High temperature effects on corn and soybean
What is the ideal temperature for soybeans? Soybeans are a temperate leguminous plant with an ideal daytime temperature of 85°F. 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. Temperatures exceeding 85°F can result in a decreased number of pods while temperatures above 99°F severely limit pod formation. Heat stress at the R5 growth stage (beginning seed), has the greatest impact on soybean yield. During seed fill, daytime temperatures of 91 to 96°F result in fewer seeds per plant. Daytime temperatures greater than 85°F during seed fill can result in decreased soybean weight.
How do high nighttime temperatures affect corn and soybean production? High nighttime temperatures (in the 70s or 80s) can result in wasteful respiration and a lower net amount of dry matter accumulation in plants. The rate of respiration of plants increases rapidly as the temperature increases, approximately doubling for each 13 degree F increase. With high night temperatures more of the sugars produced by photosynthesis during the day are lost; less is available to fill developing kernels or seeds, thereby lowering potential grain yield. High night time temperatures result in faster heat unit (GDD) accumulation that can lead to earlier corn maturation, whereas cool night temperatures result in slower GDD accumulation that can lengthen grain filling and promote greater dry matter accumulation and grain yields.
Past research at the University of Illinois indicates that corn grown at night temperatures in the mid-60s out yields corn grown at temperatures in the mid-80s. Corn yields are often higher with irrigation in western states, which have low humidity and limited rainfall. While these areas are characterized by hot sunny days, night temperatures are often cooler than in the Eastern Corn Belt. Low night temperatures during grain fill have been associated with some of Ohio’s highest corn yields in past years. In 2009, when the highest corn average yield to date was achieved, 174 bu/A, Ohio experienced one of its coolest Julys on record. The cool night temperatures may have reduced respiration losses during early grain fill and lengthened the grain fill period.
Compared to corn, soybeans are less sensitive to high nighttime temperatures. Warm night temperatures do not appear to increase respiration in soybean plants as much as corn. 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°F before any noticeable reduction in soybean yield is experienced. In an experiment conducted by the USDA, soybean plants subjected to a night temperature of 85°F resulted in a 10% yield loss. Corn subjected to 85°F at night experienced grain yield reductions of 40%.