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The Bulletin

Mid-June Notes on the 2021 Corn Crop

Emerson Nafziger

Department of Crop Sciences
University of Illinois

June 16, 2021
Recommended citation format: Nafziger, E. "Mid-June Notes on the 2021 Corn Crop." Department of Crop Sciences, University of Illinois, June 16, 2021. Permalink

The 2021 corn crop was planted ahead of normal, although “normal” includes the very late planting in 2019 and late planting in 2020, so “typical” might better describe 2021 planting progress. There were no extended wet periods in April or May, but wide swings in temperatures affected emergence and early growth. Crop ratings are about average for mid-June, but have slipped some, probably due to dry conditions in some regions. June temperatures have been favorable for crop growth, and sunlight amounts have been good.

Concerns about dryness continue, with most of Illinois north of I-80 currently designated as abnormally dry or in moderate or severe drought, worsening as we move from the Quad Cities to the northeast corner of Illinois. There are some small pockets of abnormally dry conditions elsewhere, but about 80% of Illinois is designated as free of drought. June rainfall has been below normal over most of Illinois, though, and as crop demand for water increases, a change to more widespread rainfall would be welcome.

High crop prices have heightened expectations, but have also brought pressure to maximize yield, despite weather uncertainties in some regions. Following are some observations regarding this year’s corn crop, as we consider in-season management and take lessons from this year’s experiences.

Crop progress

Stands are good to very good in most fields, and warmer temperatures have increased growth rates and improved uniformity in plant size. Parts of April and May were cool, but growing degree day accumulations were close to average in April, a little below average in May, and above average during the first half of June. From April 1 through June 15, GDD accumulations range from about 900 (100 above normal) in northern Illinois, 950 (near-normal) in central Illinois, and 1050 (50 below normal) in southern Illinois. Totals are about 200 to 250 GDD lower that this for May 1-planted corn. Nearly all of the Illinois corn crop has entered its most rapid growth and nutrient-uptake phase that begins at about stage V8. Canopies are closing rapidly as leaf area expands, and we expect sunlight interception to reach 90% by the time plants are at stage V12, or about chest-high. Getting there by the longest day of the year helps boost yield potential by increasing kernel set during pollination.

If the next month brings average temperatures, corn planted in early April should begin to pollinate the last few days in June, and corn planted around May 1 should begin to pollinate the second week of July. Below-normal rainfall in most regions recently has stimulated root growth due to good soil aeration and receding soil moisture. Less cloudiness has meant good sunlight amounts, so high photosynthetic and growth rates. Increasing crop demand for water as canopies fill out will become a problem if rains don’t return, at least in drier areas and on lighter soils. As long as leaves remain flat and not curled up during the day, growth is continuing. Smaller corn plants with smaller root systems often show curling that begins earlier in the day, but such stress has less effect than on larger plants. Leaf curling in larger plants may be accompanied by shorter internodes due to lack of water for cell expansion. As long as the water supply returns to near normal by a week or ten days before pollination, kernel set and yield potential should be good.


Nitrogen: Temperature fluctuations after emergence caused canopy color to fluctuate in May before improving under warm conditions in June. Night temperatures in the 30s or low 40s cause loss of chlorophyll and paler leaves; it takes several days of warmer temperatures to regenerate chlorophyll and for photosynthetic and growth rates to return to normal. The amount of N available to the nodal roots as they emerge can affect how quickly plants recover, and starter fertilizer might have helped such recovery this year. Warmer soils also help, both by causing roots to grow faster and by increasing the rate of mineralization to provide N to the roots. With the warm soils and little movement of N as soils remain relatively dry, both crop growth and crop canopy color have recovered well in most fields. Depending on planting date, soil type, and N fertilizer practices, many fields are taking on the dark green color that signals a plentiful supply of N.

Good canopy color reflects both improved crop growing conditions and lack of saturated soils that can compromise roots and result in N loss. The 2016 season began much like this current one, and that season was marked by good yields and by below-normal need for high N rates. Over 34 on-farm trials in corn following soybean in 2016, the average optimum N rate was only 157 lb/acre, which produced an average yield of 225 bushel per acre. The MRTN rate in place in 2016 was 175 lb/acre in central Illinois, and about 75% of the trials needed less N than that. The MRTN rate has risen with addition of responses from recent years, and there is no reason to think that the 2021 crop is going to need more N than the current MRTN rate of 175 to 200 lb N per acre (N rate calculator). When the canopy has good color during the second half of June following application of N rates in this range, there is little chance that adding “insurance” N will increase yield, and even less chance that it will increase profit. N uptake slows after pollination, and with N in the soil and more being released by mineralization, chances of developing N deficiency more or less disappear once ear formation begins, although dry soils during grainfill can limit N uptake and so can cause deficiency regardless of how much N is in the soil.

Phosphorus: Purpling of plants that is often associated with phosphorus deficiency was fairly common earlier this year, especially on hybrids genetically prone to showing this. Purple color develops when sugars produced by photosynthesis are unable to move out of the leaves quickly enough, and their buildup leads to the formation of purple pigment. A major “sink” for sugars is roots, which grow and function slowly in cold or dry soil, causing sugars to accumulate. Low plant P content can slow sugar movement out of leaves, so can also contribute to a buildup of sugars. The good news is that this condition goes away quickly once the cause of sugar buildup diminishes—which usually means once soils warm up and soil moisture is restored—with no lasting effect on the crop.

