Читать книгу Bovine Reproduction - Группа авторов - Страница 356

Puberty is a critical physiological milestone in a heifer's reproductive life. In general terms, puberty can be defined as the process whereby animals become capable of reproducing themselves [1]. At the onset of puberty, a rise in plasma progesterone concentrations indicates cyclic ovarian activity before the first observed estrus [2]. Thus, puberty can be defined as the first day that serum progesterone (determined in blood samples collected at weekly intervals) exceeds 1 ng/ml [3]. Other authors define puberty as when ovulation is accompanied by visual signs of estrus with subsequent normal luteal function as evidenced by serum progesterone concentration above baseline for over two consecutive weeks [4]. Puberty involves a complex series of interactions between genetic, nutritional, and environmental factors that direct endocrine events that culminate in the attainment of reproductive capacity. In heifers, puberty is triggered when the hypothalamic–pituitary–gonadal axis first loses its sensitivity to the negative feedback effects of estradiol‐17β, allowing a surge of luteinizing hormone (LH) to occur [5]. It is now accepted that puberty and first ovulation are not necessarily coincident since in most heifers “silent” ovulations and short luteal phases may occur during the peripubertal phase [5].