Assessment of vaginal electrical resistance (VER) as an indicator of follicular maturity and suitability for timed AI in cows subjected to a synchronization of ovulation protocol.

Objectives were to use retrospective analysis to determine the efficacy of VER for identifying cows with and without a mature preovulatory follicle (defined as ≥ 10 mm) at timed AI (TAI) and as a prospective decision aid for determining cows that should not be bred. Brahman x Hereford (F1) females (n=233) were synchronized with the CO-Synch + CIDR protocol that consisted of an Eazy-Breed CIDR and i.m. injection of GnRH (GnRH-1; 100 μg) on d 0, removal of CIDR and i.m. injection of Prostaglandin F2α (PGF; 25 mg) on d 7, and i.m. injection of GnRH (GnRH-2, 100 μg) and TAI 66 h after CIDR removal (d 10). A commercially-available device (Ovascan; Animark Inc., Aurora, CO) was used to determine VER at d 0, 7, and 10. Transrectal ultrasonography was used on d 10 to assess ovarian morphology at TAI in all cattle and in a subset of females (n=98) on d 0 and 7. Mean (± SEM) age, BCS, BW and d postpartum were 7.2 ± 0.3 yr, 5.2 ± 0.1, 538 ± 5.3 kg, and 77 ± 1.1 d, respectively. Mean VER (ohms) was greatest (101.8 ± 1.1) on d 0 and declined (P < 0.01) to 95 ± 0.7 and 82.4 ± 0.7 ohms, respectively, on d 7 and 10. We observed a low negative relationship (r = - 0.35; P < 0.001) between follicular size and VER on d 0, 7, and 10. Timed AI conception rate was correlated positively (r = 0.16; P < 0.05) with follicular size and negatively (r = - 0.15; P < 0.05) with VER at TAI. Timed AI conception rate was also correlated with changes in follicular size (FollSizediff; r = 0.24; P < 0.05) and VER (VERdiff; r = - 0.18; P < 0.01) between d 10 and 7. However, VERdiff and FollSizediff were not correlated. Vaginal electrical resistance at TAI and VERdiff did not differ between females that became pregnant and those that did not. Females without a large follicle on d 10 were not identifiable with VER. [ABSTRACT FROM AUTHOR]