Drought and heat put soybeans to the test

MT. PLEASANT — While soybeans can thrive in more harsh conditions than cornstalks, the high temperatures last month following drought like conditions through the summer may negatively impact soybean yields this year.

With a week in August registering excessive heat index values of up to 113 degrees, according to the National Weather Service, Beck’s Hybrid Agronomist and Mt. Pleasant soybean farmer Greg Shepherd expressed concern about the potential effect on crop yields.

According to the Farm Progress Network, “When air temperatures exceed 85° F, soybeans can experience heat stress regardless of reproductive stage. When soybeans experience heat stress, yield reductions can begin to occur, especially when soil moisture is limiting. Heat stress during flowering can result in pollen sterility and reduced seed set.”

Shepherd stated that prior to the extremely hot week, his opinion for their yield outcome was that they would yield a really good to above average crop. He seemed less certain of that prediction as those temperatures just began to drop at the time of this interview.

“But they can handle the heat right now pretty well,” he said. “It’s also combine with the dry weather. I mean, we’ve been short on moisture, short on rain, all year. So, if we had good rain, if we had gotten an inch on Monday and then gotten this 100 degree weather, I would say yea, the beans are probably going to be just fine. They can handle heat.”

Considering the timing of the heat with the lack of moisture, however, Shepherd worries about the beans currently being produced in the plants’ pods.

According to the Iowa Soybean Association, the lack of moisture is causing some nutrient deficiencies in soybean plants.

Shepherd says he has already seen signs of potassium deficiency in the fields.

ISA conservation agronomist in Eastern Iowa Evan Brehm explains that the first signs of this deficiency starts with the lower leaves on the outer leaf margins.

“Dry weather increases the likelihood of potassium deficiency, even in fields with adequate fertilizer applied,” he says.

“This is probably going to make the size of beans smaller,” he said and attributed the reason to heat stress and lack of water despite soybeans being a relatively more versatile crop.

While this causes yellowing around the edges of some leafs, Shepherd says some of the change in color across soybean fields also comes from the part of the life cycle the soybeans are in at this time year.

“What drives soybeans' life cycle and their growth is actually day length,” Shepherd shared. “They’re photosensitive in terms of their maturity and their growth.”

“Because of the soybeans being photosensitive, you know, as you go further south, they tend to be able to grow higher yielding soybeans because they get a longer growing season and longer days,” Shepherd said.

“It’s just literally they’ve got to have sunlight to live, obviously, but then corn grows according to heat units or growing degrees,” he explained. “Soybeans are more driven by how long the days are and as the days are shortening, then that will trigger soybean into starting to set seed and try to produce those bean pods and the actual soybeans which we harvest.”

Following the summer solstice June 20, soybeans begin this different phase in growth.

“Summer solstice is a really big deal for soybeans because that's the longest day of the year, most sunlight hours,” Shepherd said. “And so that really, the soybean can just kind of sense how many hours of daylight it has and as it gets fewer hours of daylight, it, well, it shifts into, you know, producing the actual beans and pods into reproductive growth stages, as we call it.”

Comments: AnnaMarie.Ward@southeastiowaunion.com