EFFECT OF TREATMENT WITH AN IVERMECTIN SUSTAINED-RELEASE BOLUS ON PRODUCTIVITY OF STOCKER BEEF-CALVES

Three groups of 30 crossbred beef steers, 8-10 months of age and rangi ng in weight from 158 to 320 kg, were used to compare effects of treat ment with an ivermectin sustained-release bolus or two ivermectin inje ctable treatments on parasite control and productivity in relation to untreated controls during a 168 day winter-spring grazing period, Each group of 30 consisted of five cattle on each of six separate 1.6 ha p astures. Treatments on Day 0 (12 December) were: Group 1: untreated co ntrols; Group 2: ivermectin injectable at 200 mu g kg(-1) bodyweight, s.c,, on Day 0 and Day 56; Group 3: ivermectin sustained-release bolus to deliver ivermectin at 12 mg day(-1) over approximately 135 days. A ll cattle were weighed at 28 day intervals and fecal samples were coll ected for egg per gram counts (EPG). Geometric mean EPG for Group 3 re mained consistently less than 1.0 after Day 0 and were highest (2.4) o n Day 168. All group EPG were significantly different (P < 0.01) by Da y 56, and EPG of Group 2 had increased to 10.5 following initial treat ment and to 42.8 on Day 112. With the exception of a low mean EPG of 6 .8 for Group 1 on Day 112, EPG of the group were consistently highest (range 24.9-36.0) to the end of the experiment, Ostertagia ostertagi w as predominant, along with smaller proportions of Haemonchus placei an d Cooperia spp. Throughout the experiment Group 3 had highest liveweig hts and gains that were most often different from those of Group 1 at P < 0.01 or greater. Group 2 had a similar advantage over Group 1, but at P < 0.05. On Day 168, Group 3 had liveweight and cumulative gain a dvantages over Group 2 of 9.3 kg and 10.8 kg, respectively, but these were not significant. Advantages over Group 1 were 31.1 kg and 30.1 kg , respectively.