No growing season is ever going to be perfect, but with only about 10% of the days during July and August above 86 degrees F, the daytime highs have been favorable for soybean growth in Ohio. If air temperature exceeds 85 degrees F, soybeans will experience heat stress that can impact yield potential. This is often compounded by a lack of soil moisture. Heat stress can result in a decreased number of pods set, while temperatures above 99 degrees F severely limit pod formation.
Because of their long flowering period, soybeans can often compensate for short periods of stress, but its ability to make up ground dwindles as it approaches R5. Elevated temperatures at the R5 growth stage (beginning seed fill), has the greatest negative impact on soybean yield. During seed fill, daytime temperatures greater than 85 degrees can cause decreased soybean weight while temperatures 91 to 96 degrees can result in fewer seeds per plant. During this period, pods are filling at a maximum rate, making the plant more susceptible to stresses and causing it to move nutrients from other areas. Moving those nutrients will weaken it and open it up to invasion by diseases through the roots and foliage.
Compared to corn, soybeans are less sensitive to high nighttime temperatures. In fact, soybeans like it warmer in the evening so that they can “burn” (respire) the stored energy (photosynthates that are stored as starch) and grow.
During the day, soybean plants accumulate starch in their leaves. At night, the starch is broken down and exported from their leaves. When nights are cool, the amount of starch exported is reduced resulting in high leaf starch the following day, which can disrupt photosynthesis. Nighttime temperatures have to exceed 85 degrees before any noticeable reduction in soybean yield is experienced. A string of nighttime temperature less than 60 degrees can result in reduced pod set, seed formation, and seed size. Studies have shown greatest growth rate for soybeans at mid-80 degree daytime and 72 degree nighttime temperature resulting in good seed size and no delay in maturity.
Several factors affect the rate at which crops develop — photoperiod, heat, moisture, and fertility. Heat and photoperiod are the two primary factors influencing soybean maturity. Soybeans are considered short-day plants, meaning that physiological development is accelerated as daylength shortens. However, the rate of maturity is sped up by hotter temperatures and slowed down by cooler weather. Soybeans can compensate for stresses/shortcomings that occur during early to mid-reproduction provided ample sunlight, adequate temperatures, and soil moisture is available. Favorable late-season temperatures (not too hot) and rainfall during late stages of development (R5 to R6) can create larger seed weight by extending the seed fill duration.
For more information, contact your local Pioneer sales representative or visit Pioneer GrowingPoint agronomy at pioneer.com/agronomy.