Corn and soybean crops limp towards the finish line

After the worst start to a cropping season in decades, mid-season lack of rain in parts of Illinois, and season-long low crop ratings, it’s time to take a look at what comes next as the 2019 cropping season moves into its final stages.


To no one’s surprise, various crop tours in recent weeks have confirmed that corn yields in parts of Illinois are likely to be disappointing. If there is a positive, it’s that the crop may look a little better than we thought it would by now after more than half of it was planted after June 1. While canopy cover and color in early July were a little better than expected, lack of rainfall and a less vigorous root system on late-planted corn meant that water stress began to show up in July. In areas where the dryness continued through August, some fields now show little green leaf area, and ear tips have dropped in drier parts of fields.

The driest parts of the state are the counties around the Quad Cities and in east central Illinois, with rainfall totals in July and August only about half of normal in these areas. This region shows up as being abnormally dry or in moderate drought on the U.S. drought map. Much of northeastern and southern Illinois received at least normal rainfall amounts over the past two months, and a band from St. Louis east along I-70 in south central Illinois shows rainfall totals of 150% or more of normal. Although late planting has gotten most of the attention, rain amounts, including lack of rain in some areas, will be a big part of the 2019 cropping story. That would have been the case even if planting had been early.

Late planting made the lack of adequate water a bigger problem. Many fields showed early water stress symptoms, and ended up with shorter-than-normal plants; both point to soil compaction as a major issue. Soil compaction was certainly an issue this year, but the smaller root systems and drying soils meant that plants weren’t as able to get access to water deeper in the soil as they would have been with earlier planting, even into compacted soils. Soil compaction is always present after tillage and planting using heavy equipment, but roots of early-planted corn can usually make connections to tap water from deeper in the soil even when there is compaction.

Temperatures this season have tracked very close to normal: from May 1 through September 1, the statewide GDD accumulation was about 2,580, 15 GDDs above normal. This total ranges from about 2,300 in northern Illinois, to 2,500 in central Illinois, and to more than 2,700 GDD in the southern part of Illinois. Had the crop been planted at the normal time, some fields in southern Illinois would be starting to dry down by now, and those in central Illinois would be getting close to black layer. But corn planted on June 1 instead of May 1 in northern and central Illinois lost about 350 and 450 GDDs, respectively, and so accumulated only about 1,950 and 2,050 GDDs by September 1. Corn planted on June 15 lost an additional 250 GDDs or so, and so accumulated only about 1,700 and 1,800 GDDs by September 1 in northern and central Illinois, respectively.

If we assume for simplicity that hybrids normally grown in northern and central Illinois require 2,550 and 2,700 GDDs from planting to maturity, corn planted on June 1 would need to accumulate roughly 600 and 650 GDDs, respectively, from early September through maturity. Normal GDD accumulations in September in northern and central Illinois are about 450 and 500, respectively, and accumulating the number of GDDs needed to reach maturity would, with normal temperatures, take until about October 20 in northern Illinois and about October 15 in central Illinois. September temperatures have been above normal in four of the last five years, but we can’t count on that in 2019. Corn planted after June 10 requires more GDD to mature than it is likely to get before the average date of first frost, which is around October 20.

What if the corn doesn’t get enough GDDs to mature fully? According to the Iowa State University publication Corn Growth and Development (PMR 1009), dry matter accumulation slows considerably near the end of the grainfilling period: it takes 380 GDDs to accumulate the last 10% of kernel dry weight, and 205 GDDs to accumulate the last 3% of dry weight. So having the corn stop filling with 200 GDD yet to go should not cost a lot of yield. That depends somewhat on how grainfill ends, though: a hard freeze (28 degrees or less) stops grainfill and starch formation in the kernels quickly, while slow deterioration of the leaf area before grainfill ends allows more sugar to move into the kernels and be converted to starch to add dry weight. Kernels that don’t fill completely tend to have a constricted base where they attach to the cob, and that can mean lower test weight. If frost stops the conversion of sugars to starch, kernels remain unfilled to the tip and also accumulate sugar there, which can slow field drying and can make kernels discolor more easily during heated-air drying.

