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When are soybeans done for the season?When are soybeans done for the season?

Soybean Watch: Here’s how to determine if rain or irrigation could still help yield.

Tom J. Bechman

September 22, 2023

3 Min Read
hands holding two open soybean pods
LOOK FOR MEMBRANES: A farmer holds an open pod from a plant nearing maturity (left). The beans are no longer covered with a membrane, while the soybeans in the greener pod in Steve Gauck’s hands are (right). Tom J. Bechman

The Purdue Corn and Soybean Field Guide states that at full maturity, known as the R8 stage, 95% of soybean pods have reached that variety’s mature pod color. But until then, it’s still “game on” as far as soybean plants still trying to pack the last bit of yield into beans.

“We’ve discovered that soybeans are still adding weight to individual beans inside of pods even if plants are turning yellow,” says Steve Gauck, a regional agronomy manager for Beck’s, sponsor of Soybean Watch ’23. “Many people have traditionally thought that once soybeans start turning yellow, the season is over, but that’s not really true. In fact, there is evidence suggesting that soybean plants can be 50% yellow or more but still adding weight and size to individual beans within pods.”

Why is that important to know? “If you get rain at that stage after it’s been dry, soybeans may be able to use the moisture to top off yield,” Gauck says. “If you irrigate, it’s even more important. If it’s dry late in the season, this information means you don’t want to stop irrigating and applying moisture for the crop until soybean plants are at least 50% yellow.

“Work by several agronomists recently has shown that curtailing irrigation too early can cost you up to 10 bushels per acre, depending upon how early you pull the plug on irrigating. There is still a lot more going on inside soybean plants very late in the season, compared to how we used to assume if leaves were yellowing and some were falling, the season was over for those plants.”

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Other clues

If you’re interested in pinpointing when soybeans no longer benefit from either rain or irrigation water, Gauck suggests analyzing pods more closely. Open a few and look inside.

“If beans are large and no longer have a membrane attached to them, they’re probably done taking in materials from the plant’s vascular system,” Gauck says. “It probably would not make sense to irrigate after that point, if plants and pods were consistently at that stage.

“However, if there is still a whitish, membrane-like material attached to each seed in the pod, there is likely still time to add yield. If you find many pods with green soybeans attached through membranes, then bring on the irrigation.”

Whether or not the bean itself has separated from the membrane in the pod is a good clue to plant progress toward maturity. However, if you’re irrigating, you also need a good handle on soil moisture levels, Gauck says. That’s where a soil moisture probe installed in the field can play a big role.

Ceres Solutions installed a CropX moisture sensor in the Soybean Watch ’23 field very early in the season after planting. It provides instant readouts on moisture levels down to 4 feet.

Through an app, the moisture sensor displays levels throughout the season to the agronomist’s computer. If it indicates that plants need moisture, you’re not at 50% yellow, and soybeans inside pods are still connected by membranes, it’s likely irrigation would pay, Gauck concludes.

About the Author(s)

Tom J. Bechman

Editor, Indiana Prairie Farmer, Farm Progress

Tom J. Bechman is editor of Indiana Prairie Farmer. He joined Farm Progress in 1981 as a field editor, first writing stories to help farmers adjust to a difficult harvest after a tough weather year. His goal today is the same — writing stories that help farmers adjust to a changing environment in a profitable manner.

Bechman knows about Indiana agriculture because he grew up on a small dairy farm and worked with young farmers as a vocational agriculture teacher and FFA advisor before joining Farm Progress. He works closely with Purdue University specialists, Indiana Farm Bureau and commodity groups to cover cutting-edge issues affecting farmers. He specializes in writing crop stories with a focus on obtaining the highest and most economical yields possible.

Tom and his wife, Carla, have four children: Allison, Ashley, Daniel and Kayla, plus eight grandchildren. They raise produce for the food pantry and house 4-H animals for the grandkids on their small acreage near Franklin, Ind.

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