# Evaluation of Extended-Release Eprinomectin Injectable and Doramectin Injectable with the Inclusion of Refugia on Performance Outcomes and Fecal Parameters in Stocker Cattle

## TL;DR

This study compares two anthelmintic treatments in cattle to improve growth and reduce parasite egg shedding while maintaining refugia to prevent drug resistance.

## Contribution

The study provides large-scale field data on the efficacy of extended-release eprinomectin versus doramectin in stocker cattle with refugia.

## Key findings

- Extended-release eprinomectin showed lower fecal egg counts at two time points compared to doramectin.
- A tendency for improved average daily gain was observed in the eprinomectin group.
- Maintaining refugia with untreated cattle was part of the experimental design to mitigate anthelmintic resistance.

## Abstract

Gastrointestinal nematodes may infect young, growing calves resulting in significant performance losses. Cattle entering stocker operations known for continuous grazing patterns may be at increased risk for the development of anthelmintic resistance. As such, tactics to maximize performance and mitigate the risk of anthelmintic resistance including the maintenance of refugia and/or combination treatment must be explored in these systems. The purpose of this experiment was to compare the effects on performance and parasite egg shedding of extended-release eprinomectin (ERE) and doramectin injectable (DOR), administered to cattle grazed with untreated cohorts serving as a source of refugia (REF). Steers procured by a commercial stocker operation were administered either ERE or DOR. A subset of animals (REF) was selectively not treated and commingled with the experimental treatment groups for the duration of the grazing period. A tendency for improved average daily gain was observed in the ERE group when compared to the DOR group. In addition, average fecal egg counts were lower at two time points in the 10% of calves sampled in the ERE group in comparison to the 10% sampled in the DOR group.

Evaluating different tactics to mitigate the effects of gastrointestinal nematode infection in growing stocker cattle is essential to better understand opportunities to optimize cattle health and performance. Due to the potential development of anthelmintic resistance, parasitologists and industry stakeholders have proposed maintaining refugia in cattle populations and combination treatment as tactics to delay anthelmintic resistance yet limited large-scale field data are available for practitioners to make evidence-based decisions. The objective of this experiment was to compare the effects of extended-release eprinomectin injectable and doramectin injectable on growth and fecal shedding of parasites in stocker calves grazing with non-treated (refugia) cohorts. Steers (n = 995; 243.38 kg) were randomized to one of two experimental treatment groups on Day 0, extended-release eprinomectin (ERE) or doramectin injectable (DOR). A subset of animals (n = 47) was selectively not treated with an anthelmintic to maintain refugia (REF). Individual body weights were recorded on days 0, 105, and 130 to calculate average daily gain and overall body weight gain during each of those time periods. Fecal samples were collected per rectum from approximately 10% of the same animals in each group on days 0, 105, and 130. Mean fecal egg count was significantly lower in the 10% of animals tested in the ERE group on days 105 and 130 when compared to 10% of the animals in the DOR group (Day 105—ERE: 46.45 eggs per gram, DOR: 155.30 eggs per gram, p < 0.01; Day 130—ERE: 9.65 eggs per gram, DOR: 22.51 eggs per gram, p = 0.02). From day 0 to 105, the mean average daily gain in the ERE group was 0.87 kg/day, which tended (p = 0.055) to be higher than the mean for the DOR group, 0.845 kg/day.