Vaginal Electrical Resistance and Size of Dominant Follicle in Beef Cows Subjected to Synchronization of Ovulation Protocol

Abstract

The objectives were to determine the efficiency of vaginal electrical resistance (VER) to indicate synchronization success and its correlation with the diameter of the largest follicle (DLF). Crossbred beef cows with more than two lactations from the beef farm at the Kamphaeng Saen campus, Kasetsart University were synchronized with the Co-Synch plus controlled internal drug release (CIDR) protocol that consisted of a CIDR insert and an intramuscular injection of gonadotropin-releasing hormone (GnRH; first GnRH; 2 mL) on Day 0, removal of the CIDR insert and an injection of Prostaglandin F2 alfa (PGF_2α; 2 mL) on day 7 and a second injection of GnRH (2 mL) at 48 or 66 h after PGF2α injection. VER was determined with an Ovatec device at Days 0, 7 and at the time of the second GnRH injection. Trans-rectal ultrasonography was performed on the same day to assess ovarian follicular development. The DLF (mean ± SE) was highest (16.3 ± 0.5 mm; P < 0.05) at the second GnRH injection and lowest at Day 0 (9.8 ± 1.0 mm). The VER (mean ± SE; measured in ohms Ω) was significantly highest (93.8 ± 1.7 Ω; P < 0.05) on Day 0 and lowest (79.1 ± 1.7 Ω) at the time of second GnRH injection. Mean VER was inversely correlated (r = -0.49; P < 0.05) with DLF at the second GnRH injection. The significant correlation obtained between VER and DLF indicated that VER can be used to predict the stage of follicular maturity and the response to synchronization treatment.