Choosing Soybean Maturity Group for Late Planting

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Everything is behind this 2013 season due to the excessive rain. For those planting soybeans late, you may be wondering which maturity group to choose. Dr. Jim Dunphy from NCSU offers his advice below:

Another question that arises with the extremely late soybean planting dates our farmers are hoping for is whether you should change maturity groups, and if so, to what. I would stick with the later maturing end of the range of maturities that you’re normally comfortable with. With planting dates of mid-July or later, I think getting the plants big enough to get the job done is a bigger challenge than beating a frost or avoiding a hurricane. So I’d stick with the varieties which mature later, and more importantly quit growing later, and thus have a few more days to get their vegetative growth. Since they’ll probably be less than 3 feet tall, every extra leaf is a pretty valuable leaf. That’s the reverse of the recommendation we usually make for corn.

As a rule of thumb, if you plant a variety a month later (e.g. July 15 instead of June 15), you delay maturity about 10 days. A mid group VII variety will mature, and more importantly quit growing, about 10 days later than a mid group VI variety. An indeterminate variety will flower for a longer period of time, and will keep growing for several weeks after it starts flowering, but mostly because it starts flowering earlier, not because it keeps growing longer. Planted on the same date, a group III variety will usually quit growing earlier than a group IV variety will, which, even though it is typically indeterminate, will quit growing earlier than a typically determinate group V variety will, which will quit growing earlier than a group VI variety, etc.

A soybean doesn’t have to be out of the field before a frost. It just has to be physiologically mature (safe from frost) before a frost. While physiological maturity isn’t as easy to determine on soybeans as it is on corn (black layer), it typically occurs about 10 days before it is ready to harvest. And it doesn’t have to beat the first frost, it just has to beat the first killing frost, which typically occurs 7 to 10 days or more after the first frost.

I’m pretty sure I’ll get a serious yield reduction from plants that are less than 3 feet tall. I may or may not get an early frost. I may or may not get a hurricane while my beans are still in the field. The hurricane may or may not keep me out of the field until the beans start rotting in the pods. If I don’t produce the yield, it won’t matter much whether a frost or a hurricane damages the crop.