It's commonly perceived that if summer weather is too hot and humid for people, then it must be ideal for corn since corn — in contrast to a crop like wheat — is a tropical crop. Up to a point, that perception is correct, says Tony Vyn, Purdue University agronomist.

Daytime temperatures up to 86 F are beneficial to corn plant photosynthesis and, when corn plants have adequate soil water and high-humidity air, the leaf openings (stomates) are more likely to stay open. That, Vyn says, facilitates the exchange of carbon dioxide from the air for photosynthetic fixation by green chloroplasts in the cells of corn leaves.

However, the rather continuous heat and high humidity conditions since the grain-filling period started in much of the Midwest have not been ideal for achieving high grain yields in 2010, he says.

Vyn points out 10 negatives that are associated with excessive heat and humidity during the grain-filling period:

1. Leaf diseases are more likely to spread in high humidity situations. On susceptible hybrids, and in fields not sprayed with effective fungicides, Vyn says the leaf area available for photosynthesis during the critical grain-filling period can decline too rapidly. Corn plants are also more susceptible to stalk rot infection with high humidity and wet surface soil.
2. At daytime temperatures above 86 F, the corn plant has lower net photosynthetic energy production and, therefore, fewer assimilates (e.g., sugars) available for corn-kernel development.
3. With warm nights — and particularly when nighttime temperatures exceed 70 or 80 F — corn plants expend more of the sugars they gain during the daylight hours on nighttime maintenance respiration, sometimes referred to as “dark respiration." "Nighttime temperatures of around 60 to 65 F during grain fill will result in higher grain yields than those we have been experiencing of late," he says.
4. High temperatures during grain fill shorten the grain-fill period between flowering and physiological maturity (i.e., black layer formation on the bottom of corn kernels). The primary driver of the rate of kernel filling is air temperature and, when air temperatures are high, sugars from current photosynthesis as well as remobilized sugars from leaf and stem reserves go to the ear sink even faster. "Grain fill is a 24-hour, 7-day-per-week process, so both day and nighttime temperatures matter," Vyn says. He adds that remobilization of sugars makes it more difficult to maintain healthy leaves for late-season photosynthesis, and it also lowers corn-stalk strength. Lower overall temperatures during the grain-filling period, such as those experienced in 2009, and longer grain-filling periods — provided they are not cut short by frost — are the preferred conditions for achieving high yields when corn plants have a sufficient, healthy leaf area.
5. Hazy and misty mornings during periods of high humidity mean a delay and reduction in sunlight energy received by the corn production factory, Vyn says. Less photosynthetic active radiation results in lower daily carbon dioxide fixation and, therefore, less sugar to help fill the corn kernels.
6. Ear rots can be exacerbated by high humidity. Recent observations of diplodia ear rot, even when corn ears are only halfway through their grain-filling period, are a direct consequence of excessive and prolonged humidity around the junction of the leaf sheath and ear shanks. Vyn says problems with ear rot will limit corn yields most on susceptible hybrids already under stress, such as those fields with deficient nitrogen.
7. High humidity also can reflect conditions of excessive rainfall, and excessive rainfall results in nitrogen losses to both groundwater via leaching and to the air via production of nitrous oxides. "Excessive rain in many parts of Indiana in June already compromised nitrogen availability to corn plants, and heavy rains in July and early August have further limited the plant availability of mineral nitrogen, whether from residual fertilizer nitrogen or from mineralization of the soil organic nitrogen," Vyn says. Modern corn hybrids have much more “stay green” of their leaves than hybrids of previous decades, but the expectation of corn being able to take up 40% of its total-season nitrogen uptake during grain fill is less likely this year than what was measured last year, Vyn adds. "That means that there will be more mobilization of leaf nitrogen and stalk nitrogen to the ear during grain fill than under more ideal weather conditions, and too much nitrogen mobilization will impair late-season photosynthesis rates," he says.
8. Prolonged wet soil conditions, especially in fields with inadequate drainage, have resulted in shallow root systems, and these fields are particularly sensitive to stalk lodging. "The ear placement on corn plants are generally high because of the warm temperatures experienced earlier during vegetative growth, but the combination of thin stalks at high plant densities, higher stalk reserve remobilization to ears during this summer’s weather conditions, shallow roots after earlier floods and high ear placement may cause more harvest losses in 2010 than in previous years," Vyn points out.
9. Each additional stress results in more than a simply additive effect from that stress alone, Vyn says. Pale green corn plants resulting from flooding or excessive rain in the pre-silk period are going to lose proportionately more yield from a high-temperature stress than plants that were a healthy dark green at the beginning of the grain-filling period. "The same could be said for plants with considerable gray leaf spot or blight damage to their most important leaves — such as those around the ear itself," Vyn adds. "I am more concerned about the negative consequences of excessive heat and humidity this season than I would normally be because of the other stress factors corn has experienced in 2010." Overall corn hybrids are much more stress tolerant today than those of 30 years ago, Vyn adds, but compounding stresses, especially during the grain-fill period, can impose significant restrictions to final grain yield.
10. High kernel set does not guarantee high yields. "Warm and wet conditions through late vegetative growth in Indiana meant that we had high potential kernel numbers this season," Vyn says. "We have had some kernel abortion occur in the first 2 weeks after pollination, and time will tell what the final kernel weights are going to be for the kernels that were retained." Final kernel size has considerable flex — 20% plus variation in weight per kernel within hybrids at a particular plant density — so Vyn says final yields are going to be affected by how much the current and future weather affects the supply of assimilates from the plant to the grain.

Vyn says corn fields are highly variable in 2010 and that greatly complicates yield forecasting. Excessive heat and humidity have taken their toll on the potential grain yield, but the extent of the loss is unknown at this point because of the host of interacting factors affecting actual kernel growth.

"Corn development is well ahead of normal, and these high temperatures will mean an early maturity and an early harvest," Vyn says. "Had a drought occurred, grain yield reductions might have been more severe than those from excessive heat and humidity.

"However, just because we have avoided drought in 2010 does not guarantee above-trendline yields. Too much heat and too much humidity can be tough on corn. Let’s hope the remaining weeks during the grain-filling period are cooler."