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 So, what happens to soybean plants that are stressed during the summer?

The 2017 season has been stressful and July looks to be warmer and drier than normal. First it was wet and cold early this spring, which prevented planting or stressed emerging soybean seedlings. When they finally got planted it turned dry and hot.

Soybean development is behind and plants are smaller than they normally would be by now. However, they are tolerating the conditions quite well according the weekly USDA National Agricultural Statistics Service (NASS) crop condition report. Nearly 60 percent of the soybeans were rated as being in good condition and 20 percent in fair condition as of July 9. 

Source: USDA NASS crop report.

 We can appreciate that soybeans are fairly drought tolerant, mainly because of their ability to compensate with their yield components. The soybean plant produces a lot of flowers and potential pods over a six-week period, but only keeps as many of them as it can support. An intermittent stress, say at R1, can reduce flower set in late June. Since soybeans continue to flower through July, flowers lost early will be replaced by those that come later. The same applies to pods and, finally, seed weight. With fewer pods and seeds, seeds will be bigger.

Compensatory reproductive growth won’t occur under moderate to severe stress. However, the ability to compensate returns once a rain falls, creating an opportunity to regain some lost yield. During the severe drought in 2012, rains returned in August. Surprisingly, soybean yields came in just below average. However, we couldn’t say the same for corn yields.

Stresses also impact nitrogen fixation. Fixation can be severely limited or stopped by moderate drought, essentially shutting off the nitrogen supply to the plant. Once fixation ceases, water and time are required to reinitiate the process. Fixation may never return to before-stress levels, greatly impacting yield.

If your soybeans have been stressed during the R1 to R4 stages, but got off to a good start and built a normal node number, you will be surprised by the plant’s ability to produce pods, seeds and seed weight that easily compensate for early loss. Don’t write off the crop just yet.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.