Drought and Heat Effects on Corn and Soybean• Drought and high temperatures occurring at specific growth stages can reduce corn and soybean yield potential.• Managing stress from insects, disease, and nutrient deficiencies can reduce the degree of overall stress.• Corn is most sensitive to stress during pollination and grain fill, while soybean is most sensitive during the second through

fourth week of seed-fill.

Corn and DroughtMoisture stress during corn grain fill increases the chance for leaves to dieand plants to lodge while reducing kernel weight and potentially shorteningthe time period available for grain fill. Corn plants are most sensitive todrought stress during the pollination process where delays in silkemergence and elongation relative to the shed of viable pollen can result inpoor pollination success or even complete plant barrenness, leading tosignificant loss of yield potential. However, kernel abortion and reduced dryweight accumulation in the kernels can occur after pollination. Yield lossduring grain-fill may still be 3.0 to 5.8 percent per day of stress.Developing kernels, especially those near the tip of the ear, can be proneto abortion if temperatures are high and moisture is limited during the twoweeks following pollination.

Figure 1. Corn plants are most sensitive todrought stress during pollination and grainfill.

The potential number ofcells that can accumulatestarch is determined bycell division that occurs inthe endosperm during thefirst seven to ten daysafter pollination.1 Dryweight accumulation isthe yield component thatis affected after thekernels have reached thedough stage. Limitedamounts of photosynthateto nourish the developingkernels can cause kernelsto be smaller and lighter,also known as “shallowkernels”. Prematureformation of black layer during high temperature periods can also reducegrain fill because further kernel development is terminated.

Corn and Heat StressEven with sufficient moisture, high temperatures can cause a high degreeof stress on the plant. Both high day and night temperatures can have aneffect on corn yield potential. A one percent corn yield loss can occur afterfour consecutive days of temperatures at 93 °F or greater.2 On the fifth dayof these high temperatures, another two percent yield loss can occur, andon the sixth day another four percent can be expected. A heat wave thatlasts longer than six days often results in firing of leaves and lower yieldpotential may be expected, especially when the heat wave coincides withsilking.

High temperatures stimulate respiration, and sugars that could have beenstored in grain are burned up. This can be especially true when nighttimetemperatures remain high and sugars are being used while nophotosynthesis takes place. Thus, high nighttime temperatures can reduceyield potential without plants showing visible signs of stress.3 Highhumidity can compound problems associated with high daytimetemperatures by slowing the ability of the plant to cool down in theevening.

Corn ManagementFuture management decisions should be made based on the success ofpollination. If kernel set is good, the crop has some potential to producegrain. However, if potential yield is less than 25 bushels per acre,harvesting for silage/hay may be the best option.4 Corn for silage ispreferred over hay, and plants should have 65 to 75 percent moisture.

Fields that are drought stressed to the point that plants have lost somebottom leaves, and the top leaves have browned off or turned white maybe candidates for silage or hay. However, plants that do not grow normallycan have high nitrate levels, especially in the lower portion of the stalk.Haying high nitrate corn will not reduce the level of nitrates, and cuttingheight should be at least six to eight inches above the ground to help avoidnitrate toxicity. It is strongly recommended that the hay be tested fornitrates before feeding. The level of nitrates in corn can be estimated bypuchasing a test kit either online or from local Extension offices. Samplescan be taken before harvest or in the corn after ensiling.5 Additionalsamples can be sent to a lab for further analysis if kit results indicate highlevels of nitrate.

Soybean and DroughtMoisture stress during the soybean reproductive stages causes floralabortion, reduced pod number, fewer seeds per pod, and reduced seedsize. Moderate drought stress can significantly reduce or irreversibly stopnitrogen fixation, disrupting seed development.6 From the second throughthe fourth week of seed fill, a 39 to 45 percent yield decrease can occurwhen there are four days of visible moisture stress.7 Soybean floweringstops, and plants cannot compensate for lost pods when drought stressoccurs during the R4 through R6 (full pod through full seed) growthstages.8

   

Figure 2. Soybean plants are unable tocompensate for lost pods if they are drought-stressed during the R4 through R6 growthstages.

Soybean andHeat StressIt can be difficult toseparate effects of hightemperature from theeffects of water stress insoybean plants. Oftenthese stresses occurtogether and magnify theeffects of each other.Extension SoybeanSpecialist Jim Dunphy,North Carolina StateUniversity, indicated,"when temperatures getabove 95 °F, soybeanplants simply can’t pump enough water to keep up with transpiration andevaporation. The plants close the stomates in their leaves and water can’tget out. That also means carbon dioxide can’t get in, and the plants can nolonger get the carbon they use to make the sugars that fuel everything thatgoes on inside the plant.”

Soybean ManagementEffects from drought are expected to be less on soybean plants comparedto corn plants. If adequate rainfall occurs and photosynthate is availableafter R5, the plant may compensate for earlier losses by producing largerseeds (within its genetic capacity).8 Once the plant reaches R6, pods arenot normally aborted. Managing stress from insects, disease, or nutrientsources can help reduce the overall stress load on the plant and potentiallylimit yield losses.

Drought MonitorThe U.S. Drought Monitor is a tool that can help track local droughtconditions and can be accessed with the following website: 

http://droughtmonitor.unl.edu/ 

The U.S. Drought Monitor is produced through a partnership between theNational Drought Mitigation Center at the University of Nebraska-Lincoln,the United States Department of Agriculture, and the National Oceanic andAtmospheric Administration.

Sources1 Lauer, J. 2006. Concerns about drought as corn pollination begins. University of Wisconsin.Field Crops 28.493-42. http://corn.agronomy.wisc.edu/.2 Elmore, R. and Taylor, E. 2011. Corn and “a big long heat wave on the way”. Iowa StateUniversity. http://crops.extension.iastate.edu/.3 Wiebold, W.J. 2012. None like it hot. University of Missouri. http://ipm.missouri.edu/.4 Crunch time for Kansas corn crop. 2011. The Pratt Tribune. http://www.pratttribune.com/.5 Wright, D.L. Mayo, D.E., and Jowers, H.E. 2015. Using drought-stressed corn for silage, hay,or grazing. University of Florida. SS-AGR-274. http://edis.ifas.ufl.edu/.6 Lenssen, A. 2012. Soybean response to drought. Iowa State University.http://crops.extension.iastate.edu/.7 Wright, J. Hicks, D., and Naeve, S. 2006. Predicting the last irrigation for corn and soybeansin Central Minnesota. University of Minnesota Extension. http://www.extension.umn.edu/.8 Hall, R.C. and Twidwell, E.K. 2002. Effects of drought stress on soybean production. SouthDakota State University. Ex8034. http://pubstorage.sdstate.edu/. Woodhouse, C., Webb R.S., Overpeck, J.T., et al. 2003. North American drought: a paleoperspective. https://www.ncdc.noaa.gov/.Web sources verified 05/19/16. 140602060604

For additional agronomic information, please contact your local seed representative.Individual results may vary, and performance may vary from location to locationand from year to year. This result may not be an indicator of results you may obtain aslocal growing, soil and weather conditions may vary. Growers should evaluate datafrom multiple locations and years whenever possible.ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. All othertrademarks are the property of their respective owners. ©2016 Monsanto Company.140602060604 052016MEC

Drought and Heat Effects on Corn and Soybean