In our trying to imagine how the corn plants must "feel" in response to the weather, we tend to think that, because they grow so fast when it's hot, they must "like" it that way. In general, corn is a warm-season crop, and it does thrive in warm weather. But it also tends, like people, to do a bit better when the humidity is lower than it has been this past week. High humidity during the day ( above 60 or 70 percent humidity is very high when it's warm, raising the "heat index" to tell us how miserable we should feel) signals a high dewpoint. The low temperature at night during the summer is almost always at or just slightly above the dewpoint, so a high dewpoint means high night temperatures. The dewpoint is the temperature at which the air is saturated with water vapor, and it thus serves as an indirect measure of the actual amount of water vapor in the air. As such, it changes much more slowly than relative humidity; dewpoint values usually build over a period of days and then drop when a cold front moves through.|
There are several reasons that we pay attention to dewpoint and night temperatures in corn, particularly at and after pollination. First, high night temperatures mean more respiration by the plant, and it is generally thought that at least some of this respiration uses up plant carbohydrates without producing anything in return. Plant carbohydrates tend to reach a low point at about the time of pollination in any event, so further reductions by respiration may well decrease the success of pollination. High temperatures at night often tend to increase plant pest activity. The leaf stomata usually close at night to stop water loss, and so plants are usually able to rehydrate at night regardless of night temperature. But lower night temperatures tend to result in very heavy dew, and this can help to rehydrate and to delay the onset of stress as the day proceeds. On balance, corn plants tend to do better when night temperatures are less than about 65°F than when they are above 70°F.
Do we expect problems from the temperatures in the 90s this time of year? That depends almost entirely on the soil moisture status. Although the growth of corn doesn't tend to increase as temperatures increase above the upper 80s, growth and development take place at a rapid rate at, say, 95°F, if there is an adequate supply of soil moisture. When soil moisture in the zone of active roots is deficient, then photosynthesis will be decreased as stomata close and leaves curl, and this decrease will result in less carbohydrate production. This in turn can reduce the success of pollination. When soil moisture is adequate, as it is in most parts of Illinois, the plants are doing fine as they move into the pollination stage. The official report as of Monday was that 5 percent of the state's crop is pollinating. From observations I made traveling late last week, that is probably a bit low, since most fields are pollinating in much of central Illinois. With early pollination and adequate soil moisture in most cases, the crop should get through this critical period in good shape. After that, how well it maintains its canopy will be a primary determinant of yield.--Emerson Nafziger