Variable weather shows as corn begins to silk

2017-07-14T10:13:00Z Variable weather shows as corn begins to silk Illinois Farmer Today
July 14, 2017 10:13 am

Editor’s note: The following was written by Emerson Nafziger, University of Illinois Extension crop production specialist, for the Illinois Pest Management Bulletin July 3.

While the record will show corn planting progressed at a more or less normal rate this spring, wet, cool conditions that developed after nearly half of the crop had been planted resulted in a great deal of replanting, especially in the flat-soil areas of Illinois.

The weather so far in the 2017 growing season will look more or less average in retrospect, but has been more variable than usual.

Even though they took a roundabout way to get there, growing degree day (GDD) accumulations were close to normal by the end of June, and corn planted in mid-April in central Illinois had accumulated enough GDD to be at or near silking.

The concern about loss of nitrogen and not having enough N for the crop has faded over the past month, as leaf color has deepened under warmer conditions and as plant growth has taken off. By mid-June, our measurements of soil N have shown levels almost as high as we saw in mid-June in 2016.

The largest concern now, as it almost always is at this time of year, is having enough water and sunshine to maintain photosynthetic rates in order to get the high kernel numbers we need to produce high yields. It is possible that the rollercoaster conditions over the past two months have had a negative effect on how many kernels will set per ear.

Any such effects would likely be subtle, often related to such factors as leaf area or effects of stress on the number of kernel rows now developing.

Very good pollination conditions — plenty of rainfall, good sunshine and average temperatures — can overcome such pre-tassel effects, but will need to last for two weeks or so after pollination to keep kernels from aborting.

One notable feature of the corn crop as we approach the critical pollination period is the short plant height in most fields. Plants in some fields are only 5 feet tall or so as tassels begin to emerge.

This is widespread in central Illinois, though the degree of shortening depends some on how much rain has fallen in the past few weeks.

I traveled in southwestern Illinois early last week, and early planted fields there were of normal height (about 6 feet tall) right before tassel emergence. Those in northern Illinois have a little more time before they tassel, and they might get to more normal height as well, especially if they were planted in May.

Why are early planted plants short this year? It’s an unusual combination of factors, starting with cool, wet soils in May that both restricted root growth and slowed plant growth, causing roots to grow slowly out into the bulk soil.

Then came warm and dry weather in early June, with widespread afternoon leaf-rolling caused by high evaporative demand and root systems unable to take up water fast enough to meet the demand.

Low temperatures during vegetative growth in June also worked to keep internodes short, even if there was adequate water. Night temperatures fell into the upper 40s for a day or two during the last week of June, and coming after the high temperatures and lack of rainfall earlier in the month, this likely contributed to having some upper internodes stay shorter than usual.

If we look at internode length after pollination (when plant height is fixed) we will be able to tell when stress occurred by which internodes are shortened.

Is below-normal plant height in corn a problem? If the plant has a normal amount of healthy leaf area (at 32,000 plants per acre, that would be in the neighborhood of 6 square feet of leaves per plant), high yields would be possible with plants only 6 feet tall or so after pollination.

But leaves have to compete for water in order to enlarge just like stems have to compete for water to elongate, so leaf area on short plants is often less than it is in taller plants.

Because the sun is never directly overhead, having leaves a little farther apart on the stem (that is, longer internodes and so taller plants) also improves light interception a little bit.

Most people who have watched the corn crop for many years observe that, while good yields are possible on short plants, really high yields (250 bushels per acre or more) are, all else being equal, more likely on plants that are 8 or 9 feet tall than on plants that are 6 or 7 feet tall.

In the same way that short plants have likely experienced some stress that might affect yield, early planted corn that grows tall has experienced little if any stress. That means it has been able to maximize its size and its ability to produce high kernel counts based on the leaf area, roots and stalks that it has developed.

That is not to say that tall corn always yields more than short corn. Late-planted corn often grows taller than early planted corn because it’s warmer when the stem is elongating. Some replanted corn this year will escape the conditions that shortened early planted corn, and so may be a lot taller.

But just being taller does not mean higher-yielding — late-planted plants tend to have less dry weight by the time of pollination than early planted plants, and so less capability for forming and filling the large number of kernels that high yields require.

Copyright 2017 Illinois Farmer Today. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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