Corn Earworm, European Corn Borer, Fall Armyworm, or Western Bean Cutworm: Which One Is Causing the Injury I’m Finding on My Corn Ears?

Several questions about injury on corn ears has made it way to my desk the past week.

Insect injury to corn ear (photo courtesy of Duane Frederking).

Damaged ear tips, missing kernels, and fungal pathogens are all being reported. Several insect pests in Illinois could be the culprit. Corn earworm, fall armyworm, European corn borer, and western bean cutworm are pests of Illinois cornfields. Their larvae all feed on the ears of corn plants.

So how does one determine the cause of ear damage this late in the season? The answer is simple: You really can’t. At this time in the season, it is rare to find any larvae still feeding on corn ears. Without larvae, you can’t be positive if injury was caused by earworms, corn borers, fall armyworms, or bean cutworms, as they cause very similar injury. Let’s look at each insect individually.

Corn earworm. Two generations of corn earworm infest Illinois cornfields each year. Because earworms generally do not overwinter in Illinois, summer populations arise primarily from immigration of moths from southern states in late spring and early summer. Infestations of earworm larvae can cause injury to corn plants, including slight defoliation of leaves, damage to the tassel, and consumption of silks and kernels. The second corn earworm generation usually occurs during pollination. Larvae enter the ear primarily through the silk channel, unlike European corn borer and fall armyworm, which enter through the husks or cob. As silks dry, corn earworm begin feeding on kernels. Larvae feed at the tip and along the sides of the ear near the tip, continuing to feed until they mature. At that time the larvae drop to the ground to pupate. When leaving the ear, corn earworm may drop from the ear tip or create exit holes by chewing through the husk. These exit holes can be mistaken for entrance holes caused by other larvae.

Corn earworm larvae.

Corn earworm injury to corn ear.

European corn borer. Two to three generations of European corn borer occur in Illinois each year. Injury to corn ears is caused by the second and third generations. Loss of grain to larvae’s direct feeding on kernels has not recently been an issue in field corn, but in sweet corn and seed corn, losses can be significant. We’ve also received reports of corn borer feeding in non-GMO corn. Larvae feed on pollen and silks before entering the ear. Entry to the ear is also gained by tunneling through the shank and cob. Ear feeding by corn borer larvae is not focused on any one area. Injury can be found at both ends and along all sides of the ear. Larvae feed until mature; they overwinter as fifth-larval instars in stalks and plant debris.

European corn borer larva (photo courtesy of Marlin Rice, Iowa State University).

European corn borer injury to corn ear (photo courtesy of Marlin Rice, Iowa State University).

Fall armyworm. Like the corn earworm, fall armyworm moths migrate north into Illinois each year. Fall armyworms are a concern for cornfields from mid- to late summer. They cause serious leaf-feeding damage and feed directly on corn ears. Late-planted or later-maturing hybrids are more susceptible to fall armyworm injury. Most common is pretasseled corn. Larvae consume large amounts of leaf tissue, but as corn plants develop, larvae move to the ear. Unlike the corn earworm, the fall armyworm feeds by burrowing through the husk on the side of the ear. Larvae also enter at the base of the ear, feeding along the sides and even tunneling into the cob. They usually emerge at the base of the ear, leaving round holes in the husks.

Fall armyworm larva.

Fall armyworm injury to ear.

Western bean cutworm. A mid- to late-summer pest of corn, western bean cutworm moths begin to emerge in early July. Though some leaf feeding occurs, larvae feed primarily on silks, tassels, and developing kernels. Larvae of the western bean cutworm are not cannibalistic, and several larvae may infest one ear. Entry to ears is gained through silk channels or by chewing through husks, injuring the tip, base, and sides of the ear. Larvae feed on kernels until about mid-September, when they exit through husks. Reports of western bean cutworm injury have been very sporadic the past couple of years.

Western bean cutworm (photo courtesy of Marlin Rice, Iowa State University).

Western bean cutworm injury to corn ear (photo courtesy of Marlin Rice, Iowa State University).

Any one or combination of the aforementioned insects could be the cause of the injury being seen in cornfields. As much as we would like to be able to pinpoint the direct cause of injury, that is often impossible this late in the season. Summer scouting is the key to determining the potential insect culprits.


(Updated from 2005 article)

How can we improve your experience with the Pest Degree Day Calculator?

Insects require a certain amount of heat to develop from one stage in their life cycle to another (eggs to larvae to pupae to adults). Degree-days measure insect growth and development in response to daily temperatures. The accumulation of these degree-days can be measured over a period of time and used to estimate growth and predict insect development. Calculating degree-days allows us to predict when significant biological events such as the appearance of insect pests may occur or when they may reach a life stage that is damaging to a particular crop.

