No. 21 Article 7/August 14, 2009

Corn Crop Canopy 2009

It takes about 60 days from the end of pollination to physiological maturity (black layer) in most corn hybrids grown in Illinois. By the middle of August the Illinois corn crop is normally about halfway through the grain-filling period, or mostly in the dough stage, with some early-pollinated fields in or approaching early dent. By that time, it's generally possible to assess how well the canopy has held up, how well the grain has filled so far, and the prospects of how well the second half of grain-filling will go.

Things won't be quite as clear this year, given the late start for the crop and an unusually slow rate of growing degree-day (GDD) accumulation due to cool temperatures in July. The warm temperatures of the past week have helped bring the crop around some in terms of development. This is a mixed blessing. Average high temperatures during August are in the mid-80s, not too far below the cutoff temperature of 86 degrees, above which we say that GDDs do not continue to increase. That means that the only way to have above-normal GDD accumulation during August is for night temperatures to be above normal. The crop is never very well served by night temperatures above normal during August, so it's not clear whether catching up through increased GDD accumulations will be a net positive. The crop in parts of Illinois where planting was very late may end up needing the extra GDD to reach maturity before frost, but overall the crop will likely be harmed more than helped by warmer-than-normal August weather.

We think that pollination was successful in most fields and that the water supply was adequate to keep to a minimum the abortion of kernels that were successfully fertilized. In a few fields I have seen a few late-emerging silks like we have observed in previous years. These have stayed fresh after the rest of the silks turn brown, and the lack of pollen means that they won't be fertilized. But their number is relatively low--only 15 to 30 or so silks remain attached at the tip when the husks are removed--and the number of developing kernels is high enough in most cases that a few tip kernels won't be missed.

Our biggest concern now is the current state of the canopy and whether it will stay in shape to fully fill the grain that has formed. Kernels that have reached the milk stage are entering the period of linear, rapid dry weight accumulation, and the only possible way for them to reach their full potential will be with "care and feeding" by a canopy that can photosynthesize at the maximum rates allowed by the sunlight and temperature.

Water supply is a critical part of this. The stomata--tiny openings that allow water vapor out as CO2 enters the leaf--need to remain wide open during the day, and they can do this only when there is enough water available to the roots. Water use rate, which is closely tied to the health and activity of the crop canopy, should be as high as 0.2 to 0.25 inches per day now, with higher rates soon after pollination and with high temperatures, windy conditions, and maximum sunlight. Plants can generally hydrate at night even if soils are getting dry, but as water uptake begins after the stomata open, deficits develop when the demand for water exceeds to ability of the crop roots to take up water. Well before you see leaves starting to wilt or to take on the silver-gray appearance indicating that they're starting to dry out, the stomata start to close. When soils get very dry, stomata may open in the morning but close again before noon. In such cases most of the sunlight that falls that day will do the crop no good.

We have started to get some reports of upper leaves starting to lose their color (and to see them--overpasses are a good vantage point for this). This may be due to loss of N and to availability of water in the soil; water uptake moves N to and into the roots, and lack of water almost always means some lack of N. In some cases lower leaves are showing some firing as well. Loss of color from upper leaves is more problematic, since upper leaves are in a position to receive maximum sunlight. Upper leaves are the farthest from the water in the soil, and under hot, dry conditions they may simply not get enough water even if there is water in the soil. In such cases rainfall may help restore color (and photosynthetic capacity) in these leaves. The lowermost leaves are typically shaded, so their firing may not mean as much direct loss. If the leaves in the vicinity of the ear remain healthy and active, the plant can still take up much of the sunlight, and yields can be good. But it's still best if all leaves on the plant, other than perhaps those that are completely shaded, retain their green color to near the end of grain-filling.

As I've said many times before, the real health of the corn crop canopy can best be seen by noting how well the canopy intercepts sunlight during midday. Any sunlight that hits the ground is lost forever to the crop--the grain will continue to develop almost regardless of how much dry weight is deposited in kernels on a given day. So the fewer patches of sunlight that hit the ground the better. The amount of light getting through the canopy is never zero, but it can be less than 2% to 3%, and for best yields it needs to be this low.

We normally point to the pollination period as the most critical stage for the corn crop; yields are certainly limited without adequate kernel. But it's the grain-filling stage that usually makes the difference between good, very good, and outstanding yields. Normally, the linear phase of kernel weight accumulation starts about 15 days after pollination (this might have been closer to 20 days this year with the cool temperatures) and lasts about 40 days, slowing as the crop approaches black layer. We have measured yield accumulation rates as high as 11 bushels per acre per day during the middle part of the grain-filling period.

To put this in perspective, note that a 200-bushel crop averages only about 5 bushels per day for 40 days, and if we can move that to 7 or 8 bushels per day we can approach 300-bushel yields. Why does this happen so rarely? To a large extent it is because of events we can't control. Filling rates can be decreased by hail damage to leaves, for example, and a windstorm can lodge plants and decrease light interception. But the larger causes are loss of sunlight, lack of adequate water, and temperatures that are too low during the day or too high at night. High night temperatures do not directly affect daytime photosynthetic rates, but they do mean higher-than-normal rates of respiration at night, which uses up some of the sugars produced during the day, so they have the same effect as reducing photosynthesis.

Unless we can turn a switch to put water on a field when it's dry, there is not a great deal that can be done at this point to assure maximum rates of photosynthesis and grain-filling over the next month. It is very useful to look at canopy completeness and health, and in a few cases (mostly in late-developing crops) it might pay to apply fungicide if diseases are starting to affect leaf health, or even insecticide in that rare case when insects might be eating a lot of leaf area. You will also be able to see where stands are too thin for maximum sunlight interception and where row spacing might be too wide for the hybrids used. "Managing for canopy" means using such observations to fix management-related problems, and it is a sound approach, even if it's after the fact.--Emerson Nafziger

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