Seeing Crops Through a Hot, Dry June

After a slow start, corn and soybean planting in Illinois proceeded at a normal pace in 2022. Rainfall in May was at 85 percent of normal statewide. While we remember the very warm days from May 10-15 and a few days later in the month, the statewide May temperature was only about 2 degrees warmer than normal.

The weather during the first half of June continued the trend from May, with temperatures averaging about 2 degrees above normal, and statewide rainfall about 70 percent of normal. Apart from an area of above-normal rainfall in western Illinois, rainfall during the first half of June ranged from 85 percent of normal in the northern crop reporting districts to 60-65 percent of normal in central Illinois to around 40 of normal in southern Illinois. A system that moved through southern Illinois this morning (June 17) brought some unexpected and welcome rain. June temperature has been above normal across the state, with larger departures in southern than in northern Illinois. The current forecast is for dry weather to continue, along with temperatures again rising to above average following a brief respite this weekend.

Good stands, little or no standing water, and warm soils combined to produce early June crop condition ratings above 80% good + excellent for both corn and soybeans. Ratings slipped by several points from June 5 to June 12, likely reflecting to some extent the appearance of crop stress symptoms in places where dry weather continues.

Low rainfall and high temperatures during the time crops are getting established do not typically lower yield potential on their own. In fact, above-normal rainfall often results in poor stands, drowning of the crop in low spots, seedling diseases, restrictions on roots due to low soil oxygen, movement of mobile nutrients (nitrogen, primarily) to below the rooting zone, and N loss from denitrification in saturated soils. We have avoided all of these so far in 2022. Also, dry weather brings plentiful sunlight, and warm temperatures speed up crop development.

An extended period of low rainfall and high temperatures after crop establishment brings some negatives as well: difficulty in activating herbicides; limitations on plant growth in soils, including compacted areas, where roots can’t reach enough water; better survival of soil insect pests such as corn rootworm; and stress on humans as we see crops begin to show symptoms of water limitations.

The above suggests that there should be a point in time when the effects of low rainfall and high temperatures turn from positive to negative as these conditions continue. That point has not yet been reached in Illinois, except perhaps in soils that do not supply as much water, and in places where symptoms of dryness have been appearing daily for most of June. But most corn and soybeans have not yet begun to lose yield potential. How long it will take before yield potential begin to diminish as dry conditions continue is not very predictable, but we may be able to get some clues from the crops in coming weeks if this weather pattern persists.

Corn

The accumulation of growing degree days by which we monitor corn development averaged 479 in Illinois in May, modestly above normal, especially in northern Illinois. Over the first 15 days in June, the statewide total was 319, only 16 GDD above normal. Corn planted in central Illinois on May 1 accumulated about 800 GDD by June 15, bringing it to growth stage V9. Corn planted on May 15 accumulated about 600 GDD and reached V6-V7 by June 15. Corn plant development is tracking closely to predicted; this indicates that stress has not slowed plant development so far.

One GDD per day above normal like we have had over the past six weeks does seem like very much, given that average temperatures have exceeded normal by two to three degrees. This is because the GDD calculation has a ceiling at 86 degrees: any daytime high temperature above 86 is taken as 86 in the calculation. So two days with the same morning low temperature, one with a high of 98 degrees and one with a high of 86 degrees, both produce the same number of GDDs. Although well-watered plants photosynthesize and grow a little faster at, say, 92 degrees compared to 86 degrees, high temperatures typically mean that plants are under a little more water stress, so using the ceiling was found to make the method work better to track development.

Daily water use by the crop can be estimated using crop stage and daily evaporation rate, which are affected by temperature, relative humidity, windspeed, and other factors. Evaporation is estimated by models, and is often called “potential evapotranspiration” (PET) in weather data and forecasts. It is the amount of water that evaporates from an open water surface in a day. Actual evapotranspiration (ET) includes evaporation from the soil surface and water vapor loss through plant leaves (transpiration); this is usually not the same as PET. As the crop grows and leaf area increases, transpiration outpaces water loss from the soil surface; with plant roots extracting water from the soil, daily ET (mostly transpiration) equals or can even exceed PET by the time the crop has a full canopy. The ratio of ET to PET is called the “crop coefficient” and can be used to estimate actual water use by crops based on growth stage and daily PET values.