Fortunately, we have at our fingertips a calculator that can help us calculate degree-days for selected pests. The Pest Degree Day Calculator is a result of a collaborative scientific effort that combines daily weather data collected by the Water and Atmospheric Resources Monitoring (WARM) Program (Illinois State Water Survey, Prairie Research Institute, University of Illinois) and the Integrated Pest Management (IPM) Program (Department of Crop Sciences, University of Illinois) to provide daily, up-to-date information about pest and crop development in Illinois. Many of you may utilize the calculator to predict cutting dates in your area for black cutworm or to identify when rootworm hatch may be occurring.

We are working hard to improve our Degree Day Calculator. Let us know what you like, don’t like, or what you would like to see changed. Take our survey to share your thoughts: Thanks in advance for your help!

Agronomy Day 2016

Agronomy Day is a collaborative field day hosted by the Department of Crop Sciences in partnership with several academic units in the College of Agricultural, Consumer and Environmental Sciences (ACES). From nitrogen management to drone demonstrations Agronomy Day shares cutting-edge research with practical implications for your farm or business. CEU and CCA credits are available during tour stops. Want to know more about Agronomy Day? Sign up now for the Agronomy Day mailing list!

For directions and a list of field tour presentations, please visit the Agronomy Day web site.

Soybean: crunch time to come

The 2016 Illinois soybean story is similar to the corn story; current (July 24) crop ratings for both crops are similar to those we saw in 2014, when we produced the highest-ever yields for both crops. Illinois producers matched the 2014 soybean yield (56 bushels per acre) in 2015, despite the crop’s getting off to a very rocky start last year. Few surprises in crop production have been greater than that of seeing fields that looked marginal in July 2015 go on to produce 60, 70, or even 80 bushels per acre.

Compared to the 2015 soybean crop, the 2016 crop took off well and has looked good the whole season in most Illinois fields. Stands are good, growth is generally uniform, and unlike most seasons, there are few drowned-out spots in many areas if the state. In some places where June was dry then heavy rains came in July, Phytophthora has developed. But overall, the 2016 soybean crop is the most visually appealing one we’ve seen in a long time.

While we appreciate the outstanding appearance of this year’s soybean crop, we saw last year that soybean appearance even in late July is a poor predictor of yield. In fact, it’s possible that the soybean crop might even look (and be) “too good” for this time of year. We don’t say that about the corn crop, so why is soybean different?

Corn is a highly efficient crop that makes enough leaf area to form a complete canopy but not much more than that. Corn also has a single ear attached to the stalk, and all leaves feed sugars into that same stalk, so every leaf contributes to both forming and filling the ear. Pollination takes place over the course of a few days, and high yields depend on the number of kernels, so it’s “all hands on deck” for corn leaves, and having too much canopy or plants too tall is not an issue.

Soybeans differ in that flowering takes place over a period of several weeks, and the success of flowering (that is, number of seeds/pods formed) is closely tied to the ability of the leaf at each flowering node to photosynthesize at a rapid rate while the flowers are forming. When leaves are large and plants are tall, not all leaves can compete successfully for light, and they can be partially shaded at critical times, resulting in fewer pods formed and less ability to fill the pods that form.

Petioles (stems that support leaflets) in soybean can be as long as 18 inches to help leaves get up into sunlight. But when stems get to be more than 40 inches tall (to the tip of the stem, not the top of the canopy), lower leaves will be shaded or partly shaded much of the time.

Shading of leaves attached to the lower stem nodes is a problem not only in terms of pod numbers that form at those nodes, but also in these leaves’ getting enough sunlight later in the season to fill those pods. Tall plants with a lot of leaf area in mid- to late August may have few pods forming on the lower stem, and while this might be partly offset by having more pods at mid- to upper nodes, total pod numbers per plant is often decreased.

This “overgrowth” phenomenon in soybean has been known for a long time, and has given rise to various attempts aimed at reducing plant height or leaf area to “help” the soybean plant get over this problem. Dinging leaves with herbicide or growth regulator, “topping” plants by hand or mechanically, and generally undoing what (large) plants spent time and effort building has been the common theme. I’ve seen this work in the subtropics where intact soybean plants turn into vines, but in the Corn Belt, such treatments have almost always done more harm than good. It’s just difficult to get consistently positive results by beating up soybean plants.

