Content area
Abstract
Endocrine mechanisms which control puberty in heifers were evaluated in two experiments. Prepubertal changes in cytosolic estradiol receptors (preoptic area, hypothalamus and pituitary), pituitary receptors for LHRH, hypothalamic LHRH content, ovarian follicle growth and uterine growth were evaluated in the first experiment. Fifty prepubertal heifers which averaged 249 days of age were used. Secretion of LH was monitored in six heifers from 249 days of age until puberty and frequency of LH pulses detected was regressed on days prepubertal. The resulting regression equation was used to predict puberty for heifers (n = 30) which were sacrificed at 253, 302 or 351 days of age (frequency of LH pulses was determined 3 to 5 days before sacrifice). The concentration of receptors for estradiol in the anterior and medial basal hypothalamus and in the anterior pituitary declined as puberty approached. Uterine weight increased rapidly during the 50 days preceding puberty. Other endocrine and morphological characteristics evaluated did not change as puberty approached.
The second experiment evaluated the effect of restricted intake of energy during the prepubertal period on secretion of LH and pituitary responsiveness to LHRH. Starting at 299 days of age, heifers (n = 23) were fed two levels of energy which either allowed spontaneous onset of puberty (.79 kg weight gain/day; CONTROL) or delayed puberty (.21 kg weight gain/day; DELAYED). These diets were provided for 175 days. Eleven of 12 CONTROL heifers attained puberty during the feeding period whereas none of the DELAYED heifers reached puberty during this time. Serum concentration of LH and frequency of LH pulses increased rapidly during the 175 day feeding period in CONTROL heifers. In DELAYED heifers, serum LH increased less rapidly and no increase in pulse frequency was detected during the experimental period. Amplitude of LH pulses was higher in CONTROL than DELAYED heifers. Responsiveness of LH secretion to LHRH was lower in DELAYED than CONTROL heifers.
