EFFECT OF INDUCED SUPRABASAL PROGESTERONE CONCENTRATIONS ON FOLLICULAR DYNAMICS IN HEIFERS

To investigate the influence of induced suprabasal plasma concentrations of progesterone after luteolysis on follicular dynamics, heifers were allocated to receive subcutaneous silicone implants containing 2.5 (n = 4), 5 (n = 4), 6 (n = 3), 7.5 (n = 3) or 10 g (n = 4) of progesterone, or a blank implant (controls, n = 5). The implants were intended to produce plasma progesterone concentrations under 3 nmol/L. They were inserted on day 8 of the cycle (day 0 = ovulation) and left in place for 17 days. Ovaries were ultrasonically scanned daily until day 14, at 12-h intervals from day 15 to 26, and daily again until day 30. The growth pattern of the dominant follicle of the first follicular wave was not different between control and treated heifers. The ovulatory follicle emerged on days 11-15, and in control heifers and heifers treated with 2.5 and 5 g of progesterone, it reached its largest diameter on days 19-22, just before ovulation. In heifers treated with 6, 7.5 and 10 g, the ovulatory follicle prolonged its growth for a longer period (P < 0.05) and to a larger diameter (P < 0.05), after which it reached a static phase before ovulation. The interovulatory interval was consequently prolonged (P < 0.05) as compared to controls. During the extended dominance of the ovulatory follicle there was a total absence of follicular recruitment, and a new follicular wave emerged shortly after ovulation. In heifers treated with 10 g, emergence of the postovulatory wave was delayed (P < 0.05). Suprabasal plasma concentrations of progesterone (above 1 nmol/L) were able to disturb the growth pattern of the ovulatory follicle. It is suggested that the model used is suitable for studies of the dysfunctions of follicular development that spontaneously occur in repeat-breeding heifers.