The effect of a single subcutaneous injection of extended-release eprinomectin for parasite control on cow/calf performance was evaluated in a pair of studies. In the first study, a fall-calving Angus herd was utilized. Mature cows were treated with either extended-release eprinomectin (EPR) or a conventional injectable anthelmintic (CONV). Cows treated with EPR had better maintenance of BCS through calving and into breeding. Subsequent pregnancy rates were greater for EPR than CONV cows. Furthermore, calves from EPR dams were younger at weaning, but had greater weaning weights than calves from CONV dams. From the same fall-calving Angus herd, replacement heifers were either treated with EPR or CONV. Weights taken post-treatment demonstrated heavier BW, greater weight gain, and greater ADG, for EPR heifers. Both conception to AI and overall pregnancy rates were greater for EPR heifers. Also, a greater proportion of EPR heifers calved in the first 21 days of the subsequent calving season.

A second study was conducted to elucidate the effects of a single injection of extended-release eprinomectin on economically relevant production variables in beef cows and calves as well as subsequent feedlot health, performance and carcass traits of calves. Animals from 13 cooperator herds across 7 states were stratified within herd by cow age, calf birth date, calf birth BW, and calf sex and assigned to 1 of 2 treatments; injectable doramectin (DOR) or injectable extended-release eprinomectin (EPR). There was no difference in cow BW at the start or end of the grazing season, resulting in no differences in BCS, change in BW or ADG over the course of the grazing season. Fecal samples collected at treatment indicated no difference in fecal egg count (FEC) at the start of the grazing season. However, subsequent samples collected at the end of the grazing season showed lower FEC for EPR cows and a greater overall reduction in FEC over the course of the grazing season. EPR cows had a lower incidence of pinkeye, however, there was no differences in calf pinkeye. Fly counts conducted indicate no difference in fly burden between treatment groups. There was no difference in reproductive success including conception to AI, overall breeding season pregnancy rates, calving interval, and calving distribution. There was no difference in calf treatment BW, weaning BW, or pre-weaning ADG.

Following weaning, a subset of calves from each herd at the discretion of the cooperator were shipped to Tri-County Steer Carcass Futurity (TCSCF) for the finishing phase. At initiation of the feeding phase, calf BW did not differ. While EPR calves tended to be heavier at re-implantation, final BW and overall ADG were not different between treatments. Despite a lack of differences in feedlot performance, morbidity was lower for EPR calves indicating these calves were healthier throughout the feeding phase. However, evaluation of carcass performance showed no difference in HCW, dressing percent, backfat, KPH, REA, YG or marbling score. Analysis of quality grade indicated higher average quality grade for EPR calves as well higher percentage of calves that graded average choice or higher. There were no differences in the percentage of steers that graded low choice or lower. Economic analysis indicates an opportunity for producers operating on a retained ownership platform who treated with EPR to realize a profit above the initial cost of treatment through improved health status during the feeding phase. An overall lack of performance differences observed in the current study may likely be a function of low FEC in participating herds.

In summary, results from these studies indicate that certain environmental conditions can result in improved performance following treatment with extended-release eprinomectin. Benefits from anthelmintic administration to suckling calves pre-weaning may extend to the feedlot phase. Opportunities to capitalize on initial treatment investment are evident, but are dependent on the size of the production response and the economic conditions at time animals are marketed.