Regulation of synthesis and pattern of gonadotropin secretion and reproductive function in the bovine female

Influence of estradiol on steady-state concentrations of gonadotropin subunit mRNAs during sexual maturation in the heifer was evaluated in experiment 1. Estradiol suppressed concentrations of gonadotropin subunit mRNAs and a differential effect of estradiol during sexual maturation was detected. However, prepuberal heifers had sufficient hypothalamic stimulation by LHRH to maintain gonadotropin subunit mRNAs. The influence of direction of body weight change and level of body composition on pattern of gonadotropin secretion during restriction in dietary energy was investigated in experiments 2 and 3. By the end of experiment 2, heifers attained equivalent body weight from opposite directions of body weight change. Frequency of LH pulses declined while amplitude of LH pulses increased with decreasing daily gain. Restriction in dietary energy in heifers with low body condition resulted in an increased amplitude of LH pulses and response to 750 ng LHRH. Moreover, heifers with high body condition were refractory to influences of restriction in dietary energy. Experiment 4 was conducted to evaluate the effect of bull exposure and growth rate of heifers on attainment of puberty. A greater proportion of heifers receiving bull exposure attained puberty by 12, 13, 14, 15 and 16 months of age than non-exposed heifers. There was a bull exposure by growth rate interaction for age at puberty in heifers. Earlier age at puberty was associated with increased proportions of heifers becoming pregnant to AI. The influence of varying levels of progesterone on pattern of LH secretion during the estrous cycle in the cow was investigated in the final experiment. Cows administered subnormal levels of progesterone had higher concentrations of estradiol and frequency of LH pulses compared to control cows during the luteal phase. The preovulatory surge of LH occurred 17 hours earlier in cows receiving subnormal levels of progesterone compared to control cows. Function of the corpus luteum during the subsequent estrous cycle was not influenced by level of progesterone