'Good' stress or 'bad' stress?
As the dry weather continues across Illinois, anxious thoughts increase regarding the effects of so-called stress on crops. Many agronomists have observed over the years that "some" stress in June can be favorable, in that drying surface soils tend to cause roots to develop deeper. We also link the lack of moisture with warm temperatures and a lot of sunshine, which favor corn growth.
Corn planted in central Illinois around April 20 has made phenomenal growth, especially in the area west of Springfield, where rainfall has been a little more abundant. I was in a field near Pleasant Plains on June 12 with corn as high as my head, with excellent crop color and good uniformity. The growing degree day calculator at the State Water Survey Web site calculates that Springfield received 988 GDD from April 20 to June 12, and that with normal temperatures, the total by June 26 (two weeks out) will be 1,288 GDD.
The GDD requirement to reach silking is a little more than half the total needed for a hybrid, with a greater proportion needed for early hybrids. That means that a 110-day hybrid, which would require perhaps 2,600 GDD from planting to maturity, might tassel in such fields by June 23 or 24 and silk by June 27 or 28. If it's warmer than normal until then, these events will happen even earlier.
Although we do not have a good, easy way to measure this, the fact that the corn crop has grown well in some areas where the surface soil has been mostly dry for a month or more is a strong indication that the roots are in, and are growing into, soil with more moisture than can be found in the top few inches of soil. Crop color is excellent in most fields, reflecting the effect of high amounts of sunlight and good mineralization of nitrogen from soil organic matter. Low humidity has also meant very little development of fungal disease. In general, the effect of low-rainfall "stress" on much of the Illinois crop so far this season has been more positive than negative.
At some point, of course, lack of rainfall will mean depletion of soil moisture near the roots and will decrease the crop's ability to continue to grow roots deeper into moist soil. How soon this happens is linked to the stage of the crop and to soil conditions. In eastern Illinois, where there has been little rainfall in some areas for the past month, leaves are curling this week by early afternoon, meaning that much of the afternoon sunlight is doing the crop no good.
It is easy to see where field operations such as tillage and less-than-favorable planting conditions have resulted in restricted roots in fields and parts of fields in this area. Affected plants may not be much smaller than in less-stressed areas because they've had enough water to grow on so far. But they are now showing leaf curling earlier in the day and more severely than in less-stressed areas, and their growth rate is being restricted by lack of soil water.
Rates of water use by the crop increase as plants get larger. These rates are measured using both the evaporation rate, which is calculated from weather data (relative humidity, wind speed, temperature), and the crop coefficient, which is an estimate of the percentage of evaporation that the crop actually uses in a day. The crop coefficient rises from 0 in corn at emergence to almost 1 (the maximum) at silking.