With high temperatures, little cloud cover, and moderate windspeeds, June 14 and 15 this week had PET values averaged over the 19 WARM sites in Illinois (https://www.isws.illinois.edu/warm/) of 0.29 inches per day, with values at individual sites as high as 0.34 inches. These are among the highest values we see in Illinois. A corn crop at stage V8 has an crop coefficient of about 0.65, so we can estimate water loss from the soil of a field with corn at this stage as 0.29 x 0.65 = 0.19 inches of water for each of those two days. The crop coefficient increases as leaf area increases, and reaches a high of 0.90 to 0.95 by tasseling. If every day were like June 15 and the crop coefficient averaged 0.85 between June 16 and July 15, crop water use would total 30 x 0.29 x 0.85 = 5.66 inches over those 30 days. That’s not impossibly high—the corn crop typically takes up 22 to 24 inches of water for the season, with peak rates from a week or so before pollination to about midway through grainfill. But with soil water already depleted by a couple of inches in many places, getting little or no rain over the next weeks will tax the ability of even the best soils to provide enough water to the crop.

Figure 1 shows water stress symptoms in corn following corn in a field near Urbana, Illinois, on June 15. While these symptoms are not severe, plants with leaves curled and upright intercept less sunlight—note the small leaf shadows on the soil surface. This lowers daily photosynthesis and growth rates compared to those of unstressed plants. Restricted water availability could also limit stalk growth, and plants could end up shorter than normal as a result. High temperatures during stalk elongation usually mean taller stalks (this is common with late-planted corn) but dry soils can counteract this. A more severe lack of water might restrict leaf size and canopy completeness. The once-feared effect of low soil water causing silk appearance to be delayed until pollen shed is over is much less of an issue with newer hybrids, which typically have earlier silking relative to pollen shedding. Still, silk elongation is sensitive to how much water is in the soil, and failure of silking is still possible if soils get very dry.

It is not easy to predict how dry weather and increasingly dry soils will affect the corn crop, but the effects on plant growth over the coming weeks can help us to understand how yield might be affected. Compared to the drought year of 2012, soil water levels were much higher at the time the crop was planted this year, and this will provide some buffer against dry weather. The critical period that begins at about V15 is only some 300-400 GDD away depending on when the crop was planted, so will be approaching or here by the end of June. How well the crop has maintained growth by then will tell us a lot about yield prospects.

With diseases less likely to appear early if it stays dry, we might consider revisiting the idea of routine use of fungicide on the corn crop this year. Foliar fungicides are promoted as a means to improve yield prospects even in the absence of disease. That can happen—some fungicides lower crop respiration rates modestly (and temporarily)—but that would be more likely to improve yield in a crop that is not under severe stress. The largest effect of stress on corn yield is lower kernel numbers set; if low kernel numbers result from stress that begins before pollination, attempts to reverse this may not be successful.

We expect current conditions also to affect nutrient availability and management. Dry soils that limit root growth may lead to potassium deficiency symptoms, even when soil test K levels are adequate. This has been rare in Illinois in recent years, but could appear in the next week or two, especially in no-till fields in which roots are somewhat restricted. Rainfall usually brings a quick end to such symptoms, although there can be some loss of lower leaf area if it persists long enough.