In the drier areas of eastern Illinois we’re seeing a plants starting to die in patches, with ear tips dropping and leaves drying up. Such patches are typically where soils hold less water, and in some fields also in low areas where there was damage from standing water early or perhaps more compaction at planting. If other areas in the field are still green, we expect patches where the crop died early to show lower kernel weights and yields.

The USDA-NASS will issue the September 1 crop yield estimates on September 12. The August 1 estimate was 181 bushels per acre for Illinois corn, which is down 29 bushels (14%) from the 2018 Illinois corn yield. The corn crop ratings are not very high: 19, 35, and 44 percent of acres were rated as poor or very poor, fair, and good or excellent in the September 1 report. In 2017, only 55% of the crop rated as good or excellent in early September. That year, the Illinois yield estimate went from 188 in August to a final of 201 bushels per acre. This year is not a lot like 2017 (or any other year in the last 40), so we’ll need to wait to see what the new estimate turns out to be.


With 80 percent of the 2019 Illinois soybean crop planted after June 1 and some 10% planted after July 1, we set a new record for late planting of soybean in Illinois this year as well. With such late planting, the flowering and pod setting took place at least two weeks later than normal (average of the last five years); by September 1, nearly 10% of the crop was still not setting pods.

Two main factors will combine to limit soybean yields in much of Illinois in 2019. One is that late planting has, at least in many areas, resulted in lower numbers of pods that are filling. Reasons for this are complex, but include: 1) late canopy formation, which likely limited the supply of sugars needed to set pods; 2) less favorable (lighter green) canopy color, at least in some fields; 3) lower than normal numbers of nodes with pods, especially in dry areas where plants are short; and 4) low pod numbers per node. We see very little of the three-to-six pods per node (in the central part of the stem) that we saw in the 2018 soybean crop, even in fields that appear to have made fairly good vegetative growth. Many plants have only two or three pods per node, and only 10 to 12 nodes with pods. There appear to be more productive (pod-bearing) branches than normal in some fields, possibly because main stem growth was limited so branches had more resources. In some plants I’ve seen, a third to half of the pods are on branches. We don’t know if this affects yields compared to having most or all of the pods on the main stem.

Another factor that is likely to lower soybean yields in 2019 is the late start of podsetting followed by the late start of seedfilling. This is because, compared to August, days in September are shorter and average temperatures are lower, meaning that the amount of daily photosynthesis is lower. This isn’t a problem at the beginning of September, but it is by the end of the month. Based on temperature and daylength changes, we would expect the amount of daily photosynthesis (on a day with full sunlight) in central Illinois to drop by about 55% from September 1 to September 30. Most of this is due to lower temperatures.

One of the difficult-to-predict differences between early- and late-planted soybeans is the timing of crop maturity. We gauge the end of seedfilling by when the crop canopy loses its color, which signals that the leaves have exported their nitrogen to the seed with the last of the sugars. That happens quickly—often over only two days or so—and we don’t have a very good way to guess when it’s going to happen. We think that the signal to end seedfilling originates in the pods—plants without pods stay green, and even the leaf that feeds a single node without pods may stay green. It’s possible that the timing of this signal is related to how many pods are on the plant and to what extent the seeds in these pods have filled.

We’re now starting to see the loss of leaf color in early-maturity varieties, even those that were planted late. We know, of course, that early varieties mature earlier than later ones, both because the period of pod formation is shorter in early varieties, and because seedfilling starts earlier. Based on the low pod numbers we’re seeing this year, it appears unlikely that early-maturing soybeans are going to produce high yields. That can indicate that fields that mature later may not have great yields, either.

We’ll need to wait to find out how well seeds fill before leaves lose their color, but we should keep in mind that soybean yields are more closely tied to seed numbers (per acre) than to seed weight, and in the parts of Illinois most affected by late planting and dry weather, we aren’t seeing the high seed numbers we’d need for high yields. At 120,000 plants per acre and 3,000 seeds per pound, yield in bushels is the number of seeds per plant divided by 1.5. At 2.5 seeds per pod, each pod per plant would mean 1.67 bushels per acre, and 30 pods per plant would mean 50 bushels per acre.

