The anthesis date that we use for soybean is first flower, i.e., when 50% of
the plants have expressed their first flower (R1 stage). For a uniform
cultivar, this happens virtually in 1 day, yesterday there was no or little
flowering, and today more than 50% of the plants are flowering. Tomorrow,
more than 80% of the plants will have flowered. See the Fehr and Caviness
reference, Crop Sci. 10:929.
Good luck,
Ken Boote