371 Evaluation of parasite resistance in cattle treated with extended-release eprinomectin (LongRange)

Abstract

The objective was to determine parasite resistance in cattle treated with extended-release eprinomectin (LongRange) from a herd with 3 years of prior eprinomectin use. Fall-born Angus x Simmental heifers (224 ± 22 d of age) were randomly assigned 1 of 3 treatments: LongRange (LR; n = 21), LongRange and Synanthic (COMBO; n = 21), or saline-treated control (CON; n = 21). At trial initiation, all heifers were administered treatments according to label instructions. All heifers grazed endophyte-infected tall fescue in groups (n = 3 per treatment) and supplemented daily modified wet distillers grain (0.91 kg as fed per heifer) and soybean hulls (0.91 kg as fed per heifer). Prior to trial initiation and monthly thereafter, BW, packed cell volume (PCV), and fecal egg counts (FEC) were determined. There was a treatment × date interaction ( P = 0.001) for fecal eggs counts. There were no differences ( P ≥ 0.53) in the FEC in May or June. In July and August, the LR and COMBO heifers had decreased ( P < 0.01) FEC compared with CON. By September there were no differences ( P = 0.84) in FEC. At d 28, there was no difference ( P ≥ 0.44) in FEC reduction between LR (91% FEC reduction) and COMBO (98% FEC reduction). There was no treatment or treatment × date interaction ( P ≥ 0.34) for PCV. There was a treatment × date interaction ( P = 0.01) for BW. There were no differences ( P ≥ 0.16) in BW during May, June, and July. In August the LR and COMBO heifers tended ( P = 0.09) to have greater BW than CON. In September and October, the LR and COMBO heifers had a greater ( P ≤ 0.01) BW than CON heifers. Parasite resistance to extended-release eprinomectin was not observed based on FEC reduction tests and similar BW of LR and COMBO heifers.