Fields that have received lots of rain may be showing sulphur deficiency. Rain leaches sulphur lower in the soil profile. Plants can usually access this leached sulphur later in the season, but the roots have to reach that far. Often with heavy rain throughout the season, plants will not root as deep. In this case, a top dress up to 10-20 pounds before or even at flowering can help.
If plants are not showing signs of deficiency but heavy rains and/or high yield potential make it highly probable, an application ahead of symptoms may be helpful. The earlier the better for a top dress. You probably don’t want to go later than 20-30% flower with a sulphur top dress as it will start to delay maturity, and leaves are starting to fall off at that stage and not very efficient. Earlier applications — at bolting or earlier — are much more effective.