CHANGES IN OVARIAN-FUNCTION AND GONADOTROPIN-SECRETION PRECEDING THE ONSET OF NUTRITIONALLY INDUCED ANESTRUS IN BOS-INDICUS HEIFERS

The aim of this study was to monitor endocrine and ovarian changes imm ediately preceding the onset of nutritionally induced anestrus. Daily blood samples were obtained from 14 postpubertal heifers for one estro us cycle (initial estrous cycle). Subsequently, heifers designated ''r estricted'' were given a limited diet (n = 9), and daily blood samples were obtained for approximately 21 days preceding onset of anestrus ( anovulatory cycle). Controls were allowed ad libitum dietary intake (n = 5), and daily blood samples were collected for a complete estrous c ycle during a time period that coincided with that preceding onset of anestrus in restricted heifers. Plasma samples were assayed for LH, FS H, progesterone, and estradiol-17 beta. The ovaries of all heifers wer e examined daily using transrectal ultrasonography from the initial un til the final or anovulatory estrous cycles to determine changes in gr owth of follicles and corpora lutea. Anestrus was defined as failure o f ovulation of the dominant follicle following luteolysis. When anovul atory and initial estrous cycles in restricted heifers were compared, mean concentrations of LH were lower (p < 0.01), and diameters of domi nant follicles were smaller (p < 0.01); mean concentrations of estradi ol-17 beta were also lower in the three days following luteolysis (P = 0.06), but concentrations of FSH appeared to be higher (p = 0.003); m aximum diameters of corpora lutea were smaller (p < 0.001), but durati on of luteal phases and concentrations of progesterone preceding luteo lysis were similar (p > 0.10). In controls, no differences were found between estrous cycles for any of these variables. It is concluded tha t failure of ovulation, following reduced dietary intake, resulted fro m insufficient circulating LH to stimulate maturation of the ovulatory follicle.