Follicle stimulating hormone secretion and dominant follicle growth during treatment of Bos indicus heifers with intra-vaginal progesterone releasing devices, oestradiol benzoate, equine chorionic gonadotrophin and prostaglandin F2α

The aim of this study was to investigate the effects on follicle stimulating hormone (FSH) secretion and dominant follicle (DF) growth, of treatment of Bos indicus heifers with different combinations of intra-vaginal progesterone releasing devices (IPRD), oestradiol benzoate (ODB), PGF2α and eCG. Two-year-old Brahman (BN; n = 30) and Brahman-cross (BNX; n = 34) heifers were randomly allocated to three IPRD-treatments: (i) standard-dose IPRD [CM 1.56 g; 1.56 g progesterone (P4); n = 17]; (ii) half-dose IPRD (CM 0.78 g; 0.78 g P4; n = 15); (iii) half-dose IPRD + 300 IU eCG at IPRD removal (CM 0.78 g + G; n = 14); and, (iv) non-IPRD control (2 × PGF2α; n = 18) 500 μg cloprostenol on Days −16 and −2. IPRD-treated heifers received 250 μg PGF2α at IPRD insertion (Day −10) and IPRD removal (Day −2) and 1 mg ODB on Day −10 and Day −1. Follicular dynamics were monitored daily by trans-rectal ultrasonography from Day −10 to Day 1. Blood samples for determination of P4 were collected daily and samples for FSH determination were collected at 12 h intervals from Day −9 to Day −2. A significant surge in concentrations of FSH was observed in the 2 × PGF2α treatment 12 h prior and 48 h after follicular wave emergence, but not in the IPRD-treated heifers. Estimated mean concentrations of total plasma P4 during the 8 days of IPRD insertion was greater (P < 0.001) in the CM 1.56 g P4 treated heifers compared to the CM 0.78 g P4 treated heifers (18.38 ng/ml compared with 11.09 ng/ml, respectively). A treatment by genotype interaction (P = 0.036) was observed in the mean plasma P4 concentration in heifers with no CL during IPRD insertion, whereby BN heifers in the CM 1.56 g treatment had greater plasma P4 than the BNX heifers on Days−9, −7, −6, −5, and −4. However, there was no genotype effect in the CM 0.78 g ± G or the 2 × PGF2α treatment. Treatment had no effect on the DF growth from either day of wave emergence (P = 0.378) or day of IPRD removal (P = 0.780) to ovulation. This study demonstrates that FSH secretion in B. indicus heifers treated with a combination of IPRDʹs and ODB to synchronise ovulation was suppressed during the period of IPRD insertion but no significant effect on growth of the DF was observed.