SIGN UP FOR OUR EMAIL LIST!
To receive weekly email notification when the latest issue of the Bulletin is online, click on this link and fill out the form.



Dry Weather and Crops

May 18, 2001
Wheat

While the wheat crop has generally benefited from the dry weather up to now, there is increasing concern about how the continuing lack of rainfall is affecting the crop's chances. We have very little experience with conditions this dry at this time of the year in Illinois, so what we say here is based more on "principle" rather than on experience.

In general, wheat is well adapted to a Mediterranean-type climate, with little rainfall following heading, but with stored soil moisture tapped by roots to bring the crop to maturity. Low humidity and good sunshine are helpful but lack of adequate soil moisture will not allow the crop to fill out the kernels as well as it should. Do we have enough stored soil moisture to get this crop through to maturity? It has been a good year to develop a root system to tap water that's in the top foot or two, but it is doubtful that we will fully fill kernels with only the water present in the soil now. Still, it only takes perhaps 3 inches of water from flowering to maturity, so some fields that are well tapped into existing soil water may fill better than we expect.

To assess how well the crop is extracting soil water, note plant height. Plant height is sensitive to water shortage, so if rainfall has been low for the past month, yet plant height seems almost normal, that's a sign that the roots have done a good job of water extraction. Photosynthesis, which we need now to fill the grain, is less sensitive to water stress than plant height, but we do not have a good way to see if photosynthesis is affected as soils continue to dry. Some varieties tend to curl their leaves somewhat like corn when water is inadequate, and this usually indicates that photosynthetic rates are low. It takes rather severe stress to curl the leaves in most varieties, however, so crops that show this symptom may be filling grain slowly, if at all.

Added to the lack of soil moisture is the armyworm problem affecting some southern Illinois fields. We normally say that if the upper two leaves of the crop remain intact, good yield should still be possible. In some cases where armyworms have stripped the entire plant of leaf tissue, the questions arise as to whether such plants can fill kernels at all and on how the dry weather might be interacting with leaf loss.

The head and stem tend to increase in photosynthetic capacity if leaf area is lost and can do a surprising amount of grainfilling on their own. However, if the crop canopy is such that little light is being intercepted, then we can't expect much in the way of yield. Dense wheat canopies intercept almost all of the sunlight, while those that have reduced interception can probably expect to produce yield only, as a rough guess, in proportion to the amount of sunlight they are still able to intercept. You can estimate sunlight interception by seeing how much of the ground is shaded around noon when the sun is high. Because small amounts of remaining green tissue can have higher efficiency, yields of plants that are intercepting only small amounts of sunlight might yield more than the proportion of sunlight that they intercept, but they can hardly be expected to yield well.

It is dry enough in most areas where wheat is a main crop that destroying a defoliated or drought-stressed crop to plant soybeans will not be successful until it rains enough to germinate soybean seed. As you continue to monitor the wheat crop in such fields, watch carefully for signs that the crop is starting to lose its green color. Once that happens, there is little if any chance that the crop will revive to produce more yield, even if it rains. Kernels have accumulated little dry weight up to now, and dryness and defoliation are probably slowing the rate of fill. If the crop starts to lose its green color, the weight of the kernels is probably as high as it will get, and at that point the crop can be assessed to see if it is worth harvesting. Besides weight (yield) loss, premature crop death will also result in light test weights, with substantial discounts likely.

Corn

Almost all of the corn crop has been planted in Illinois, completing another year of early planting. In most fields, stands are good, though dry soils have caused some uneven emergence in some places. Following emergence, the crop in fields with dry surface soils has struggled to some extent in trying to establish its root systems. The root system arising from the seed (the seminal root system) grew normally in most cases, though it may have struggled in some shallow-planted fields.

By the time the second leaf collar is visible, the crop needs to initiate the nodal root system to support the rapid growth that is coming. Roots do not grow into dry soils, so in many fields the nodal roots, which emerge from the crown area less than an inch beneath the soil surface, have had difficulty growing as rapidly as they normally would. This problem compounds itself in that plants then begin to suffer from inadequate water uptake, which further reduces growth rates. Photosynthesis is affected less (and later) than plant growth rates under drying conditions, and in many fields, the sugars produced by photosynthesis have had no place to go because of limited root growth (and root function). Excess sugars tend to build up and cause anthocyanin levels to increase, turning the leaves purple. Some hybrids show this much more than others, but the lack of purple color does not mean that growth is normal.

While we normally say that purple color goes away once root growth returns to normal, the purpling still is an indication that there is a problem, and if that problem continues, it might eventually affect yield potential. Plants have been slow to grow out of the purpling this spring, and it may take some substantial rainfall before we see the plants return to normal. The cool night temperatures and bright sunlight this past weekend (May 12 and 13) caused some loss of chlorophyll in leaves, removing some of the green color and bringing out the purple color even more vividly.

There have been a few reports of defoliation of corn by armyworm, and questions about regrowth of cutoff plants. In general, this tends to be fairly "clean" injury, and the regrowth potential should be very good, with little or no yield loss from plants that were in the 3- or 4-leaf stage (collar visible) when they were cut. To make a prognosis, simply watch these plants to see if new leaf tissue is appearing. Note that leaf growth will be slower in fields where the roots are in dry soil, but if new, green leaf tissue does not appear within 2 days, dig and split plants to see if the growing point is discolored.

Soybean

With dry soils at planting in many fields, getting soybeans up is a real challenge this year. There are different philosophies on this: do we plant into dry soils and wait until it rains to bring the crop up; do we plant deep to try to reach moisture; or do we wait until it rains enough and then plant into moisture at the normal planting depth? There is no magic answer to these questions, though most producers are probably doing some of each, depending on the soil type and rainfall to date, and on their past experience. The fact that seed supply is limited this year further complicates this situation, in that replant seed may not be widely available, at least for the varieties that people want to plant.

It helps to look at the calendar and to note that we are only in the middle of May. In northern Illinois, soybean yields will start to decline slowly as planting is delayed past this point, while in the southern part of the state, yield potential will decline only if planting is delayed past late May. Of course, these are "average" statements, and the chances that they apply in any given year are not high. But with rainfall our only real hope for getting a soybean crop up and growing in Illinois, we can at least take some comfort in the fact that there is still some time for this to happen.--Emerson Nafziger

Author: Emerson Nafziger


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

Subscription information: Phone (217) 244-5166 or email acesnews@uiuc.edu
Comments or questions regarding this web site: s-krejci@uiuc.edu