Efficacy of mid-summer doramectin pour-on application for parasite control in spring calving cow-calf herds in the Northern Great Plains

Abstract

One-hundred-twenty cow-calf pairs grazing native pasture in southcentral North Dakota were used in a prospective study to evaluate efficacy of mid-summer topical doramectin application for control of lice compared to non-treated controls. Pairs were allocated to one of two treatment groups and placed on eight separate pastures. On day -40, cows were weighed and body condition score (BCS) evaluated. On day 0 (day of treatment) pairs allotted to the treatment group were treated topically with 227 mcg/lb of doramectin in mid-July. Cows were examined for lice monthly from day 134 through day 238. Fecal samples collected from cows and calves were examined for parasite eggs, and horn fly counts were taken on selected cows in each replicate. Weights and BCS (cows only) were taken for cows and calves. Steer calves were committed to a finishing program to evaluate effect of doramectin on feedlot performance and carcass characteristics. Percentage of cows positive for lice in the doramectin-treated group was lower on days 161-162, 184-185, 210-211 and 238-239 than non-treated cows. Doramectin-treated cows and calves had lower fecal egg counts on days 21 and 42 than non-treated controls. Mid-summer application had no effect on cow BCS, cow body weight, calf average daily gain or steer carcass characteristics. Mid-summer application of doramectin was effective for control of lice through the winter. Internal parasite load in this study may not have been high enough to detect a difference in performance in spring calving cow-calf pairs.