The rule of thumb amongst agronomists is that you plant the fullest- season variety or hybrid that you can in your area with legitimate hopes that it will mature before a typical killing frost. The idea is that the fuller the season the variety, the more it should benefit from the extended growing season, and the higher the yield should be.
It sounds good in theory, but it doesn't always bear itself out. Based on his research plots at the Southwest Purdue Ag Center near Vincennes, Chuck Mansfield says 2011 was one of those years. He plants a late group III, about a 4.0 and a mid to later group 4 at the Vincennes locations. Based on the location in southern Indiana, one would expect the latest maturing variety to take advantage of the extra growing season and yield the best.
"That's not what we saw this year," he says. "It appears it was likely our lowest yielder. The soybeans in the pods were much smaller in comparison to the soybeans in the pods of the other varieties."
Mansfield speculates that the earlier-maturing varieties flowered somewhat earlier than the full-season variety. Since it did not rain late in the season at their location, when the full-season variety was flowering and reproducing, there was less than the desirable amount of water available.
As a result, soybean size in the latest Group four variety in the test was smaller than normal, and smaller than the size of soybeans in the other two varieties. Seed size is a major component of determining soybean yield.
Fuller-season varieties tend to begin flowering somewhat later than earlier-maturing varieties, Mansfield notes. While response and initiation of the reproductive phase in soybeans is largely tied to the amount of light the plants receive per day, the fullest season varieties still starts flowering somewhat later than earlier-season varieties. At Vincennes, at least, it appears that the fullest season varieties drew the short end of the straw, and found less moisture available to finish forming beans and filling pods at key times this year.