REPRODUCTIVE-PERFORMANCE OF HEIFERS INDUCED TO ESTROUS ASYNCHRONY BY SUPRABASAL PLASMA PROGESTERONE LEVELS

Suprabasal progesterone concentrations around oestrus have induced dis turbances in oestrous behaviour and ovulation. To determine whether fe rtility in such an altered oestrus can be maintained at normal levels with additional inseminations (AI) until ovulation, fertility was comp ared in heifers (n = 11) inseminated in normal oestrous cycles and the reafter in cycles in which the animals were treated with progesterone in order to create suprabasal concentrations after luteolysis. The tre atment consisted of silicone implants containing 10.6 mg kg(-1) of pro gesterone inserted subcutaneously on Day 8 of the oestrous cycle (day of ovulation designated Day 0) and removed on Day 25. Both in control, oestrous cycles and oestrous cycles under progesterone treatment, gro wth of the ovulatory follicle and ovulation were determined by frequen t ultrasound scanning. Blood was collected frequently for further anal ysis of progesterone, oestradiol-17 beta and luteinising hormone (LH). Insemination was performed 12 h after onset of standing oestrus. If o vulation did not occur 24 h after AI, heifers were inseminated again u ntil ovulation. Pregnancy was diagnosed by ultrasound 25 days after ov ulation. In control oestrous cycles, plasma progesterone decreased to 0.3 +/- 0.3 nmol 1(-1). Duration of oestrus was 22.9 +/- 2.0 h, the in terval from onset of oestrus to ovulation was 32.4 +/- 2.3 h and the i nterval from LH peak to ovulation was 28.6 +/- 1.4 h. The interovulato ry interval was 20.7 +/- 0.6 days. In oestrous cycles in treated heife rs, progesterone decreased to 1.0 +/- 0.3 nmol 1(-1) (P > 0.10) and th e interovulatory interval was prolonged to 23.5 +/- 1.0 days (P < 0.05 ). Standing oestrus lasted 47.2 + 12.0 h (P = 0.09, n = 7). The interv al from the onset of oestrus to ovulation was 59.4 +/- 13.0 h (P = 0.0 8) and the interval from LH peak to ovulation 25.8 +/- 1.3 h (P > 0.10 ). The prolonged oestrus was associated with increased (P < 0.05) grow th of the ovulatory follicle and higher (P < 0.05) release of oestradi ol-17 beta. Conception rates were 90% and 46% (P < 0.05), and the numb ers of AI per heifer were 1.1 +/- 0.1 and 3.4 +/- 0.6 (P < 0.01) for c ontrol oestrous cycles and after treatment, respectively. The inductio n of suprabasal concentrations of progesterone caused asynchronies sim ilar to those observed in cases of repeat breeding. The repeated AI di d not maintain fertility at normal levels. It is suggested that the ex tended growth of the ovulatory follicle may cause impaired oocyte matu ration or it may alter the maternal milieu owing to the prolonged rele ase of oestradiol.