Will the August 1 NASS estimate of 55 bushels per acre for Illinois soybeans hold up? I don’t have a basis to judge, but there are both some very good and some very poor soybeans in Illinois fields. Having a stretch of warm, sunny weather in September would help to fill the green pods on green plants in many of the late-planted fields. But pod numbers are not going to increase, and that means that many fields will not produce yields this year as high as those we saw in many areas in 2018.

Let us know if you see these diseases in Illinois!

There are two, fairly new and / or important diseases  to keep an eye out for in 2019.  We are actively seeking samples of symptomatic plants for research to help us understand the biology, ecology, and management of these pathogens.  If you have a suspect sample, please send to the UIUC plant diagnostic clinic for confirmation (cost will be covered), and or contact me via email, telephone, or twitter.


The first is a disease that we started working on in late 2017- Tar spot on corn.  I mentioned in earlier articles that I expect this disease will start to show up more in after canopies close, provided we get sufficient rain.  It has been dry, so unless you have seed fields under irrigation, it is likely that tarspot thusfar is extremely low in incidence or nearly absent in your fields in Northern Illinois.  Look for raised black spots that typically have a small yellow halo.  These spots can expand along the vein, giving them a diamond shape, but also can take various amorphous forms. Make sure you pay attention to the lower canopy.

Example of black tar spot on a corn leaf. N Kleczewski


Below is the current map showing counties where we have confirmed the presence of tar spot in at least one field.  If your county is in this region of the state, ensure that you are keeping an eye on your crops and their growth stage.  There is still a fair amount of corn that has yet to tassel and plenty of crop development that needs to take place before the season is finished.

tar spot incidence map at the county level in Illinois as of 8/7/19.


The second disease is Red crown rot on soybeans.  This disease is something we want to keep an eye on because it is new, it shouldn’t be here, and it can look very similar to Sudden Death Syndrome.  Red crown rot likes it warm and wet.  The fungus that causes this disease colonizes the roots of the plant, and produces a toxin that accumulates in the foliage, typically after R3.  This results in interveinal chlorosis that resembles SDS, or other diseases such as brown stem rot, or stem canker.  As the plant nears senesce, the fungus will produce red fungal structures at the base of the stem that give the disease it’s name.  When you split the stems roots will often be black and rotted and the center pith of the lower stem will have a grayish appearance.   This disease caused significant losses in a field near Pittsfield last season, so those in that area should pay particular attention for this one (but so should everyone!)

Signs of Red crown rot on soybean.


A split stem showing gray inner pith characteristic of Red crown rot.

Corn and soybean crops at mid-season, 2019

The 2019 Illinois corn crop reached 50% planted during the first week of June, more than a month later than the average of the past five years. The soybean crop reached 50% planted a few days later than corn, and more than three weeks later than the average of the past five years. May rainfall was above normal over most of Illinois, and June brought near-normal rainfall over much of the state. Still, the late planting coupled with too much or too little rainfall after planting produced July crop condition ratings of only about 40% good + excellent for both crops, compared to an average of some 70% over the past three years.


While corn crop condition ratings haven’t improved over the past month, the warm temperatures and good sunlight have resulted in good growth of the crop in most fields, except in low, wet spots. Leaf color of the crop has also improved, as soils have dried enough to allow the roots to reach N from both fertilizer and from mineralization. This has helped the crop plants and crop canopy to recover some from the rough start.

The 20% of the corn crop planted between mid-April and mid-May has reached or completed pollination in most fields, in line with growing degree day accumulations: at Champaign, about 1,400 GDDs accumulated between May 15 and July 15, enough to get a 110-day RM hybrid to, or close to, pollination. That number is only about 1,140 at DeKalb, where little corn was planted by mid-May anyway. With little disease or insect pressure and with dark green leaves, most of the fields that have reached or finished pollination should end up with high kernel counts, which means high yield potential.

