Effects of Spring Administration of Extended-Release Eprinomectin (LongRange) on Fescue Toxicosis, Performance, and Reproduction of Fall-Born Beef Heifers

The objective of this experiment was to assess the effects of eprinomectin (LongRange), an extended-release injectable parasiticide, on fescue toxicosis and its impacts on beef heifer performance and reproduction. Fall-born, Angus x Simmental heifers (9 mo; 264.8 ± 21.1 kg BW) were randomly assigned 1 of 2 treatments: LongRange (LR; n = 100) or control (CR; saline; n = 99). Prior to trial initiation, heifers were dewormed with oral fenbendazole (SafeGuard) to minimize parasite load. At trial initiation (May 29, 2015), treatments were administered at a rate of 1 mL / 50 kg BW. All heifers grazed endophyte-infected tall fescue as a single group and were offered a supplement (1.36 kg DM / heifer·d^-1) for the duration of the experiment. Body weights, BCS, hair coat score (HCS), blood, and fecal samples were collected 10, 10, 6, 4, and 3 times, respectively throughout the trial. A subset of 40 heifers were randomly selected (20 from each treatment) to assess respiration rate (RR) on d 1, 54, and 110. On d 138, heifers began a 14-day controlled internal drug release-PG synchronization protocol. Following AI, heifers were exposed to 5 bulls for 71 d. On d 214 and 291, AI and overall pregnancy were determined. Body weight, ADG, BCS, HCS, RR, and fecal egg counts were analyzed using the MIXED procedure of SAS. Reproductive data were analyzed using the GLIMMIX procedure of SAS. Heifer served as the experimental unit. Heifer ADG was greater (P ≤ 0.01) for LR heifers throughout the study. From d 112 to 291, LR heifers had greater (P ≤ 0.01) BCS than CR. On d 83 and 112, LR heifers tended (P ≤ 0.10) to have slicker hair coats than CR; however, neither serum prolactin levels nor RR differed (P ≥ 0.58) between treatments. Fecal egg counts did not differ (P ≥ 0.16) on d -1, or 55; however, on d 111, LR heifers had decreased (P < 0.01) fecal egg counts compared to CR (1.5 vs. 13.6 eggs / g). LongRange heifers tended (P = 0.10) to have greater AI pregnancy rates (69% vs. 58%) and had greater (P = 0.01) overall pregnancy rates (84% vs. 68%) than CR. LongRange administered in the spring increased ADG, BCS, AI, and overall pregnancy in fall-born beef heifers. However, the underlying mechanism is still unclear as there were minimal to no differences in HCS, RR, serum prolactin, and fecal egg counts.