It's become the excuse of last--and sometimes first--resort: When we are stressed about the condition of a crop and by unusual weather conditions, we tend to say that crops must be under stress as well. (Crops tend not to respond very noticeably to what we think or say about them, giving us a free hand to believe or say what we might.) Sometimes crops look as if they are under stress (growing more slowly than we expected or not the color we would like) and sometimes they look normal, but we tend to think that they must be under stress if it has been wetter, cooler, warmer, or drier than "normal." By normal we really mean average; those of us who have lived in Illinois for a long time know that it is normal for weather conditions to deviate considerably from average on any given date. In fact, weather conditions in recent weeks have precedence--they have happened at this time of the year in some previous years--and so they are within the "normal range."
Even if soil moisture and temperatures are within the normal range, however, they can still cause plant stress. In the past week (May 28 to June 3) at Urbana, we have accumulated the grand total of 70 growing degree-days (GDD), and it has rained some on 4 of the 7 days, with much less bright sunlight than average. Still, the crop has shown some growth, seen mostly as some filling of the row middles rather than as an increase in plant height. Corn planted here on April 23 is in stage V6, and so continues to develop faster than Bob Nielsen's guidelines predict. This crop has experienced only about 480 GDD since it was planted, while the Nielsen method (125 GDD to emerge and 85 GDD per leaf through V10) would suggest that corn with 480 GDD should have reached only past V4.
The fact that the GDD growth-stage formula is underestimating development can be seen as a positive factor, in that the plant is obviously making the most of the temperatures it is getting. In a way, this probably tells us that the crop is undergoing less stress than we might think it is. My working definition of stress is anything that reduces the rate of photosynthesis, which is the rate at which green plant leaves convert sunlight to sugars. Sugars are the means by which plants grow and develop, so if growth has been better than we expected, stress may have been less than we thought. This does not mean that the crop would not look better and grow faster under warmer conditions with more sunlight, but I think we can conclude that the root system is in place and functioning well, and that the leaves are making good use of what solar energy they are receiving. If we think back over the years, having a corn crop with good color and four to six leaves by June 1 tends to indicate good potential for the season, though there is no guarantee about that potential being realized.
Unfortunately, not every field is the picture of health that I described above. Some fields were planted late, and often in less-than-ideal conditions, so their root systems will probably struggle to establish well and their top growth may develop slowly, especially if cool, cloudy conditions continue. A much more widespread problem has been leaf-wrapping (buggy-whipping) in plants. Kevin Black of Growmark reported that some fields with plants as small as 6 inches tall are showing this symptom. Because this phenomenon is somewhat related to growth rate, and growth rates in plants that small are relatively slow, we expect that some of these fields may be showing effects related to acetanilide-class herbicides (which tend to slow growth) and/or to growth regulator-type herbicides (dicamba, 2,4-D), which tend to increase growth rates. But many fields that show this symptom had neither type of herbicide applied. Anecdotally, hybrids showing this symptom that did not have either type of herbicide applied tend to be those with higher growth rates following emergence. There might be other genetic factors, but these are not well-defined.
We think that the basic cause of leaf-wrapping is the inability of the older leaves to relax quickly enough to allow the younger leaves to push up through the center of the whorl. One way this could happen is for the older leaves to simply be growing more slowly than the newer ones. Cool weather may, for example, reduce the rate of growth of the outer, older leaves more than that of the inner, newer leaves. Or the older leaves may stay more "folded" than normal, or they might develop leaf surface characteristics that make it hard for young leaves to slide past. Once a leaf gets caught up and unable to push out, then the "tie-up" takes place rapidly, and all leaf tissue on the plant can become a tightly rolled mass. At some point, this restricts the growth rate of the newest leaves due to physical restriction. More importantly, curling up all the leaves into a tight roll cuts the rate of photosynthesis of the green leaves, by making the leaves upright and thus unable to intercept sunlight and by cutting down on movement of carbon dioxide into the leaves. Thus tightly rolled leaves probably do almost no photosynthesis; the longer plants stay this way, the more sunlight is wasted. Plants do not grow when there is no photosynthesis: This is a true example of "stress."
It is possible that leaf-wrapping can cut yield potential, especially if it persists much past V6 or so, when ears initially start to form. If we get some warm weather, the pressure from the new leaves will eventually cause the whorl to break open. When this happens, the leaves that have been rolled up with no access to sunlight will suddenly appear as pale yellow or white "flashes" as you look out over the fields, especially when it is breezy. Leaves can green up only when they encounter sunlight, so it's no surprise that such leaves aren't green when they first break out. They might have inadequate nitrogen as well. But they will recover quickly, especially if sunlight and temperatures are both favorable. So, appearance of these yellow-white leaves is a sign that the crop is starting to break out of this problem, but it also is a reminder that the crop has not been able to function fully for a period. Leaves should green up within a day or so, but as new ones appear, the yellow-leaf symptom may continue to show up in the field for some days.
Cool weather by itself also represents a stress, in that photosynthesis is low when temperatures are low. There has also been some light frost damage in low parts of fields, especially where there is crop residue that keeps the soil from radiating heat to help protect the leaves. Temperatures in the low 40s can also cause some leaf damage that may not show up very well, but that can reduce leaf function for a day or two. While we don't have a good way to link these early-season problems with yield potential, we do think that temperatures in the 30s last year when some corn was in the V4 to V5 stage may have caused some abnormalities in plants later, including loss of yield. This year, crop condition is considerably better in most fields than it was a year ago, so we are optimistic that the crop can grow out of its present condition without loss of yield potential. In the meantime, both the crop and those who watch it will continue to be stressed.--Emerson Nafziger