As the dry weather continues in some parts of the province, 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.

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 Ontario 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. Along Lake Erie's north shore, where there has been little rainfall in some areas for the past month, leaves can be seen 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. The crop coefficient is about 0.9 in the most advanced corn now and 0.4 to 0.5 in corn that is knee-high. Evaporation on a warm, windy day is as high as 0.28 inch, ranging down to 0.2 inch if it is warm but with moderate wind. Thus the crop is using perhaps 0.75 inch to 1.5 inches of water per week now, depending on its size. Where the leaves are curling in early afternoon, water loss is decreased considerably, but photosynthesis and the ability to grow are also decreased.

The stress indicated by leaf curling in corn is negative for crop growth during the time the leaves are curled, but the overall effect of such stress on yield potential depends on how long it lasts and the crop stage when the stress is taking place. In fields where stress has been severe since the plants were small, such that they are not growing well, the crop will behave much like late-planted corn, with yield prospects decent only if it rains soon. Such fields are likely to need more consistent rainfall to prevent stress throughout the remainder of the season as well, because root growth is unlikely to catch up even as plants develop rapidly once they get water. In the crop that has reached V7 or V8 and continues to grow well, leaf curling in the afternoon is decreasing the overall growth rate, but crop prospects have not been compromised greatly up to now.

Overall, then, dry weather and dry soils have not greatly decreased the prospects for the corn crop so far. The remarkable ability of corn to take up water from deep in the soil, and the ability of soils to store water, means that short-term dryness tends to have minimal effects on yield.

Soybean tolerates early-season water deficiency at least as well as corn does, but afternoon wilting means that the plants are not photosynthesizing or growing much that day. As long as they are revived by rainfall before early July, and as long as they retain leaves and otherwise show some signs of growth, we don't think this crop will have suffered much loss in yield potential. We need only to go back to 2005 to see a soybean crop that was seriously stressed for most of the season before early August but ended up yielding quite well. One difference from corn, though, is the possibility that aphids, spider mites or other pests might build in soybean more rapidly if the crop remains under stress.