By this time of the season soybean plants have not yet reached their maximum height; that will happen by about the second week of August, and plants could add 25 or 30% more to their height by then. Warm temperatures and good soil moisture will keep them growing. There’s not a lot we can do about that.

There is one thing we might not do that will provide some help: application of in-season nitrogen. Application of N (and fungicides) tends to keep leaves a little greener and to increase growth rates a little. Neither of these would be a good thing for a soybean crop that is already approaching stem height of 30 to 36 inches. For those who might want photos showing how tall your soybean crop is, have the person standing in them bend his or her knees a little. Or just acknowledge that really tall soybean plants are not usually the highest-yielding and that they are no indicator of best management.

Heavy canopies tend to keep humidity higher in the canopy, which can lead to more disease development. I haven’t heard too much about white mold so far, but these are the type of conditions that favor its development if it has already infected flowers. There are some other foliar fungal diseases such as frogeye leafspot that may be favored if it stays damp.

While we’ve found a yield increase of 2 bushels per acre or more from applying foliar fungicide in about half of the trials we’ve done, we’re not able to find many clues about when it might work and when it might not. There is no correlation between response and yield level, so “making high yields higher” doesn’t work as a principle. While we can’t rule out a physiological effect, it makes sense that using fungicides to help control fungal diseases should provide the most consistent return.

What’s our best-case for the 2016 soybean crop? Some cooler, drier weather will help slow growth down a little, but that may not be enough to bring back really high yield potential. Although we are concerned about heavy canopies, we have had years when this seemed to have less effect; only by looking at pod numbers per node in mid-August will we really know. If many nodes have 4 or 5 pods like we saw in 2015, we can consider this a false alarm, or at least a case where possible negatives were canceled by some positives, even if we don’t understand how.

Is the 2016 corn crop as good as it looks?

With the exception of a few cool and wet periods in May and some areas of southeastern Illinois that stayed wet and were planted late, the 2016 growing season has been very good so far. The Illinois corn crop was planted a little earlier than normal, stands are excellent, and the crop has had outstanding leaf color throughout the spring and into mid-summer. On July 24, 82% of the crop was rated good or excellent. That matches the late July rating in 2014, the best-ever Illinois corn crop at 200 bushel per acre.

Virtually all of the corn crop has completed pollination, and the half that pollinated by July 10 is now into the “roasting ear” stage (R3), with some already in the dough (R4) stage. With good soil moisture in most fields, kernel numbers are not likely to decrease due to kernel abortion, so the number of kernels in a field now is likely to be close to the final number.

As is always the case, yields are going to depend on two things: kernel counts and canopy. Counting kernels may not be great fun on a warm day, but it’s a straightforward exercise. Simply get a good count of the number of ears per acre (most people do this in 1/1000th of an acre, which in 30-inch rows is 17 ft. 5 in. of row) and then count the number of rows of kernels and the number of kernels in an average row to give kernel number per ear for several ears. Making these counts a few times across a field, and going to those spots randomly will give more realistic estimates. If the goal is to brag, choose the best spots to count. But decency requires that you move into at least the fourth row in from the outside of the field; outside rows have “indeterminable” row spacing so can’t be used to calculate yield.

Once you have the number of ears in 1/1000th of an acre and the kernel number per ear averaged over at least three ears, simply multiply those two numbers to give the number of kernels per 1/1000th of an acre. Typical numbers in a good crop might be 34 ears x 500 kernels per ear, or 17,000 kernels (17 million kernels per acre.) The tricky part of the yield estimate is trying to guess how large the kernels will get. The default we usually use at this point in the season is 80,000 kernels per bushel. We drop the 000s since kernel count is for 1/1000th of an acre, and divide kernel number by kernels per bushel. In our example, that would be 17,000 divided by 80 = 212.5 bushels per acre.

To fill kernels to their maximum will require a full crop canopy that stays green and active up until close to maturity. With warm temperatures continuing, the early-planted crop is on track to mature (reach kernel black layer) by late August or early September. So we need the canopy to maintain its color and activity (the two are very nearly the same thing) for 4 to 5 weeks more.

Leaves that turn from dark green to yellow have lost most of their photosynthetic capacity, so any lightening of canopy color is a concern. You can see canopy color loss by using a drone or otherwise getting above the crop, but a better way is to walk into the field around mid-day to see if the amount of light passing through the leaves – that is, how well-lighted the ground seems to be – has increased from what it was a week or two earlier. Reading a newspaper placed on the ground should require some squinting if the canopy is good; if it’s easy to read, too much light is getting through to the ground. Dark at ground level means the crop is intercepting 97 or 98 percent of the sunlight. We expect yield to drop by as much as 2 percent for each additional percent of light that gets through the canopy.

