Enclomiphene does not alter the postpartum interval of suckled beef cows.

The objective of this study was to determine whether an antiestrogen (enclomiphene) would shorten the interval to first estrus and conception in postpartum beef cows. Sixty postpartum Angus beef cows were stratified by age, body condition, and calving date and were randomly assigned to one of two treatment groups. Group 1 cows (n = 24) received three silastic implants, each containing 150 mg of enclomiphene, on d 20 postpartum. Implants were removed on d 30 postpartum. Group 2 cows (n = 28), received empty implants and served as controls. Cows were artificially inseminated at first detected estrus. Estrus detection and ovulation were further verified by increased serum progesterone. Concentrations and pulse frequencies of LH were determined from blood samples collected at 15-min intervals for 6 h on d 20, 25, 30, and 40 postpartum. Hypothalami and pituitaries were collected from four cows in each treatment group on d 30 postpartum and analyzed for concentrations of estradiol receptors. Concentrations of total and unoccupied hypothalamic and pituitary estradiol receptors were reduced by enclomiphene. Neither concentrations nor pulse frequencies of LH differed significantly between treatment groups on any of the 4 d. Days to first estrus did not differ (P greater than .05) between enclomiphene-treated (57 +/- 6; n = 24) and control (56 +/- 4; n = 28) cows. Days to conception did not differ between treated (81 +/- 9) and control (79 +/- 8) cows. The dose of enclomiphene used in this study reduced hypothalamic and pituitary estrogen receptors but did not alter secretion of LH or days to first estrus in the postpartum beef cow.