Comparison of Ivermectin and Conventional External Parasite Control in Young Beef Cows

Abstract

A study was conducted in north central Montana to determine whether three consecutive annual applications of ivermectin improved performance of young cows as compared to a conventional external parasite treatment, topically applied phenyl phosphorothioate. Two hundred and four heifers were paired by breed and weight and randomly assigned to ivermectin or conventional treatments. Treatments were applied in October for 3 consecutive yr at roughly 7, 19, and 31 mo of age. Heifers treated with ivermectin did not gain more than (P=0.55) heifers receiving the conventional treatment in a 140-d feedlot test. Age at puberty and pregnancy rates of heifers from both treatment groups were also similar (P>0.05). As first-calf heifers, ivermectin did not improve (P>0.20) body condition score and weight at calving over the conventional external parasite control. Milk production and weaning weights of calves from first-calf heifers were also similar (P>0.20) for both treatments. Performance of second-calf heifers and their calves was similar (P>0.05) for both treatment groups. Fecal egg counts of heifers treated with ivermectin tended to be lower (P≤0.07) than the conventional treatment, but all fecal egg counts were low. Mean fecal egg counts of heifers 2 mo after application were 9.9 and 25.2 eggs per gram of feces for the ivermectin and conventional treatments, respectively. Consistent annual treatment of young beef cows with ivermectin also did not affect weaning weights of their calves as compared to a conventional external parasite control treatment. The internal parasite control provided by ivermectin also did not improve the reproductive performance or milk production of young cows grazing semiarid rangelands.