A number of things can cause loss of canopy light interception. One that many people worry about is having the crop “run out of nitrogen” during grainfill. The good news is that a corn crop growing on average or above-average Illinois soils, and that had enough N to stay green up to pollination, is virtually guaranteed not to run out of N during grainfill. That includes nearly all corn fields in Illinois in 2016.

Canopy color has been outstanding in most fields this year due to good mineralization, adequate (to more than adequate) N fertilizer used, and little potential for N loss in the spring. Crop N needs drop quickly once the crop is past pollination, and by now the crop in most fields is taking up no more than a pound of N per acre per day. Mineralization rates typically exceed that amount at this time of the season, so the crop needs little or no additional N from fertilizer from here on out. Much of the N applied late in fields with dark green plants was unnecessary, and we can expect some of it to exit the field through tile lines.

The main cause of loss of canopy during grainfill is having the soil get dry enough to restrict water availability to the plants. Illinois is generally well-supplied with soil water now, but with uneven distribution there are likely some areas where the water supply may be dwindling to the point that canopy function is starting to slow. Plants during grainfill may not show the distinct leaf rolling that we see with water stress before pollination. Instead, leaves exposed to the sun at the top of the plant may start to lose their green color, and this loss may continue until it rains or until leaves start to dry up.

As plants start to lose photosynthetic capacity as soils dry, they often show characteristic “firing” as leaves, starting with those closest to the ground and moving up, begin to transport their N to the upper leaves and into the ear. This is a defensive mechanism that results in some seed production as photosynthetic activity declines or stops. Because N loss from leaves means development of N deficiency symptoms, there’s a tendency to believe that having more N in the soil would have prevented firing. That’s simply not true: leaf firing results from having too little water available, and has nothing to do with the amount of N in the soil. In fact, high fertilizer N rates can increase plant and canopy size, and larger plants use water more quickly, which can trigger earlier and more severe firing.

Foliar diseases can also take a toll on the crop canopy, decreasing both yield and stalk integrity, with greater potential for stalk rot to occur later in the season. Foliar diseases seem to be fairly low this year, but scout to make sure, and consider fungicide use if diseases begin to move up the plant in the next week or so. The amount of benefit from using fungicides decreases as grainfill proceeds, and once the kernels are into the dough stage the cost may exceed the benefit.

Anthracnose leaf blight is a fungal disease that can appear during grainfill on susceptible hybrids, and when it appears in August, control by foliar fungicides may not be very good. Goss’s wilt, which can also destroy leaf area, is a bacterial disease against which fungicide has no activity. If crop canopy is physically removed by hail, no repair is possible. Insects very rarely cause extensive defoliation in a corn crop during grainfill, but scouting will help you know if there’s a problem developing.

If we continue to get enough rain and the canopy stays healthy, will the crop be as good as it looks? Yes. I’ve heard reports of kernels counts of above 20 million per acre, which would be 250 bushels at 80,000 kernels per bushel. If conditions deteriorate and the canopy declines before maturity, kernels may end up lighter and yields lower. But if conditions through August remain favorable and the canopy stays intact, kernels can get larger than this and yields higher.

What could go wrong with this year’s crop? Wind and hail always come to mind, but the frequency of these events decreases later in the season, and the fact that these have not been widespread up to now is a plus. A sudden onset of high temperatures without rainfall would limit yields in a lot of fields, but better soils in many areas have enough water today to keep the plants going for several more weeks, as daily water use rates are starting to decline. So this would be less devastating than we might expect. Late-developing foliar diseases could end grainfill early, but their larger effect, like that of late-season drought, would be to result in depletion of stalk reserves as ears take in sugars faster than the plant is producing them. This may not greatly diminish grain yields, but depleted stalks are more susceptible to stalk rots, and affected fields could lodge earlier.

With the finish line moving into sight, it’s clear that the crop has an unusually good chance to yield above trendline this year. But as in most things, there’s no guarantee.

2016 Orr Center Field Day Set For July 20

The 2016 Orr Center field day will be held on Wednesday, July 20, beginning with sign-in and refreshments at 8:00 AM. The format will be new this year, with three UI Extension specialists making presentations in indoor classrooms:

  • Weed scientist Aaron Hager will talk about weed management
  • Agronomist Emerson Nafziger will discuss crop conditions and nitrogen management
  • Ag economist Gary Schnitkey will discuss crop income projections

Indoor sessions will be followed by a short wagon tour to look at crop conditions and some of the research trials underway at the Center. The tour should be finished by 11:00 AM. Continuing education units will be available for Certified Crop Advisors.

