Many North Carolina soybean fields got off to a rough start in 2020. Early planted soybeans encountered excessive moisture and cooler-than-normal temperatures, leading to compromised stands, increased seedling disease, slow growth and marginal root systems. Many later planted soybeans in the state have encountered dry weather since planting, leading again to compromised stands, increased seedling disease, and marginal root systems. Now it is July and we are encountering our typical high temperatures and we have soybeans in the state anywhere from the early vegetative growth stages all the way to soybeans approaching physiological maturity (R7). It is estimated that half our North Carolina soybean crop is flowering and a ⅓ of our soybean crop is into pod development (USDA NASS). The current high temperatures, coupled with drought in parts of the state, are creating concern for some growers and their soybean crop. 

What happens to our soybean crop when tempeatures are high?

The ideal temperature for soybean growth is 85℉. When temperatures exceed this threshold, especially for several subsequent days, heat stress can occur. Soybeans will be particularly stressed when excessive heat and drought conditions occur simultaneously and drive up soil temperatures leading to reduced moisture and nutrient uptake. Soybeans are less sensitive to high nighttime temperatures than corn, however yield impact can still be seen if nighttime temperatures exceed 85℉. Heat can also accelerate soybean maturity, as photoperiod and temperature interact to control flowering in soybeans. 

Vegetative stage soybeans under heat stress: High temperatures can result in slowed or stopped photosynthesis due to the plant closing its stomata in an effort to conserve moisture. When the stomata are closed, photosynthesis is restricted due to limited CO2 uptake. This will inhibit new vegetative growth which is critical, especially on late planted soybeans, to ensure we get adequate vegeative growth to drive reproductive development. Temperatures above 86℉ can also reduce nodulation, and ultimately N-fixation in soybeans, and the effects could extend into the reproductive growth stages. 

Reproductive stage soybeans under heat stress: High temperatures can result in any of the following in reproductive stage soybeans: aborted flowers, aborted small pods, aborted seeds in larger pods, and production of smaller seeds. Leaf loss can even occur under severe stress. It is important to remember that soybeans do not have the genetic capacity to turn all flowers into pods, even under perfect environmental conditions. Depending on the level of stress, 20-80% of soybean flowers will be aborted. Indeterminate soybean varieties will typically flower for 3 to 5 weeks and determinate soybean varieties will typically flower for 2 to 4 weeks. Moisture and drought stress may lead to shorter flowering times. Regardless of growth habit, soybeans flower for several weeks so they have the opportunity to withstand short-term periods of stress. Dr. Dunphy would emphasize ‘soybeans do not particularly care which flowers become pods’, highlighting the ability of this plant to compensate for periods of stress. Heat stress has the most adverse impact on soybeans in the R5 (beginning seed) growth stage.

With rainfall chances across most of the state this week, we are hopeful that these soybeans will get a much needed rainfall boost. This coupled with slight reductions in temperature will hopefully move our soybean crop into more favorable conditions for reproductive development. 

Additional Resources

Impacts of Extreme Heat Stress and Increased Soil Temperature on Plant Growth and Development

High Temperature Effects on Corn, Soybeans