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Stresses Build on Crops

July 12, 2002
The last rainfall of more than 0.05 inch at Urbana came on June 14, and 9 of the 10 days through July 8 at Urbana have had high temperatures of 90 deg F or higher. Night temperatures over those same 10 days have ranged from 67 deg to 76 deg F, with an average of 71 deg F. The early corn is beginning to pollinate, while the later-planted corn is rolling its leaves, often before mid-morning. Although we know that some other parts of Illinois have enjoyed more rainfall than we have received in the east-central area, we know that prospects for the 2002 corn crop in Illinois are starting to be seriously compromised by weather. Here are some thoughts and observations:

How much has the corn crop been hurt so far?

Despite the gloom expressed in the opening paragraph, we still have the potential to produce a good crop in Illinois in 2002, though the chances of its being as good as the 2001 crop are probably zero. Stands are mostly good, and in areas that have received some rainfall root systems have probably gotten enough of a boost to extend into the water that is stored in the soil. Compared to very dry springs (such as 1988), stored soil moisture is in good supply from the spring rainfall. The early-planted crop has been able to tap this moisture and is entering the pollination period in reasonable shape in many fields. Still, a month without rain takes its toll, and it is likely that the stress so far will have some effect on yield of the earliest corn. Corn planted in early April at Urbana had silks emerge about July 6, which was about 2 or 3 days after pollen shed began. That's not unusual, but pollen shedding was heavy, and it probably will not last as long as it would have under cooler temperatures. Silks are also under pressure from Japanese beetles and rootworm adults, so fewer kernels could set than might have set without this additional stress. The biggest problem with early-pollinating corn may well be kernel abortion, which could be substantial if we continue for another week or more without rainfall.

Unfortunately, the late-planted crop, much of which was planted with some degree of soil compaction, was not able to grow its root system deep enough to tap the soil moisture very well. As a result, it has undergone visible stress (leaf rolling) "early and often," and it's fair to say that it has probably not been able to photosynthesize at full rates for most of the past 2 weeks. A corn plant with rolled leaves is surviving until the next day but isn't doing much else; it can't grow if it can't photosynthesize. Plants whose leaves roll in the morning are not taking any advantage of the brightest sunshine that day. Warm nights (minimum temperatures above the low 60s) also hurt in that they speed up the respiration, thus wasting some of the sugar that the plant was been able to produce during the day. About the only hopeful thing we can say about the late-planted crop is that it still has some time before it reaches its critical pollination stage. Rainfall anytime in the next week or 10 days will help this crop a great deal. But late pollination means late grain filling, and unless the weather pattern changes drastically kernel number and final size will be reduced in the late-planted crop.

Is it the heat or the dryness that's causing the problem?

Although I see little evidence that we can do much about either rainfall or temperature, the answer to that question is definitely "dryness." Daily high temperatures in the low 90s are very favorable for photosynthesis, providing that the crop has adequate soil moisture to keep water streaming through the plant and out through the leaves (as water vapor) during daylight hours. A full canopy in July requires almost as much water as an open pan would evaporate on that day, which is between 0.25 and 0.3 inch on a warm, sunny day. That's why we have corn still able to pollinate after 25 rainless days--it has probably tapped about 5 inches of stored soil water so far. Even the best soils, though, cannot store much more than a month's worth of water for the crop, so we are dangerously close to running out of stored water for the largest corn in the driest areas.

How much longer can the crop "hold out"?

Under extreme and long-running lack of rainfall in 1988 (it rained less than a half inch from May 22 to July 14 at Urbana that year), the crop survived and was even able to pollinate and form small ears after several weeks during which it did little but survive. That's more extreme than we have experienced (or that I hope we experience) this year. It does give us a hint that the crop may have more resilience than we credit it with. Part of the reason for this is that the research that has been done on the effects of short periods of drought on corn yield potential has been done by "artificial" imposition of water deficits in the plant, usually by methods that allow water supply to be cut off and restored very quickly. Plants in the field usually develop water deficits slowly and so are probably able to make some adjustments that they can't make in the research studies that have been done. Still, a week or two with little photosynthesis means that the crop simply is not using the resource (sunlight) it's designed to use, and that cannot help but decrease yield potential unless the weather turns extremely benign during the next two months.

Is there anything we can do?

We can only watch and hope for rain (and, if we're so equipped, throw the switch on the irrigation pump). It is instructive to walk in the fields to see what effect lack of water has had on the crop so far. Some of this is subtle, but if the internodes are shortened, it means that the crop has undergone several weeks of serious stress, during which plants either do not fully hydrate during the night or photosynthesis rates are inadequate for growth even if there is water "recharge" at night. Internode shortening does not by itself decrease yield potential much, but very dry conditions can decrease even the size of leaves and the lack of a full canopy usually decreases yield potential. The key thing to look for, though, is the appearance of tassels, pollen, and silks. Dry weather often throws off the normal timing of the appearance of these, and in extreme cases, tassels may emerge and shed all their pollen (it usually takes 5 or 6 days but can be a shorter time if temperatures are very high) before any silks emerge.

The primary key for success in pollination, though, is the appearance of ear shoots and silks. If tassels are out and shedding pollen but ear shoots do not seem to be emerging from the ear leaf sheath, then there is little hope that the plant can "push" ear shoots and silks in time to receive pollen. When plants are able to muster enough water to push ear shoots, they usually can push some silks as well; but studies have shown that silk elongation is one of the most water-sensitive processes in the corn plant. Once silks do struggle out, of course, they need to be protected from insects that can chew them off. Cutoff silks can receive pollen, but the "target" offered by the cut ends of silks is much smaller than that offered by the normal mass of silks; so successful capture of pollen by the silks is much less certain, even if the plant still has pollen to be shed. One very slight positive effect of early-season unevenness in growth might be a modest extension in the pollen-shedding period this year. It isn't much, and it won't solve yield loss by the late-emergers; but it might provide pollen to some last-minute silks.

Is there anything we should have done differently to prevent these problems?

No. Until and unless we can get accurate seasonlong weather forecasts (and with the forecast for the next 12 hours changing as it often has in the past weeks the chances do not look good), we need to manage in anticipation of average weather conditions. We warned about planting before soils were dry enough when planting was delayed in May, and most people exercised reasonable care in getting the crop planted, even though many fields were compacted. We are reminded once again that weather can override almost everything we do to manage a crop. We still have to manage astutely, though, or poor weather will cause even more damage; and, worst of all, a poorly managed crop won't be able to take advantage of good weather.

What about soybean?

With smaller root systems than corn, soybean is also not in a good position to tap soil water very well. Fortunately, its delayed canopy development has meant lower water-use rates, and so far the crop seems to be growing, at least slowly. Much of the crop has reached flowering, even though many fields are less than 12 inches tall and probably have only four or five trifoliolate leaves emerged. In our planting date study at Urbana, the plots planted on April 5 are almost 30 inches tall and are starting to set pods, but marginal water supplies have caused the leaves to be smaller than normal, and the plants have not filled the middles of 30-inch rows. Narrower rows will fill better and should not have this problem. And, if we get some rainfall, soybean will respond well. In general, soybean plants are not yet into critical stages, and yields have probably not been limited much, if any, by weather so far.--Emerson Nafziger

Author: Emerson Nafziger


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