For more information, please contact Mike Vose at 217-236-4911 or at

The Orr Center is on State Hwy 104 approximately 4 miles west of the junction of IL Routes 107 and 104 north of Perry, Illinois.

How might soybean yield be affected by hail damage?

In the early morning hours on Wednesday, June 22 a severe storm moved through western Illinois affecting crops throughout much of Henderson, Warren and Mercer Counties, including those at the University of Illinois’ Northwestern Illinois Agricultural Research and Demonstration Center in Monmouth.  Preliminary data collected by instruments maintained by the Illinois Climate Network at the center had the wind gusting to 78.1 mph and more than 1 inch of rain falling in a 10 minute period contributing to the nightly total of 3.34 inches. The National Weather Service models showed that ¾ inch diameter hail fell over the area as well. Corn plants were blown over and both corn and soybean plants were damaged by hail.

Soybeans in a planting date trial were at different stages of growth and development when the hail damage occurred; soybeans planted on April 18, May 7, May 19, and June 7 are at R2 (flowers at the upper nodes), V6, V5-6, and V1, respectively.

While plant damage occurred regardless of growth stage, the damage appeared to be most severe on the latest-planted soybeans, with stems of some plants broken over and many of the primary growing points severely damaged (Photos).

Picture1While hail damage on soybean can be shocking, the growth habit of soybean and the timing of the recent damage provide encouragement that plants will recover well. Soybeans we grow have an indeterminate growth habit, meaning that they continue to add new leaves for some weeks after flowers have begun to appear. Even early-plated soybeans have only 15 or 20% of their final leaf area now, so most leaf area is still to come. Damage to existing leaf area is thus a relatively minor problem, providing that plants remain alive and capable of forming leaves and pods. Those leaves that remain will contribute to the growth of new leaves. Leaf loss does set back plant development, similar to having planted at a later date, but the soybean canopy typically doesn’t finish developing for another 6 weeks. New leaf area should be starting to appear soon, and a few weeks from now it may be difficult to see any lingering effects of leaf loss.

A full canopy is required for a soybean crop to fully intercept sunlight to produce sugars, fill pods and maximize yield. Those fields in which hail damage lowers stand to fewer than 100,000 plants per acre or so may not be able to develop a full canopy to intercept all of the sunlight that they need to produce higher yields. Provided that the primary growing point of most plants remains unaffected, populations are unlikely to suffer. Most plants that had experienced hail are damaged but not killed, but after flowering, plants with most of their leaf area missing may not recover fully. Fortunately, a full stand of healthy soybean plants can produce more leaf area than they might need for full yields, so some leaf loss now should have minimal effect on yield potential.

It’s been warm enough that flowering in early-planted soybeans began before the longest day of the year this year. While yield loss due to hail damage starts to accelerate as plants pass flowering and enter podsetting stages, we do not believe there is much irretrievable yield loss yet, as long as plants are able to get enough water and the canopy stays healthy.

-Angie Peltier and Emerson Nafziger

Storm Damage in Corn

High winds hit parts of central and north-central Illinois on June 22 and 23, flattening corn that was at stages V10 to V13 or so (4 to 7 feet tall.) Hail damaged leaf area in some places as well, but hail was not as widespread as wind damage.

Figure 1 shows corn completely flattened at our Monmouth Research & Education Center, following wind gusts up to 78 mph between 2:45 and 3:00 AM on June 22. The detailed weather record indicates that rain started to fall at about that same time, and by 6:00 AM more than 2.5 inches had fallen.

Figure 1. Corn flattened by wind in the early morning of June 22, 2016. Photo taken in mid-afternoon on June 22 at the Monmouth Research & Education Center by Angie Peltier.

Figure 1. Corn flattened by wind in the early morning of June 22, 2016. Photo taken in mid-afternoon on June 22 at the Monmouth Research & Education Center by Angie Peltier.

Even though “steamrollered” corn is a disheartening sight, several factors converged to make this much less damaging than we would often see with such events at this time of year and with corn this size. Rainfall during the first half of June has been limited in most of Illinois, and warm temperatures have meant rapid growth and water uptake. This has meant relatively dry surface soils, which has encouraged roots to grow deeper. So the crop was well-anchored by its root system when the wind blew.

