PSIV-26 Effects of extended-release eprinomectin on heifer growth and reproductive performance while grazed on endophyte-infected tall fescue or housed in drylot

Abstract

The objective was to assess the effects of an extended-release eprinomectin (LongRange) on beef heifer reproductive performance when grazed on endophyte-infected tall fescue or housed on drylot. In a split-plot design, fall-born, Angus × Simmental heifers (n = 153) were stratified by BW and divided into 6 groups. Groups were randomly assigned environmental treatments: grazed endophyte-infected tall fescue (pasture) or maintained in drylot and fed alfalfa hay. Within a group, heifers were randomly assigned to LongRange (LR) or saline (control). Heifers were dewormed with oral fenbendazole (SafeGuard) to minimize parasite load before trial initiation. On d 140 and final pregnancy check, body weight (BW) and body condition score (BCS) were collected; BW was also collected at time of artificial insemination (AI). Hair coat score (HCS), fecal egg count (FEC), packed cell volume (PCV), and respiration rates (RR) were collected on d 140. Data were analyzed using the MIXED procedure of SAS. Environment × treatment effects for BW were detected (P ≤ 0.01) at d 140 and at overall pregnancy confirmation; LR increased BW to a greater magnitude in pasture than in drylot heifers. At breeding, LR increased (P = 0.01) heifer BW compared to control. On d 140, LR increased (P = 0.01) heifer BCS compared to control. On d 140, pasture heifers tended (P = 0.08) to have greater RR than drylot heifers. Heifer AI pregnancy rates were not affected (P ≥ 0.21) by treatments. Environment × treatment effects for overall pregnancy were detected (P = 0.03); LR increased overall pregnancy rate in pasture compared to drylot heifers and LR tended to increase overall pregnancy rate compared with control in pasture heifers. In conclusion, LongRange increased BW to a greater extent in pasture than in drylot and tended to increase heifer final pregnancy rate compared to control on pasture.