EFFECT OF INDUCED SUPRABASAL PROGESTERONE LEVELS AROUND ESTRUS ON PLASMA-CONCENTRATIONS OF PROGESTERONE, ESTRADIOL-17-BETA AND LH IN HEIFERS

A controlled study was carried out to investigate the effects of supra basal plasma progesterone concentrations on blood plasma patterns of p rogesterone, LH and estradiol-17 beta around estrus. Heifers were assi gned to receive subcutaneous silicone implants containing 2.5 g (n=4), 5 g (n=4), 6 g (n=3), 7.5 g (n=3) or 10 g (n=4) of progesterone, or i mplants without hormone (controls, n=5). The implants were inserted on Day 8 of the cycle (Day 0=ovulation) and left in place for 17 d. The time of ovulation was determined by ultrasound scanning. Blood was col lected daily from Days 0 to 14 and at 2 to 4-h intervals from Days 15 to 27. Control heifers had the lowest progesterone concentrations on D ays 20.5 to 21 (0.5 +/- 0.1 nmol L(-1)); a similar pattern was observe d in heifers treated with 2.5 and 5 g of progesterone. In the same per iod, mean progesterone concentrations in the heifers treated with 6, 7 .5 and 10 g were larger (P < 0.05) than in the controls, remaining bet ween 1 and 2.4 nmol L(-1) until implant removal. A preovulatory estrad iol increase started on Days 16.4 to 18.4 in all the animals. In the c ontrols and in heifers treated with 2.5 and 5 g of progesterone, estra diol peaked and was followed by the onset of an LH surge. In the remai ning treatments, estradiol release was prolonged and increased (P < 0. 05), while the LH peak was delayed (P < 0.05) until the end of the inc rease in estradiol concentration. The estrous cycle was consequently e xtended (P < 0.05). In all heifers, onset of the LH surge occurred whe n progesterone reached 0.4 to 1.2 nmol L(-1) The induction of suprabas al levels of progesterone after spontaneous luteolysis caused endocrin e asynchronies similar to those observed in cases of repeat breeding. It is suggested that suprabasal concentrations of progesterone around estrus may be a cause of disturbances oestrus/ovulation.