SERUM PROGESTERONE AND INITIATION OF OVARIAN ACTIVITY IN PREPUBERAL HEIFERS TREATED WITH PROGESTERONE

The influence of progesterone on the initiation of ovarian activity in 32 prepuberal Holstein heifers (250 kg) was studied by inserting subcutaneous single or double Silastic implants (60-cm 2 silicone rubber tubing containing 1 g progesterone or 120-cm 2 tubing with 2 g progesterone) or Silastic intravaginal devices (SID) (120-cm 2 tubing with 2 g progesterone) or a progesterone-releasing intravaginal device (PRID) (210-cm 2 coil with 2 g progesterone). Blood samples were collected every 6 h on days −1, 4, 8, 12, 16 and 20 after insertion. Serum progesterone, quantitated by radioimmunoassay, varied with treatment (P < 0.01), with the day following treatment (P < 0.01) and with the time of day (P < 0.01). The highest concentrations (mean ±SE 2.1 ± 0.06 ng/mL) appeared at 0600 h and the lowest (mean ± SE 1.7 ± 0.08 ng/mL) at 1800 h. Serum progesterone was consistently greater (P < 0.01) in the PRID group. Location (subcutaneous vs. intravaginal) of the 120-cm 2 tubing, i.e. double implant or SID, had no influence per se on serum progesterone. Heifers treated with the 60-cm 2 tubing, i.e. single implant, had lower (P < 0.01) serum progesterone concentrations than heifers in the other three treatment groups. When the implants and devices were removed on day 20, two females in each of the four treatments received either saline, 5 mg estradiol-17β (E 2 ), 500 IU PMSG, or 100 g GnRH. None of the heifers treated with the single implant ovulated. Of the saline-, E 2 -, PMSG- and GnRH-treated heifers, 0, 5, 6 and 2 showed estrus and 0, 2, 6 and 2 ovulated, respectively. PMSG-treated females had a mean of 2.5 ovulations. This study further demonstrates that adequate progesterone priming increases the response of prepuberal heifers to an ovulatory hormone stimulation. Key words: Progesterone, ovarian activity, prepuberal heifers, hormonal stimulation, silastic implants