ASSESSMENT OF PARASITE CONTROL AND WEIGHT-GAIN AFTER USE OF AN IVERMECTIN SUSTAINED-RELEASE BOLUS IN CALVES

Objective - To assess parasite control and weight gain after administr ation of an ivermectin sustained-release bolus over 135 days to calves grazing in the midwestern United States. Design - Replicated pasture study. Animals - 56 Bos taurus calves. Procedure - Calves were matched for body weight and randomly allocated to remain untreated or to rece ive an ivermectin sustained-release bolus before turnout on day 0. Cal ves were grazed by treatment group on 8 pastures (4 replicates). Body weights and fecal egg counts were recorded on days -1 and 28, and then at 28-day intervals until day 168. Results - Parasitism was not clini cally evident prior to or during the study. In treated calves, mean fe cal egg counts were at or near 0 at all posttreatment evaluations. Alt hough the mean egg count exceeded 20 ova/g only once in control calves , the cumulative egg output was > 42 million/calf. For the treated gro up, it was < 0.1% of this number. Mean total weight gain was 33.9 kg ( 74.6 Ib) greater for ivermectin-treated carves than for untreated cont rol calves (P < 0.02); a 34% increase. Clinical Implications - Fecal t richostrongyle eggs from calves can accumulate over a grazing season t o provide enormous potential for augmenting pasture infectivity. An iv ermectin sustained-release bolus (administered to calves being placed on pasture) controls parasitism, limits pasture infectivity, and can s ubstantially influence growth by limiting the impact of subclinical pa rasitism.