Comparison of ivermectin and extended-release eprinomectin deworming treatment on stocker and subsequent feedlot performance and carcass characteristics of fall-born Angus heifers

The effect of deworming beef heifers with either ivermectin or extended-release eprinomectin on performance and carcass characteristics of stocker and feedlot cattle was assessed. Sixty Angus heifers (610 +/- 50 lb initial body weight; 277 +/- 23 kg) were blocked by body condition score and body weight, and allotted to 1 of 2 injectable deworming treatments after grazing summer pasture for 35 days: (1) ivermectin (IVM) or (2) extended-release eprinomectin (ERE). After 63 days grazing, heifers were placed in a feedlot and fed a finishing ration for 150 days. Heifers were stratified by BW within grazing treatment and allotted to receive ivermectin (dewormed; DWRM) or no treatment during processing (NO) at feedlot entry. Although fecal egg counts did not differ at treatment initiation, egg counts were greater in the IVM group (5.138 eggs per gram) than in the ERE group (0.073 EPG) at the end of the grazing period (P less than 0.01). Heifers in the ERE group had greater average daily gain and greater body weight change (P=0.01) during the grazing period. There were no differences in feedlot performance or carcass characteristics (P greater than or equal to 0.09) between treatment groups. Based on this study, there may be no benefit to deworming at feedlot arrival when cattle are effectively managed to minimize internal parasite infection during the grazing period. However, even at extremely low levels of infection during grazing, parasites can significantly impact performance.