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Crunch time for corn

Emerson Nafziger
July 3, 2017
Recommended citation format: Nafziger, E.. "Crunch time for corn." Department of Crop Sciences, University of Illinois at Urbana-Champaign, July 3, 2017. Permalink

While the record will show that corn planting progressed at a more or less normal rate this spring in Illinois, 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. Some fields damaged by water and some that were too wet to plant before late May likely were planted to soybeans instead of corn. The June 30 acreage report shows Illinois corn acreage dropping by 500,000 from 2016 to 2017 (to 11.1 million acres) and soybean acreage increasing by 290,000 acres, to 10.4 million acres in 2017.

The Illinois corn crop condition ratings (from NASS) reflect both the poor growing conditions during the first weeks of May and the fact that so much replanting took place. The May 14 rating showed that only 42 percent of the corn crop was in good or excellent condition. This rose to 52 percent by May 28, and to 59 percent by June 4. It has remained around 60 percent for the past month, and was 62 percent on June 25. That’s lower than in any of the previous four years, and is lower than the 70 percent or more that is typical for a corn crop on its way to high yields.

The weather so far in the 2017 growing season will look more or less average in retrospect, but has been more variable than usual. April was relatively warm with average rainfall, and May was wetter to much wetter, and a bit cooler than normal. The first half of June was dry with temperatures 2 to 5 degrees above normal, while during the second half of June, rainfall varied from below to above normal, and temperatures were 2 to 3 degrees below normal. 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.

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 five 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 Illinoi 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. Having leaves roll indicates a shortage of water in the plant, and cells in any internodes that were developing at that time were not able to compete very well for water. Such cells elongated less than they normally would, and once the cell walls hardened after that, these internodes stayed short.

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 than early-planted corn. 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.

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. There is a little less growth of the crop this year than last year, so a little more N yet may need to be taken up this year. But soil N levels aren’t low enough to ring any alarm bells, and as pollination approaches and canopy color remains good, it’s unlikely that the crop is going to run out of N, especially if soil water supplies remain adequate. Adequate water not only carries N to the roots for uptake, but also helps maintain mineralization needed to make N available from soil organic matter.

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. We simply can’t know how this will end until we can count kernels and assess the state of the canopy by the time kernels start to add dry weight, about a month from now. We remain optimistic.

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