Corn was originally a tropical grass from the high elevation areas of central Mexico about 7,400 feet above sea level, 2,000 feet higher than Denver. Today, corn still prefers conditions typical of that area — warm daytime temperatures and cool nights. Areas that consistently produce high corn yields share some significant characteristics. These areas — central Chile, the west slope of Colorado, etc. — are usually very bright, clear, high light intensity areas with cool nights.
Corn maximizes its growth rate at 86°F. Days with temperatures hotter than that cause stress. In the high yield areas, cool night temperatures — at or below 50°F — reduce respiration rates and preserve plant sugars, which can be used for growth or reproduction, or stored for yield. These are optimum conditions for corn, and interestingly, are fairly typical for areas around central Mexico where corn is native.
This year, in the prairie states and in the Cornbelt, conditions have been dramatically less than optimal.
In years when we get high day and nighttime temperatures coinciding with the peak pollination period, we can expect problems. Continual heat exposure before and during pollination worsens the response. Daytime temperatures have consistently stayed in the upper 90s to low 100s.
The high humidity, which helps reduce crop water demand, also increases the thermal mass of the air—and provides extra stored heat and insulation at night.
Corn is a “C4 Photosynthesis” plant, making it extremely efficient at capturing light and fixing CO2 into sugars. One drawback of this system is that with high daytime temperatures, the efficiency of photosynthesis decreases, so the plant makes less sugar to use or store. High nighttime temperatures increase the respiration rate of the plant, causing it to use up or waste sugars for growth and development. This results in the plant making less sugar but using up more than it would during cooler temperatures.
Heat, especially combined with lack of water, has devastating effects on silking. If plants are slow to silk, the bulk of the pollen may already be shed and gone. Modern hybrids have vastly improved “ASI” or anthesis-silk interval (the time between mid-pollen shed and mid silk). Regardless, in some dryland fields we see seed set problems because of “nick” problems between pollen and silking.
Even in some stressed areas within irrigated fields (extreme sandy spots, hard pans or compaction areas where water isn’t absorbed and held, and some “wet spots”) we can see stress-induced slow silking and resulting seed set issues. Historically, this has been the most important problem leading to yield reduction, particularly in stressful years. Once silks begin to desiccate, they lose their capacity for pollen tube growth and fertilization.