Evaluation of the impact of parasite control with the IVOMEC SR Bolus given at breeding age on first lactation yield in Holstein heifers.

This field trial was conducted to test the effect of treatment with a sustained-release ivermectin bolus on first lactation milk production (projected to 305 days) and milk composition (percent fat and protein) in breeding-aged Holstein heifers. From 12 herds, 96 heifers were randomly allocated at turnout to treatment with a commercially available ivermectin bolus (n=49) or untreated control (n=47) groups. Animals were weighed at turnout, mid-season, and at the end of the grazing season. Fecal samples were taken and nematode eggs counted. Projected 305 days milk production, fat and protein content were retrieved from PATLQ reports (Quebec DHI records) for heifers that calved during the subsequent 10 months. The heifers had a mean weight of 409 and 404 kg at turnout, respectively, for control and treated groups. Nematode egg excretion was relatively low (mean of 10 egg/g (EPG) for controls at housing) throughout the pasture season. The average daily gain (ADG) and total weight gain (TWG) over the entire pasture season was found to be higher in treated heifers compared to controls (ADG: difference=0.08 kg per day, P=0.010; TWG: difference=12.82 kg, P=0.013). Taking into account a possible farm effect, the 305 days milk production for IVOMEC SR bolus-treated heifers was significantly higher than for untreated animals (mean+/-S.D.; 348+/-476 kg; t-test=2.31, P<0.05). Overall, 8 of the 10 remaining farms in the study demonstrated a positive milk production difference in treated (n=34) versus control heifers (n=33). The results of this study suggest that preventive treatment of breeding-aged, grazing dairy heifers with a sustained-release ivermectin bolus provides a significant weight-gain which may translate to a significant milk production advantage, even in situations with commercial utilization of moderately contaminated pastures.