Issue No. 17, Article 8/July 21, 2006
Can Corn and Soybean Crops Handle the Heat?
The rains that fell over most of Illinois during the first two weeks of July were most welcome, and in most cases came just in time to prevent problems with corn pollination. More than 80% of the corn crop is now into or finished with pollination, on pace with last year and with other seasons when planting was early and temperatures were near normal. When July rainfall is at or close to average and pollination is early, there is a strong possibility that yields will be good. As we have noted before, however, the difference between good yields and great yields usually is determined late in grain fill, when under great conditions kernels fill to more than their normal weight.
In our planting date study here at Urbana, the March 30 planting is now in stage R3, which is the "roasting ear" stage of kernel development. This planting has received about 1,700 growing-degree days by now, which is about 65% of the total that this hybrid (109 days relative maturity) needs from planting to maturity. Weekly accumulation rates are about 200 GDDs at this point in the season, so this planting should reach black layer (physiological maturity) in about five weeks, or by late August. The planting made around April 20 is in late R2, and the May 10 planting is close to completing pollination. The late-May planting has not yet pollinated. The earliest planting is as much as a foot shorter than the later plantings, most likely because the weather was cooler on average during stem growth so that internodes did not expand quite as much. We will be able to see whether this might have a small effect on yield through a slight reduction in light interception.
The corn-yield model that has been worked out at the University of Nebraska places a premium on extending grain fill into mid- to late September, in order to take advantage of cooler nights, which help to reduce respiration loss. While that may indeed help if the late-planted crop is in excellent condition by late August, the fact that it has not yet pollinated while the early planting is one-fourth of the way through grain fill now means that the late-planted crop is not as "safe" as the early-planted crop, at least today. If good rainfall continues, though, the later-planted crop should thrive, and it could yield as much as earlier plantings, though the likelihood of this is not high. How well the crop maintains the integrity of its canopy, and later of its stalks, will determine how well it fulfills its potential. From a simple "what can go wrong?" standpoint, we'd have to give the edge to the most advanced (earliest-planted) crop today.
There remain places in Illinois where the topsoil is still dry and July rainfall has not been adequate. Water use by the crop is now at its seasonal peak, with losses as high as 0.3 inch on hot, sunny days. The growth of the root system slows greatly by the end of pollination, so if the crop is starting to show deficit in the afternoon now, the deficit will increase until there is rainfall, since the roots will not be tapping much water from deeper layers. If the crop has pollinated, the effect of water deficits now will be increased abortion of kernel at the tip of the ear. Abortion will continue up to about two to three weeks after the end of pollination, or even longer in some cases, as shown by the amount of dry matter deposited into the aborted kernels before they stop filling. After kernel abortion stops, continuing water deficits will cause final kernel size to be reduced.
A big concern this week is the effects of very high temperatures on the corn crop, even in places where soil water is adequate. As with most suspected stresses on plants, it helps to watch the crop to see what signals it is giving. Remember that corn is not human; even when we may be miserable watching the heat index rise above 100, the corn plant may show no signs of stress at all. This was the case on Sunday, July 16, when temperatures in the northern part of Illinois reached the upper 90s. I traveled from Chicago to Champaign in the afternoon that day, and the corn seemed to be unfazed by the high temperatures. When the corn seems unfazed, it probably is. In fact, corn is a tropical crop, and even though we use a "cutoff" temperature for GDD calculations that implies that corn functions about the same at 95 degrees as it does at 86, that's a simplification based on the fact that when the temperature is above 90, the crop is often under water deficit stress as well. When the crop has adequate water, it very likely functions at a higher rate at temperatures above 90 than at 86, which is the cutoff temperature.
Corn tolerates high temperatures very well, but there is a limit, of course. Work done in greenhouses has shown that even well-watered corn starts to deteriorate fairly quickly once temperatures rise above 115 degrees or so. Such damage involves actual "cooking" of plant proteins, and the plant recovers very slowly from such damage, if at all. There might have been some of this in South Dakota this past weekend, when temperatures reached above 115. But temperatures below 100 to 105 will not do much of any damage if the crop has adequate water. One exception to this is that pollen viability can be reduced in some hybrids if temperatures exceed 100. If there is not enough soil water, or if the loss rate of water under high temperatures and windy conditions exceeds the rate at which the plant can move water from the roots to the leaves, then photosynthesis will slow as stomata close. Once water loss from the leaf slows, leaf temperature rises, as the cooling effect of water evaporation inside the leaf diminishes. Hence, leaf temperatures can rise above the heat-damage threshold even if the air temperature remains below this threshold.
Because people suffer when the relative humidity is high, they often get the impression that plants must suffer under such conditions as well. Aside from the fact that higher relative humidity means that leaves remain wet from dew longer, which can encourage disease development, high relative humidity has little direct effect on plants. As I write this in the late morning of July 18, the temperature is 88 degrees, and the relative humidity is 38%. That's "high" as relative humidity goes during the warm part of a summer day in Illinois. People who talk about "90 degrees and 90% relative humidity" exaggerate, unless they are speaking about an actual sauna. The dewpoint is the temperature at which a certain concentration of water vapor in the air saturates the air. The dewpoint at 88 degrees and 38% relative humidity is about 62 degrees, which means that the relative humidity would rise to 100% if the outside air were cooled to 62 degrees. Dewpoints in the lower 70s are a sign of high water vapor content, but as the daytime temperature moves into the 90s, the relative humidity at such a dewpoint will be 50% or less. That is uncomfortable for people but of little consequence for plants, since the "demand" by the air for more water vapor is high whether the relative humidity is 60% or 30%. It's this demand, plus the rate at which air moves across the leaf surfaces, that determines the rate of water loss from plants.
Soybean does not tolerate very high temperatures as well as corn, but temperatures less than 100 degrees are not usually a problem for soybean plants that have adequate water. Most of the Illinois soybean crop is now in the R2, or full flowering, stage, though some early-planted fields might be at R3, or beginning pod. The crop is thus not quite at its most critical stage, which begins once pod numbers are being set and pods start to fill. Still, plants that undergo water stress now will lose more flowers to abortion and will need a period of better conditions within the next few weeks in order to set enough pods for maximum yield. We will have to wait until early August to assess the success of pod setting. We need to keep the canopy healthy so that photosynthesis is not the limiting factor for pod number. The yellow patches and streaks that were common a few weeks ago have become less noticeable in fields that have received rain, as new leaves grow above the older ones and so disguise earlier problems.
Wheat yields in Illinois turned out to be surprisingly high in most areas in 2006. Results from the variety trials are posted at the Crop Sciences variety testing Web site.--Emerson Nafziger