Agronomics: When to Harvest Soybeans

It’s fairly easy to predict corn maturity and harvest since development follows a calendar determined by temperature. Soybeans aren’t as easy, and with some fields of soybeans beginning to turn in Illinois, we realize that harvest can’t be too far off.

Soybeans go through eight reproductive stages; R7 and R8 are the last two.

• R7 – Beginning maturity — one mature pod found on the plant.
• R8 – Full maturity — 95% pods have reached mature pod color.

We all know corn reaches maturity when it forms a black layer at the base of the corn kernel. You can cut open a kernel longitudinally (top to bottom) and see that black layer. This black layer cuts off water, sugar and nutrient transfer into the kernel, and kernels can’t get any heavier. Grain moisture will be 34 or 35% and will dry down from there.

What about soybeans? Soybeans do not form a black layer, and for years I have thought that when soybean pods turn brown, gold or yellow, they would be mature. According to the Iowa State University soybean development guide, soybeans reach physiological maturity when 95% of the pods reach their mature color (brown, gold or yellow—but usually brown). Soybeans seeds will also contain about 35% moisture.

I have always gone by the Iowa State guideline of 95% of the pods at their mature color. However, you can split open pods to check whether the beans are separating from the white membrane that lines the inside of the pod. When seeds pull away from the membrane (which provides sugars and nutrients to the seed), they will no longer gain any more dry weight and will be mature.

Everyone knows that pods don’t mature evenly on a plant or across a field. There is a spread of 10 to 14 days between earliest-maturing and latest-maturing pods in a field. Pods usually will turn their mature brown color 4 to 8 days after reaching physiological maturity. Then it takes another 2 weeks after physiological maturity for soybeans to dry down 20 points to 13 to 14%.

While corn and soybeans seem to dry down initially at the same rate after physiological maturity, corn drydown seems to slow when it gets down to 18% and changes slowly after that.

Soybeans dry down very quickly after they get down to 14%. Beans will be 14% in the morning, 12% by noon and 8 or 9% by 4 p.m., especially on a sunny, warm and windy day. No matter how hard a farmer tries, it is hard to harvest all your beans at the 13 to 14% level where you can maximize yield.

As we approach soybean harvest, how are you tracking maturity and do you have some tricks up your sleeve to maintain moisture at around 13%?

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agrwrite.com