ivermectin bolus on average daily and total weight ga in in breeding age Holstein heifers under commercial pasture condition s in southern Quebec. One hundred and twelve heifers from 12 herds wer e 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 gra zing season. Fecal samples were collected at each of these times and n ematode eggs counted. Nematode egg excretion was relatively low throug hout the pasture season, which was abnormally warm and dry until midsu mmer. Over the entire pasture season, average daily weight gain was hi gher 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 thi s study suggest that preventive treatment of breeding age, grazing dai ry heifers with a sustained-release ivermectin bolus provides a signif icant weight gain advantage, in situations with moderate utilization o f moderately contaminated pastures.