Berardinelli and Adair (1989) observed a tendency for beef heifers treated with PGF2α at the mid-luteal stage of the cycle to have a longer interval to estrus compared with heifers treated early or late in the estrous cycle.Sirois and Fortune (1988) found that the size of the dominant follicle when luteolysis was complete in heifers was negatively related with the interval from treatment with PGF2α to the onset of estrus. Cows treated with PGF2α on d 10 to 13 of the estrous cycle would probably have smaller dominant follicles after luteolysis