The theory is simple. Plant the fullest-season hybrid you can for your area of the state. Since Indiana is a long state, maturity group ranges that can be grown successfully range from Group II near Ft. Wayne and LaGrange to Group IV near Evansville. The maturity rating system starts with low number farther north- you can actually buy a Group 0 bean for planting in the extreme northern U.S. or Canada.
Common theory holds that you want to plant the variety that will make best use of the season. The longer a variety has to grow, the more it should yield. That would mean that Group II beans would be the answer if the question said 'what is the fullest season variety to be grown in northeast Indiana?'
Since the question asked about Evansville, the goal generally is to find a variety that matures later so that it can take advantage of the entire growing season, soaking up heat units as the corps move along. However, in practice, that's not always the case. Sometimes timing of rainfalls can determine whether a field gets a key rain or not make a big difference in yield.
Phil De Villez, director of Yield Performance trials in corn and soybeans for Purdue University, says that in plots they've run so far in southwest Indiana, the Group III varieties tend to have an edge over early Group IV varieties this year. At least that's the way his test plots are catching up, he notes.
Timing of flowering and weather patterns can allow different scenarios to play out when there are differences in expected maturity, and blatant differences in weather forecasts.
De Villez said there are cases where group IV soybeans yield better than Group IIIs. This was one of those years when everything was out of kelder.