Evaluation of vaginal electrical resistance as an indicator of follicular maturity and suitability for timed artificial insemination in beef cows subjected to a synchronization of ovulation protocol

Objectives were to test the hypothesis that vaginal electrical resistance (VER) could be used to identify cows without a large (<10 mm) follicle at timed-AI (TAI) following a synchronization of ovulation protocol and thus serve as a prospective decision aid for determining cows that should not be inseminated. Brahman x Hereford (F1) females (n = 233) were synchronized with the CO-Synch + CIDR protocol that consisted of a controlled internal drug release (CIDR) insert and i.m. injection of GnRH (GnRH-1; 100 μg) on day 0, removal of CIDR and i.m. injection of prostaglandin F2α (PGF; 25 mg) on day 7, and i.m. injection of GnRH (GnRH-2, 100 μg) and TAI 66 h after CIDR removal (day 10). Vaginal electrical resistance was determined with a commercially available device (Ovascan; Animark Inc., Aurora, CO) at days 0, 7, and 10. Transrectal ultrasonography was used on day 10 to assess ovarian morphology at TAI in all cattle and in a subset of females (n = 98) on days 0 and 7. Mean (±S.E.M.) age, body condition score (BCS), BW and days postpartum were 7.2 ± 0.3 years, 5.2 ± 0.1, 538 ± 5.3 kg, and 77 ± 1.1 days, respectively. Mean VER (Ω) was greatest (101.4 ± 0.8) on day 0 and declined (P < 0.01) to 95.2 ± 0.8 and 82.0 ± 0.8 Ω, respectively, on days 7 and 10. Mean diameter of the largest follicle and VER values in females conceiving after TAI differed (P = 0.05) from those that did not conceive. Mean VER on days 7 and 10 and VER difference (VER on day 10 minus VER on day 7) did not differ between females with a small (<10 mm) or large (≥10 mm) follicle at TAI. Timed-AI pregnancy rate was greater (P < 0.01) for females with large follicles (43%) than those with small follicles (22%). Vaginal electrical resistance difference values, categorized as negative (<0 Ω) or neutral/positive (≥0 Ω), did not differ between females that conceived to TAI compared with those that did not. We conclude that VER measurements, as used in the present study, are not adequately sensitive to differentiate between females with and without a large follicle and thus are unable to serve as a prospective decision aid for determining suitability for TAI after synchronization.