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Consecutive days of high temperatures can have a detrimental effect on corn and soybean yield potential. Other factors increase the overall negative effects of high temperature, including droughty soil conditions, sensitive crop growth stages, and high nighttime temperatures when coinciding with high daytime temperatures. It is challenging to separate the impact of high temperatures from the effects of water stress in crops. Often these two stresses occur together and magnify the effects from each other.1
Corn. Even with sufficient moisture, high day and night temperatures can have a negative effect on corn yield potential. Iowa State University reports that a one percent corn yield loss can occur after four consecutive days of temperatures at 93 °F or greater.2 On the fifth day of these high temperatures, another two percent yield loss can occur, and on the sixth day another four percent loss can be expected. A heat wave that lasts longer than six days often results in firing of leaves and lower yield potential may be expected, especially when the heat wave coincides with silking.
High daytime temperatures reduce photosynthesis efficiency, so the plants make less sugar to use or store.3 High humidity can compound problems associated with high daytime temperatures by slowing the ability of the plant to cool down in the evening. High nighttime temperatures stimulate more plant respiration resulting in more sugars being used while no photosynthesis takes place. This results in the plant making less sugar but using up more than it would during cooler temperstures.3 Thus, high nighttime temperatures can reduce yield potential without plants showing visible signs of stress.1
Figure 1. Silk elongation
Corn Silking. Exposed corn silks can dry out prematurely when humidity is low and temperatures exceed 95 °F.4 Silks generally emerge at a rate of 1 to 1.5 inches a day and continue until fertilized. High temperatures alone do not significantly affect the rate of silk elongation (Figure 1).4 Consistent and timely silking is needed to allow for a viable silk and pollen grain to unite.
Kernel set can be irregular on ears when pollen shed and silk elongation are not synchronized. This is more common when moisture stress accompanies heat stress. Pollination can be successful during stretches of high temperatures if adequate moisture is supplied to the plant.
Soybean. It can be difficult to separate effects of high temperature from the effects of water stress in soybean plants. Often these stresses occur together and magnify the effects of each other. Extension Soybean Specialist Jim Dunphy, North Carolina State University, indicated, “when temperatures get above about 95 °F, soybean plants simply can’t pump enough water to keep up with transpiration and evaporation. The plants close the stomates in their leaves and water can’t get out. That also means carbon dioxide can’t get in, and the plants can no longer get the carbon they use to make the sugars that fuel everything that goes on inside the plant.�?
Plant stress, such as high temperatures, moisture deficit, etc., occurring during full pod (R4) to shortly after full seed (R6) soybean growth stages will reduce yield potential more than the same stress at any other period of development.5
1 Wiebold, W.J. 2012. None like it hot. Integrated pest management. University of Missouri. http://ipm.missouri.edu/IPCM/2012/7/None-Like-It-Hot/ 2 Elmore, R. and Taylor, E. 2011. Corn and “ a big long heat wave on the way”. Integrated crop management. Iowa State University Extension and Outreach. http://crops.extension.iastate.edu/cropnews/2011/07/corn-and-big-long-heat-wave-way 3 Hoegemeyer, T. 2011. How extended high heat disrupts corn pollination. Cropwatch. University of Nebraska-Lincoln. Cropwatch. University of Nebraska-Lincoln. http://cropwatch.unl.edu/how-extended-high-heat-disrupts-corn-pollination-0 4 How high temperatures and stress affect corn pollination. Iowa State University. http://www.agronext.iastate.edu/corn/production/management/mid/stress.html 5 Pedersen, P. 2004. Soybean growth and development. PM 1945. Iowa State University. Web sources verified 06/19/16. 160617132919.