Potassium: We haven’t heard many reports to date, but dry soils that limit root development can limit K uptake, and if soils stay dry we might begin to see K deficiency symptoms. Roots need to grow out into the soil volume in order to take up K, which moves very slowly towards the root on its own. So when roots have difficulty growing out into the soil during the time of most rapid K uptake (from stage V8 to V15), K deficiency symptoms can occur even when soil test K levels are high. This is more common with no-till, typically when roots have difficulty growing out of the planting furrow and into the soil between the rows when soils are dry. Rainfall relieves such symptoms quickly.

Sulfur: Sulfur deficiency has somewhat unexpectedly appeared in some fields this year, even in deeper prairie soils where we seldom see such symptoms. Lower-lying areas with higher soil organic matter may show not symptoms, while plants in higher parts with slightly lower OM may show paler green color that identifies S deficiency. The major function of both N and S in the plant are as components of proteins, and deficiency symptoms can look similar. S-deficient plants may show interveinal striping (lighter and darker green alternating on the newer/upper leaves), and S is less mobile in the plant than N, so lower leaves in plants with S deficiency tend to stay relatively darker green than those with N deficiency. Mineralization mobilizes both N and S from organic matter, and so we seldom expect to see S deficiency under conditions like those that prevail now. Cool soils in May might have limited the mineralization process, and since more fields get some N with early fertilizer than get S, more fields might be showing S deficiency.

Should S be applied if the crop is showing deficiency? This is a difficult question, but if symptoms are mild and not clearly identified as being due to S deficiency (applying some S is the only way to know for certain), it may be better to wait for another week to see if the symptoms disappear. If symptoms are clear and are strongly suspected of coming from lack of S, then applying S may be appropriate. Elemental S will become available to the plant too slowly to correct a deficiency, and foliar forms are less common. The best option is probably to dribble ammonium thiosulfate solution (12-0-0-26S) at perhaps 5 gallons per acre (14.5 lb S) near the row.

Cover crop

Corn following a cereal rye cover crop has become more common, and this has brought increased management challenges. Cereal rye works well as a way to take up N from the soil following last fall’s harvest, thereby keeping it from reaching tile drainage water. But we need to learn how to make this work without lowering corn yields. The aggressive growth that makes cereal rye good at taking up N also makes it an aggressive grass weed in corn, and only by making sure rye is dead before corn is planted can we minimize the chances of corn yield loss. In a spring with dry conditions like those in some areas this year, water taken up by the rye can directly limit the amount available to the corn seeds, which can affect uniformity of emergence and corn plant growth, even when rye was killed before corn was planted. But the biggest danger comes from having rye roots active in the soil during corn crop establishment. This creates competition for water and for N, and the corn plant seldom wins such a competition. This year we have heard of cases of tilling rye rather than killing it with herbicide; planting ”green” without killing the rye before corn planting; and mowing off rye plants, killed at or after planting, after corn emerges. All of these can be expected to decrease water and N available to the corn crop, and some may also lead to allelopathy—damage to corn plants from substances released as residue breaks down. Adding more N now won’t fix problems like these once the corn is damaged, but we can take a lesson for next time.

Other inputs

Dry weather has meant relatively little foliar disease development in corn, although that could change if the weather pattern changes. While low disease usually limits response to foliar fungicides, there seems to be increasing emphasis on using foliar (strobilurin) fungicides as “stress relievers” for crops. There is some physiological basis for this: some strobilurins decrease plant respiration rates for a while after application, and although plants need to respire, a temporary decrease in respiration can increase the amount of sugar available for plant growth during critical periods. This may theoretically provide more of a boost for C3 plants like soybeans and wheat than for corn, which is C4. But such are marketed as a way to diminish the effect of “stress” on corn, including stress due to things such as leaf loss as well as drought. This issue is complicated by timing and severity of drought and other stressors, and with little research data available, the conclusion is that fungicides work most consistently as fungicides and not as stress-relievers.

It’s too late to apply most other novel products such as microbes that might be sold as a way to increase root growth and thereby help the crop when it’s under moisture stress. Some of these are now being promoted for application with post-emergence herbicides, presumably as a way to lower cost, but possibly also because this makes it difficult to leave strips without the additive, since all strips need herbicide. If other products are out this year, we hope they are in strips to allow evaluation of both visual and yield effects.

When spray planes fly over corn fields, insecticides are often in the spray tank, not because there are insects known to be threatening the crop, but because including them is inexpensive and application is “free.” One certainty is that such applications will kill beneficial insects—those that help control actual crop pests. In this way, insecticides without a target can actually worsen, rather than relieve, damage form insects.


Today’s hybrids have a remarkable ability to develop and maintain a complete, dark-green canopy, which in turns results in high kernel number (per acre, not per plant) and high yield potential. Although water-gathering ability of hybrids is also very good, inadequate water remains the largest threat to the corn crop, as long as other factors such as pests are not limiting. With a full canopy, the Illinois corn crop has a daily water requirement similar to the estimated potential evapotranspiration (PET) that is calculated daily at various weather-recording sites. This number can be as high as a quarter to three-tenths of an inch in mid-summer, and totals of 6 to 7 inches for the month of July are typical. If the crop needs 6 inches and 4 inches of rain falls during the month, the soil needs to supply the other 2 inches in order to maintain yield. If only 2 inches of rain fall and the soil is unable to provide the other 4 inches, then the crop will spend days or afternoons unable to take up water, which limits photosynthesis and yield. How much of this happens on a daily basis, compared to having the canopy active all day long, is a good measure of how yields will end up. The corn crop fills grain for roughly 40 days once it gets past the lag phase, and that means that a good crop needs to add an average of 5 to 6 bushels per day, and more than that on sunny, warm days to make up for those days that are cloudy.

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