Defining the gonadotrophin requirement for the selection of a single dominant follicle in cattle

The aim was to define the pattern and physiological concentrations of FSH and LH required for the selection of a single dominant follicle in mono-ovulatory species. A series of five experiments was carried out using gonadotrophin-releasing hormone agonist-induced hypogonadal heifers. Animals were infused with different patterns of either FSH and/or LH followed by an ovulatory dose of human chorionic gonadotrophin. Follicular response was monitored by ultrasound scanning and blood samples were collected to measure concentrations of FSH, LH, oestradiol and progesterone. The main findings were: (1) physiological concentrations of FSH given as a continuous infusion and for an adequate duration, in the presence of basal LH, with or without LH pulses, are capable of inducing a superovulatory response, (2) initial exposure to FSH followed by LH pulses alone stimulate the development of multiple preovulatory follicles, confirming that ovarian follicles are capable of transferring dependence on gonadotrophins from FSH to LH, (3) while LH pulses appear not to have a major effect on the pattern of preovulatory follicle development, adequate LH pulsatile support is required for full oestradiol synthesis and (4) the duration of initial exposure to FSH and the ability to transfer the dependence from FSH to LH are critical for the selection of a single dominant follicle. In conclusion, this experimental series confirms that the duration of initial exposure to FSH and the ability of the selected follicle to transfer its gonadotrophic dependence from FSH to LH are critical for the selection of a single dominant follicle in cattle.