Читать книгу Bovine Reproduction - Группа авторов - Страница 368

It is well established that progesterone sensitizes the hypothalamus to the positive feedback effect of estradiol‐17β that results in the preovulatory LH surge. Progestin administration hastens puberty by accelerating the peripubertal decrease in estradiol‐17β negative feedback effect, thereby facilitating the LH surge [72]. Furthermore, progesterone priming has been shown to enhance follicular estradiol‐17β synthesis in ewes [70] and it has also been demonstrated that circulating estradiol‐17β concentrations were higher at the first ovulatory estrus than in the preceding silent estrus [71]. As an example of the use of an exogenous progestin, melengestrol acetate (MGA) fed for eight days followed by withdrawal, in turn, enhanced onset of puberty by stimulating pulsatile LH secretion that accelerated follicle growth to the preovulatory stage [73]. In synchronization programs for heifers that may be peripubertal and prone to premature luteolysis, the incorporation of a progestin improves pregnancy rates. Treatment with some progestins, but not others, before first ovulation was able to eliminate the occurrence of short‐lived corpora lutea [74–76]. Specifically, progesterone administered via intravaginal implant for five days was found to be more effective in reducing incidence of premature luteolysis after GnRH‐induced ovulation than oral MGA [75].