As soils have dried out, water uptake has slowed slightly. The crop has been making good growth, but drying soils mean that cells in the stalk take in a little less water. This decreased the internal cell pressure, and so lowers the tendency of plants to snap off at a node – what is called greensnap. Plants of this size and at this stage, when well-watered and growing fast, are often susceptible to greensnap. Such breakage happens at upper (younger) nodes that haven’t yet been strengthened by lignin deposits. Even a slight reduction in the amount of water moving into cells is enough to reduce the potential for greensnap.

The third factor that helped the plants was the sequence of events: wind came first then rain, instead of a lot of rain followed by wind. Soil softened by rain, especially when it’s been wet and roots haven’t grown as deep, allows plants to tip over, pulling part of the root system out of the soil, and allowing plants to lie down flat on the soil. In the photo above, it’s clear that the plants, while nearly parallel to the ground, aren’t flat on the ground like they often are when corn root-lodges.

While the picture based on the event at Monmouth probably is not accurate for some places where this type of damage occurred, I think we will see this crop recover fairly quickly, perhaps with little if any effect on yield. In a study in Wisconsin, the soil was wetted and corn pushed down to the ground, causing root lodging, at different growth stages. They found less than 5% yield loss when plants were lodged at stage V10-12 and 9% loss when this was done at V12-14.

Root-lodged corn plants will gooseneck (bend towards upright) after lodging, but gradually lose their ability to do this as the stalks become lignified. If plants are only bent over with their roots intact and still in the soil, they will recover faster and better than root-lodged plants. Figure 2 shows corn in the same field as Figure 1, with the photo taken 24 hours later. In fields that didn’t root-lodge, recovery started quickly and is proceeding fast. Moist soil and warm temperatures will speed recovery. In many fields, we dodged a bullet this time.

Figure 2. Corn flattened by wind in the early morning of June 22, 2016. Photo taken about 24 hours after the photo in Figure 1, and in the same field. Photo by Angie Peltier.

Figure 2. Corn flattened by wind in the early morning of June 22, 2016. Photo taken about 24 hours after the photo in Figure 1, and in the same field. Photo by Angie Peltier.

If hail accompanied storms, as it did at Monmouth, yield loss will be related to the amount of leaf loss, or more accurately, to the decrease in the ability of the crop to intercept sunlight over the next few weeks and after pollination. Those who have hail insurance will have an adjuster evaluate leaf loss and crop stage, and yield loss will be estimated based on the loss chart. Corn is nearing the stage when leaf loss has its maximum effect on yield, but leaf area loss of only 10 or 15%, while it looks bad, will affect yield only modestly. With some new leaf area yet to emerge, and with relatively minor leaf damage in most cases reported, losses shouldn’t to large.

Wind along with hail damage may not increase the effect of leaf area loss, but the stalk will need to come back to a more upright position before light interception returns to normal, and leaf loss will extend the recovery time. Stalks of flattened plants may also have taken some direct hits by hail and show some bruising. This can interfere slightly with sugar movement through the leaf sheaths, which could cause some reduction in kernel set. Hail loss adjustment should cover this.

With some leaf area underneath flattened plants and out of reach of fungicides, and with research that shows that that hail-damaged leaves benefit no more than intact leaves from foliar fungicide, there’s little to suggest that fungicide should be applied now. Having leaves near the soil during and after heavy rain could encourage the start of foliar diseases such as gray leaf spot. Scouting for such diseases should, regardless of plant damage, be a high priority as pollination approaches in the coming weeks.

Does the Corn Crop Need More Nitrogen?

Except for some areas of southeastern Illinois, the 2016 corn crop went in well, and on June 12 was rated at 75% good or excellent. Warm temperatures have speeded up growth, and although below-normal rainfall, especially in western Illinois, is starting to cause some concern, the 2016 corn crop is off to a very good start.

The corn crop this year has excellent stands and there are few drowned-out areas, though there is some unevenness depending on when the crop was planted and how much rain it received after planting. The most noteworthy feature, though, is the dark green color of the crop, especially the crop that was planted in mid-April. This is among the greenest corn crops I have seen in Illinois.

Not only is the crop green where N fertilizer has been applied, it is also green where no N fertilizer was applied. In a June 9 photo taken in one of our N trials, the zero-N treatment shows slightly less growth than the treatment with 200 lb. N applied on April 18 as NH3, but leaf color is about the same without N as with a full N rate (Figure 1). We don’t expect this to last as N uptake kicks into high gear, but the crop has taken up a fair amount of N that didn’t come from fertilizer.

Figure 1. Photo taken on June 9 of V7 corn in a research trial near Urbana, Illinois. The crop followed soybean, and was planted on April 18.