Most of the Illinois corn crop was planted at the end of May or during the first half of June, and little of this late-planted crop has reached pollination: only 19% of the Illinois crop pollinated by July 14, which is about the same percentage of the crop that was planted by mid-May. June 1 through July 15 GDD accumulations totaled about 930 at DeKalb, and about 1,060 at both Champaign and Mt. Vernon, in southern Illinois. At current GDD accumulation rates of 25 to 30 GDD per day (this will decrease some with the cooler weather next week), corn planted in early June will reach begin to pollination towards the end of next week. Corn planted after June 10 will probably not pollinate until early August, although early hybrids—we can estimate about 10-11 fewer GDD for each day earlier in RM, or about 2 days earlier for each 5-day drop in RM—may pollinate by the end of July. Objective yield estimates, which use kernel counts, will be difficult to make early this year, with so little of the crop through pollination by the end of July.

There is ongoing concern about how the season will end for the late-planted crop, including whether or not it will mature before frost. If fall frost comes at or after its 50% date, which is October 15 to 20 depending on location in Illinois, the late-planted crop should be at or close to the GDD needed to mature by that time, as long as the maturity of the hybrids planted was adjusted for very late planting. From June 1 through September 30, GDD accumulations for northern, central, and southern Illinois average about 2,400, 2,600, and 2,800, respectively. Including October 1 to 15 adds about 150 GDD to these totals, but delaying planting from June 1 to June 15 subtracts about 300 GDD, or about 20 GDD per day of delay. So in northern Illinois, corn planted on June 10 is expected to accumulate only about 2,350 GDD by the average date of first frost. That’s enough to get a 95-day RM hybrid close to maturity, but not a 110-day hybrid. While a decrease is the number of GDDs required for a hybrid when it’s planted late has been noted, we have not seen a decrease in the number of GDDs needed to get the crop to its current stage so far this year, and any decrease in the number of GDDs required from now to maturity is likely going to come at the cost of losing yield due to late-season stresses that limit grainfilling.

The main challenge facing the corn crop at this point, especially the late-planted crop, is having enough water to maintain growth and to set and fill enough kernels to reach its full yield potential after the difficult start to the season. While there is enough soil moisture to maintain the crop in most Illinois fields now, the late-planted crop especially is showing symptoms of water stress under high temperatures this week. We think this is mostly because the root system of the late-planted crop is somewhat limited, both due to compaction from planting into wet soils and also because late-planted corn that develops under high temperatures tends to favor top growth over root growth. So even though the late-planted crop likely has more water in the soil now than the early-planted crop (which has used several more inches of water up to now), access to this water is somewhat limited, and the result is leaf-curling by early or mid-afternoon. In drier areas, even the early-planted crop is showing some afternoon stress, especially on slopes where the soil holds less water.

The cylinder formed by a curled corn leaf under water stress cuts the windspeed across the leaf surface and lengthens the distance the water vapor has to move once it exits the leaf, so greatly decreases the rate of water loss. That might seem like a good thing, but when leaves aren’t losing water very fast they also aren’t taking in carbon dioxide very fast—that is, their photosynthetic rates are low. Low photosynthetic rates mean slow growth, and a prolonged period of low photosynthesis means lower yield potential. A few days of water stress before pollination won’t lower yield potential by very much, but a week or more of decreased photosynthesis will decrease kernel number, and so will lower yield potential. If such stress continues after pollination, more kernel abortion will lower yield potential even more.

Although the yield potential in early-planted fields without standing water damage appears to be high now, getting the crop to reach the potential with high kernels numbers—16 to 20 million kernels per acre—will require about 10 more inches of water this season. Depending on soil and current soil water supplies, half to three-fourths of that amount of water will need to come from rainfall over the next 50 days or so. The late-planted crop is likely to set somewhat fewer kernels, but it will likely need 11 or 12 more inches of water in order to pollinate and fill kernels successfully. Some rain soon might help to increase the size and competency of the root system and so help the crop better extract water that’s in the soil. But as pollination approaches, the plant shifts its allocation of sugars (from photosynthesis) away from roots and stalks to development of the ear and to completion of the pollination process. So in fields where the crop is under water stress now, the chances of getting good yields will continue to diminish the longer it goes before rain arrives. Cooler temperatures will slow the rate of this decrease by slowing the use of water, but in areas that are dry now, there needs to be water falling from the sky before pollination if this crop is to regain some of the yield potential lost with late planting.

