INFLUENCE OF DIETARY ENERGY ON ENDOCRINE MECHANISMS CONTROLLING REPRODUCTIVE FUNCTIONS IN THE BOVINE FEMALE

A series of experiments was conducted to elucidate endocrine mechanisms responsible for controlling reproductive functions in estrual and nutritional anestrous beef heifers. In experiment 1, the feedback mechanisms of ovarian steroids in controlling luteinizing hormone (LH) secretion during the follicular phase of the bovine estrous cycle were studied. Serum LH and 17(beta)-estradiol (E(,2)) increased linearly (P \u3c 0.01) whereas no change in amplitude of LH pulses was detected before the preovulatory LH surge. In experiment 2, 12 cross-bred heifers were used to determine patterns of body weight changes in heifers fed diets varying in energy, and to relate these findings to changes in concentrations and secretory patterns of reproductive hormones. At the time heifers fed a diet limited in energy stabilized body weight after rapid weight loss while heifers fed a diet high in energy did not gain body weight, body weight changes positively affected (P \u3c 0.001) mean concentrations of LH during the follicular and subsequent luteal phases of the estrous cycle. Serum concentrations of progesterone and E(,2) were not related to weight changes. During the latter part of the weight stabilization phase in heifers fed the diet restricted in energy while heifers fed the diet high in energy gained body weight, however, body weight changes had a lesser influence (P \u3c 0.05) on concentrations of LH. Serum progesterone and E(,2) concentrations were positively related to body weight changes at this time. In experiment 3, effects of dietary energy restriction and E(,2) on LH secretions were studied using untreated ovariectomized and E(,2) treated ovariectomized heifers. Frequencies of LH pulses in untreated ovariectomized heifers fed the low energy (L) diet were lower (P \u3c 0.01) than in heifers fed the high energy (H) diet. Amplitudes of LH pulses in heifers fed the L diet were higher (P \u3c 0.01) than in heifers fed the H diet. Pulses and mean concentrations of LH were suppressed (P \u3c 0.01) in E(,2) treated ovariectomized heifers fed the L diet long before nutritional anestrus was induced. In experiment 4, endocrine mechanisms during restoration of estrous cycles following nutritional anestrus were studied. Estradiol suppressed (P \u3c 0.01) LH secretion in E(,2) treated ovariectomized heifers during the period of estrual restoration from nutritional anestrus in intact heifers. Suppressive effects of E(,2) on LH secretion continued in E(,2) treated ovariectomized heifers after intact heifers resumed estrous cycles