Figure 1. Photo taken on June 9 of V7 corn in a research trial near Urbana, Illinois. The crop followed soybean, and was planted on April 18.

Soil N changes

Soils in the plots shown in Figure 1 have been sampled several times this spring to monitor changes in N. In samples taken on June 3, plots without fertilizer had 49 lb. per acre of plant-available N (PAN, nitrate-N plus ammonium-N in the top 2 feet of soil) and those with 200 lb. of N applied as early spring NH3 had 222 lb. of PAN. Unfertilized plots had about 40 lb. less soil N on June 3 than they had two weeks earlier, but most soil N numbers remained relatively constant during May. Fall-applied N has been present mostly as nitrate this spring, and, and spring-applied NH3 has moved steadily towards nitrate, going from 25% nitrate (75% of the N recovered as ammonium) after application on April 18 to 71% nitrate on June 3. Unless soils get wet soon and stay wet for some time, nitrate will stay in the soil and remain available for uptake.

We saw somewhat inconsistent changes in soil N at some of the other research centers where we’re tracking N this spring. At DeKalb, where 4 inches of rain fell on May 11, soil N following 200 lb. N applied as NH3 in April fell from 375 lb. on May 7 to 176 lb. on June 3, while the unfertilized check rose from 80 to 90 lb. of soil N per acre. At Monmouth, the unfertilized checks had 167 lb. of soil N on June 7, and plots with 200 lb. N as fall-applied NH3 had 288 lb. of N in the soil while those with 200 lb. N as spring-applied NH3 had 260 lb. of soil N.

Fields that Dan Schaefer of IFCA is sampling under the N-Watch program are showing little spring loss of soil N, and some increases. Nine sites in Sangamon County that received 225 lb. of N last fall and winter had an average of 363 lb. soil N in the top 2 ft. on June 2. Samples taken on the same date showed that eight fields in Champaign County that received about 190 lb. of fall-applied N had an average of 279 lb. soil N. Both sets of fields showed some 80 lb. of N more in early June than they had when sampled in late winter (Feb. and March). In contrast, a set of eight fields in Vermilion County that had about 160 lb. N applied in the fall had only 130 lb. of soil N on May 24, slightly less than in March. Soil N is quite consistent among fields in each group, and it’s not clear why there are such differences among the groups.

Can soil N amounts really increase by as much as 80 or 100 lb. in May without any addition of fertilizer N? If we saw this only in a few fields we might think it was sampling error. But we’re finding that soil N often increases as soils warm, and such increases tend to be greater in soils with higher organic matter. So we think this happens as nitrification – the release of N from soil organic matter by microbial action – kicks in as soils warm in the spring. Nitrogen is in the ammonium (NH4+) form when released by mineralization, but then nitrifies (is converted to nitrate NO3) quickly. Both mineralization and nitrification are microbial processes, and rates of both processes are high in warm, moist, aerated soils.

While there are some fields that seem to have less soil N than we might have expected, soil N levels are in general showing amounts at least as high as those we saw at this time in 2015. The largest difference between the two years is rainfall: May rainfall and temperatures were similar both years, but the heavy rain that fell in June, 2015 has not returned in 2016, and is not in the forecast. In fact, rainfall during the first two weeks of June has been below normal over most of the state this year. We think this will be favorable for the crop’s N supply, and expect to see a less pronounced drop in soil N as the 2016 crop moves towards pollination.

Nitrogen management

With warm temperatures and the crop just entering its most rapid growth and N uptake phase, it seems highly likely that, unless soils start to run out of water in the next two weeks, the crop growing in soils with the normal (N rate calculator) amount of fertilizer N will be able to take up most of its N over the next few weeks with little danger of developing N deficiency.

In a trial at Urbana in 2015 with 200 lb. of N applied in April, the crop was at stage V9 and had 45 lb. of N in the plants on June 12. By tasseling time on July 13, it had 159 lb. N per acre in the plants. Soil N between these two dates fell from 240 lb. to 93 lb. per acre, and total (plant plus soil) N fell from 285 to 242 lb. per acre. Rainfall totaled more than 8 inches between these two dates. Even with the drop in soil N to a relatively low level (about 6 ppm nitrate-N and 5 ppm ammonium-N) by pollination, the crop in this treatment yielded 235 bushels per acre.

At the estimated 1 lb. of N taken up for each bushel of yield, the 2015 crop would have taken up about a third of its N after tasseling. Given the low amount of soil N at tasseling, this additional N had to have come from mineralization and, possibly, from N deeper in the soil profile as the crop drew water up during dry weather late in the season. In any case, it’s clear that low soil N at tasseling did not result in low yields due to N deficiency.

