Ovarian follicular development and hormone concentrations in inseminated dairy cows with resynchronized estrous cycles

The objective was to investigate ovarian follicular development and hormone concentrations in previously inseminated cows with estrous cycles resynchronized with various resynchronization treatments. Lactating dairy cows were treated with a previously used intravaginal progesterone releasing device (IVD) for 7 d (EB + IVD 7 + EB, n = 15) or 8 d (EB + IVD 8 + EB, n = 16), starting 13 d (Day 13) after a first estrus (Day 0) and AI. Estradiol benzoate (EB; 1 mg im) was given at device insertion and 24 h after removal. Other cows were given the same treatment as the EB + IVD 8 + EB cows, but were not treated with EB at IVD insertion (IVD 8 + EB, n = 11). There were no differences (P > 0.05) between EB + IVD 7 + EB and EB + IVD 8 + EB treatments for follicle dynamics and plasma progesterone concentrations during treatment. Based on a comparison between the IVD 8 + EB treated cows and the pooled results of the EB + IVD 7 + EB and EB + IVD 8 + EB treated cows, EB at device insertion increased the number of follicular waves between Days 13 and 20 (mean ± S.E.M.; 2.3 ± 0.14 vs 2.7 ± 0.10, P = 0.033), delayed emergence of follicles that were dominant or emerging on Day 20 (17.2 ± 0.36 vs 14.1 ± 0.65 d, P < 0.001), reduced diameters of dominant or emerging follicles on Day 20 (9.0 ± 0.58 vs 12.7 ± 0.59, P < 0.001), and reduced plasma progesterone concentrations by 0.85 ± 0.44 ng/mL (P = 0.059) during treatment. Furthermore, comparison of the IVD 8 + EB to the EB + IVD 8 + EB treated cows demonstrated that treatment with EB at device insertion also reduced the diameter of ovulatory follicles (14.2 ± 0.58 vs 19.0 ± 0.71 mm, P = 0.001), delayed emergence of ovulatory follicles (17.0 ± 0.32 vs 13.5 ± 1.26, P = 0.020), and reduced the interval from emergence to ovulation (7.0 ± 0.32 vs 10.5 ± 1.26 d, P = 0.020). We concluded that administration of EB altered ovarian follicular dynamics and tended to reduce plasma progesterone concentrations during treatment with an IVD that was used to resynchronize estrous cycles. However, use of a 7-d compared to an 8-day treatment with an IVD did not significantly affect follicle dynamics nor plasma progesterone concentrations during treatment.