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 ranging in weight from 158 to 320 kg, were used to compare effects of treatment with an ivermectin sustained-release bolus or two ivermectin injectable 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 pastures. Treatments on Day 0 (12 December) were: Group 1: untreated controls; Group 2: ivermectin injectable at 200 micrograms 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. All cattle were weighed at 28 day intervals and fecal samples were collected for egg per gram counts (EPG). Geometric mean EPG for Group 3 remained consistently less than 1.0 after Day 0 and were highest (2.4) on Day 168. All group EPG were significantly different (P < 0.01) by Day 56, and EPG of Group 2 had increased to 10.5 following initial treatment 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 was predominant, along with smaller proportions of Haemonchus placei and Cooperia spp. Throughout the experiment Group 3 had highest liveweights and gains that were most often different from those of Group 1 at P < 0.01 or greater.(ABSTRACT TRUNCATED AT 250 WORDS)