It’s premature to draw a strong parallel between the 2015 results and what we might expect this year, but with drier weather this year, soil N levels similar to those we saw in 2015, the crop darker green, and a root system that is likely to be somewhat deeper this year, all signs point to the likelihood of less chance for N loss and deficiency than we saw in 2015. In 2015, yields in most of our trials were high or very high, indicating that N loss and deficiency were not yield-limiting; exceptions were in fields where root damage form standing water was severe, and crops could not fully recover. While this looked like N deficiency, adding more N to such damaged crops often didn’t help very much in 2015.

Despite the dark green color of most Illinois corn fields in mid-June and soil N numbers that show no shortage, we are continuing to hear about producers and retailers gearing up to apply more N, including in some fields that have had a full amount of N applied and where soils have not been saturated this spring. In fields that have already received their full complement of N, with most or all of the N applied this spring, there is no clear justification for adding more N.

This does not appear to be one of those years when “just in case” justifies adding more N fertilizer. It’s highly unlikely that a corn crop that is deep green at knee- to waist-high will experience N deficiency due to lack of soil N. When N deficiency symptoms do develop in late vegetative or reproductive stages, this usually results from the crop’s running short of water to keep photosynthesis going at full speed. What is called “firing” and looks like a shortage of N is really loss of lower leaf area as the plant dries out. As lower leaves start to shut down they move N out to younger parts of the plant (including the ear) to keep the plant going as long as possible. Adding more N neither prevents nor cures this.

If some or all of the N was applied at modest rates last fall or in early spring in an area that has gotten wet several times since, and if soil N sampling shows levels of less than 15 or so ppm of nitrate-N in the top foot (2-ft. samples will capture N that has moved down but aren’t always practical) then adding more N might be indicated. We can’t accurately estimate the chances that applying more N will pay its cost, but if the crop is deep green and growing rapidly despite what seem to be low soil N numbers, that’s a hint that chances of getting a return may not be very high. The crop is always a better indicator of soil N sufficiency at a given growth stage than are soil N tests.

For those heading out to apply more N, remember that applied N has to get to the roots in order to do any good. If we get average rainfall over the next few weeks, that won’t be a problem. But if it stays dry, N is likely to stay close to where it lands in or on the soil. Roots pull water from the surface soil first, and there will need to be enough rain to bring soil moisture levels up to activate roots and to move surface-applied N into the soil before root uptake can resume. Placing N close to the rows in tall corn was only slightly higher-yielding in our trials last year than applying the same amount at normal sidedress time. The soil is a good reservoir for N, and so N applied a month or more before the crop takes it up is usually available. Even in the wet June of 2015 it was neither necessary nor cost-effective to spoon-feed N to the crop. All signs point to even less benefit to that approach in 2016.

Some people are using slowed-release forms of N for applications made at or after the normal sidedress time. When N is applied when crop uptake is close to its maximum, which starts at about the V7 stage, the main risk is that N won’t be released in time for the plant roots to take it up. Any slowing of the release of N increases that risk. Uptake of N remains at a high rate for only about three weeks, and it’s unlikely that N, especially when applied as urea or ammonium, will convert to nitrate and move out of the rooting zone in the few weeks before N uptake starts to slow. That’s especially the case now, with dry soils more common than wet soils, and extended wet periods not in the forecast.

2016 Field Day Events in Illinois

Fields days organized in 2016 by Crop Sciences and Extension at the University of Illinois, Western Illinois University, and Southern Illinois University will focus on crops and pests, with speakers talking about current crop issues along with information from previous research. Each event will offer CEUs for CCAs.

Following is the schedule of crop-related field days for 2016, including locations, dates, starting times, and contact information.

  • Macomb, WIU: Thursday June 23, 12:00 PM; Mark Bernards (309) 313-5917,
  • Urbana–Weeds, UI: Wednesday June 29, 8:00 AM; Aaron Hager (217) 333-9646,
  • Belleville, SIU: Thursday July 14, 9:00 AM; Ron Krausz (618) 566-4761,
  • Monmouth, UI: Friday July 15, 8:00 AM; Angie Peltier (309) 734-1098,
  • Perry (Orr Center), UI: Wednesday July 20, 8:00 AM; Mike Vose (217) 236-4911,
  • Ewing, UI Extension: Thursday July 28, 9:00 AM; Marc Lamczyk (618) 439-3178,
  • Urbana Agronomy Day, UI: Thursday August 18, 7:00 AM; Bob Dunker (217) 244-5444,