Evaluation of the Impact of Parasite Control With the IVOMEC® SR Bolus Given at Breeding Age on First-Lactation Yield in Holstein Heifers

A sustained-release ivermectin bolus (IVOMEC® SR bolus) has been proven efficient for the treatment and prevention of nematode infections in growing beef and dairy cattle. However, its effects on growth in breeding-age dairy heifers, as well as on first-lactation yield, have not been demonstrated. This field trial was designed to test the effect of treatment with a sustained-release ivermectin bolus on average daily gain and total weight gain in breeding-age Holstein heifers under commercial pasture conditions in southern Quebec during summer 1995. A follow-up pilot study was undertaken to test the effect of the IVOMEC® SR bolus on first-lactation milk production (projected to 305 days) and milk composition (% fat and protein) in heifers that calved between October 1995 and August 1996.