Ovarian responses in Bos indicus heifers treated to synchronise ovulation with intravaginal progesterone releasing devices, oestradiol benzoate, prostaglandin F2α and equine chorionic gonadotrophin

The objectives were: (i) improve understanding of the ovarian responses of Bos indicus heifers treated with different ovulation synchronisation protocols, (ii) compare ovarian responses of B. indicus heifers treated with intravaginal progesterone releasing device (IPRD) + oestradiol benzoate (ODB) versus a conventional prostaglandin F2α (PGF2α) protocol and (iii) investigate whether reducing the amount of progesterone (P4) in the IPRD, and treatment with equine chorionic gonadotrophin (eCG) would increase the proportion of heifers with normal ovarian function during the synchronised and return cycles. Two-year-old Brahman (n = 30) and Brahman-cross (n = 34) heifers were randomly allocated to three IPRD-treatment groups: (i) standard-dose IPRD (Cue-Mate® 1.56 g P4; n = 17); (ii) half-dose IPRD (Cue-Mate® 0.78 g P4; n = 15); (iii) half-dose IPRD + 300 IU eCG at IPRD removal (n = 14), and a non-IPRD control group (iv) 2 × PGF2α (500 μg cloprostenol) on Days −16 and −2 (n = 18). IPRD-treated heifers received 250 μg cloprostenol at IPRD insertion (Day −10) and IPRD removal (Day −2) and 1 mg ODB on Days −10 and −1. Ovarian function was evaluated by ultrasonography and plasma P4 throughout the synchronised and return cycles. The mean diameter of the dominant follicle observed at 54–56 h after IPRD removal, was greater for heifers which ovulated than heifers which did not ovulate (P < 0.001; 14.5 ± 1.1 vs. 9.3 ± 0.6 mm, respectively). The prevalence of IPRD-treated heifers with ovarian dysfunction (persistent CL, failure to re-ovulate, shortened luteal phase) was 39%. This relatively high prevalence of ovarian dysfunction may explain the commonly reported, lower than expected pregnancy rates to FTAI in B. indicus heifers treated to synchronise ovulation.