It’s often noted that soybean yield is made with rain in August, but moisture has been scarce throughout the entire growing season for soybeans on the Prairies this year.

While there are areas that have received more moisture, many fields in Manitoba are showing tell-tale signs of drought stress as they move through the critical pod-filling stage — shorter plants, reduced leaf area, earlier leaf drop, earlier maturity, fewer pods, fewer seeds per pod, and likely smaller, lighter seed.

“The leaves are flipping, they’re trying to reflect that solar radiation, and they’re basically in survival mode. When that happens, they’re also closing their stomates, which means less photosynthesis is taking place in the plant, and that means less sugars for growth and development,” explains Cassandra Tkachuk, production specialist with Manitoba Pulse & Soybean Growers, in this Soybean School episode looking at the effects of drought stress.

On the plus side, stressed soybeans will put more photosynthates into root growth to access water from greater depths, she notes.

There is also research from Brazil showing soybeans subjected to drought stress earlier in the growing season could be better prepared for dry conditions during pod fill, she says. “When soybeans were subjected to drought stress at the V4 stage, they were actually more resilient later on in their growth to drought conditions, so we’re hoping that holds true with drought conditions this year.”

Unfortunately, the number of pods and the number of seeds per pod has already been reduced through flower and seed abortion in stressed fields, but if they get some rain after R5 (beginning of seed formation), plants can compensate by producing larger seeds.

As a general rule, soybeans in Manitoba typically need 16 to 20 inches of water to reach maximum yield potential, explains Tkachuk.

Historically, the soybean-growing areas of Manitoba receive between 12 and 14 inches of rain during a growing season, meaning there needs to be some soil moisture from the previous year to reach peak yields. This year, many of those areas have received less than 6 inches of rain, as of early August.

In some ways, the drought of 2021 rounds out the soybean experience in Manitoba, notes Tkachuk.

“We know what to expect from canola and wheat under conditions like this, but for soybeans we don’t really quite know yet,” she says. “We’ve had some late season drought [in the past]. We’ve had hail, we’ve had frost, but we haven’t quite had a full season of drought like we’re having this year, so we’re still learning, and it will interesting to see how the crop fares.”

Watch MPSG’s Cassandra Tkachuk discuss the effects of drought stress, water uptake timing, and more in this Soybean School episode:

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