Below-normal rainfall helps a great deal to keep N in the soil, and above-normal soil temperatures have helped speed up the mineralization process, which adds to the soil N supply. This year should be one in which normal N rates (MRTN rates, or 170 to 190 lb N/acre from all sources) should be all that the crop needs. Ongoing or increasing daily water stress and high temperatures can diminish leaf chlorophyll levels and make the crop seem N-deficient, but that is a physiological effect, not the result of low soil N levels: we would not expect high N rates or adding more N to change this. Most of the Illinois corn crop has all of its N applied by now, but if a final application is still planned, there are a few things to consider, beyond the fact that if a normal (that is, enough) N has already been applied, there is almost no chance that more is needed.

If a last N application is still needed, keep in mind that UAN or urea placed on the soil surface is unlikely to reach the roots as long as the soil stays dry. As urea-based materials stay on the soil surface and it remains dry and warm, rates of volatilization (loss of ammonia from urea) can be high. Urease inhibitor with surface-dribbled UAN or broadcast urea can slow this loss, but it might be more effective to inject UAN instead. That would move the N closer to the roots for uptake, and would also protect against volatilization, eliminating the need for urease inhibitor. There is no reason to try to delay the release of any N that’s applied now: with only a few weeks of rapid N uptake left, N needs to get into the plant as soon as possible.

Soybeans

Soybeans that were planted around mid-May have grown more slowly than expected this year, although that is beginning to be corrected in areas where at least an inch of rain has fallen in June. Figure 2 shows a soybean field that was planted in mid-May, about a month before the photo. While these emerged well and have good stands, the leaves and plants are not as large as we would expect under better soil moisture conditions. The slight, light-colored tinge over the field indicates that the leaves have lost some of their turgor pressure in the afternoon heat, so are a little limp and easily flipped by the breeze. High temperatures may also affect soybean plants more negatively than corn plants.

It’s likely that the smaller leaves on soybean plants resulted from limitations on water availability, and perhaps due to smaller than normal cell numbers as new leaves appeared. While we would like to have more leaf area as the longest day of the year approaches in order to capture more of the sunlight, we can expect the area of new leaves to increase quickly once temperatures are a little lower, especially if rain falls. Slow growth up to now means that plants have not taken up very much water, so the soil has a little more water to provide to the crop once its root systems increase in size to take up the water.

Warm nights are conducive to early flowering in soybeans, as long as soybeans have at least three trifoliolate leaves. If the crop had a few more leaves, we would expect to see flowers before the longest day (shortest night) of the season. There may be flowers on some early-planted soybeans this year, but they aren’t numerous. Some believe that flowering before the solstice signals high yield potential. While it does indicate that the plants are off to a good start, the majority of soybean flowers that turn into productive pods appear after the longest day of the year, as night length is increasing. Warm nights will cause this to start early.

There is not much we can do to manage soybean to minimize effects of current weather on growth. Plants in most fields are healthy, although dry soils can increase injury from soybean cyst nematode. In part because leaf growth is slow, herbicide injury may remain visible longer than usual: new leaf growth is how such injury “disappears” over time. The critical period for soybean yield begins later and last longer than for corn, and as long as soils have enough moisture by mid-July, yield potential should be good.

Wheat and doublecrop soybean

The 2022 wheat crop was planted relatively late last fall; as a result, both heading and harvest began a little later than normal this year, at least in the in the major wheat-growing areas of southwestern Illinois. Planting of doublecrop soybean is proceeding as soon as the crop is combined. Soil moisture has been helped some by rain in southern Illinois on June 17, but wheat uses soil water as it fills grain, so dry soils may affect planting, emergence, and possibly establishment of the crop in some areas.

Some wheat fields in central and northern Illinois appear to be ripening early, likely due to high temperatures and drying soils. A shorter grainfilling period may affect yield and test weight. While some may have planned to doublecrop these fields this year, fields in dry areas may not have enough moisture to germinate and establish soybean plants. Some may prefer to plant then wait for rain, and others may elect to wait for rain before planting. With questions about whether or not there will be enough water to germinate soybeans and to get them established, waiting for rain first might be a little safer. It will take several inches of rain to bring soil moisture up to favorable levels, so chances are low of getting too much rain to get soybeans planted quickly afterward.