Especially in the late-planted crop, there is some talk about needing to apply foliar fungicide and, in some cases, perhaps more N fertilizer. If enough N was applied earlier, there should be no need to apply more, especially if the upper leaves have a dark green color in the morning before stress begins to show. Some have suggested that late-planted corn will more likely need foliar fungicide, with the idea that protecting the canopy is more important when (with late-planted corn) there is less leaf area, or perhaps with the expectation that fungal diseases will more likely attack corn that pollinates late. The planting date studies that we’ve conducted in Illinois have, since 2010, included foliar fungicide applied at pollination as a treatment. Results do not support the idea that fungicide increases yield of late-planted corn more than it does of early-planted corn. Averaged across 21 trials in the northern half of Illinois, foliar fungicide increased the yield of corn planted in early April by 7 bushels per acre, and increased yield of corn planted in late May by 6 bushels per acre. Across 12 trials in the southern half of the state, fungicide increased the yield of corn planted in mid-April by 7 bushels, and increased the yield of corn planted in early June by only 2 bushels per acre. With corn crop prospects so closely tied to the water supply for the rest of this season, spending money on additional inputs (besides irrigation) may not be very helpful to the bottom line.


Most of what I mentioned about corn crop condition and planting above applies to soybean as well, with even fewer soybean acres planted early (in April). May was not a great month for early-planted soybeans in any case this year, even if they had good enough stands and didn’t need to be replanted. Greg Steckel and Marty Johnson were able to plant soybeans in a planting date study on April 9 at Monmouth. Those plants are now about 18 inches tall, and, in 15-inch rows, they have formed a near-complete canopy. Most of the soybeans in Illinois, though, are “canopy-challenged” now, and it’s likely that some of the 30-inch rows will not form a complete canopy at all. Late planting also resulted in a slow start to flowering—only 12 percent of the crop flowered by July 14 this year, compared to 77% on the same date in 2018, and a 5-year average of 54%.

Like corn, late-planted soybeans in the drier parts of Illinois are showing symptoms of water stress. These aren’t as noticeable as they are in corn, but are mostly seen as a slight drooping of leaves in the afternoon as the stress intensifies. The result is the same, though—plants under stress for much of the day do not do photosynthesize very well, and so they do not make very fast growth. This is compounded by the fact that late-planted soybeans have small root systems that can’t get access to very much soil water, and as a result, leaf area and plant height are increasing slowly. Root damage from wet soil conditions after planting also contributed to this in some places, and growth has been slow in these fields or parts of fields as well. As we know from past experience, soybeans are likely to “come around” and make good growth once they get past this period of slow growth, but it’s certainly better when that happens in late June than in late July.

While we’ve historically said that soybeans are less sensitive to late planting than corn, that was more true when soybeans yielded 40 to 50 bushels per acre than when they yield 70 or 80 bushels per acre, as they have done in recent years. Rapid early growth and canopy formation is key to high soybean yields, and this has not happened—and will not happen—in Illinois in 2019. With only about 20% of the Illinois soybean crop emerged by two weeks before the longest day of the year (June 21), the hours of sunshine on that day did little for the crop. The only path to good yield potential this year will be through a turn to unusually favorable weather, with good rainfall and temperatures in August and good rainfall and above-normal temperatures in September. We know from doublecrop soybeans that yields of 50 bushels per acre or more are possible when planting is late, but that does not tend to happen when growth through mid-July is as limited as it is in many field this year. We don’t have much information on how soybeans planted in late June or early July will do in central Illinois, but the slow growth of the crop since planting is a cause for concern.

Along with symptoms of water stress in some soybean fields is a lightening of canopy color that may indicate lack of adequate nitrogen. In fact, the N fixation process is sensitive to plant stress, and this could be a factor. It’s also the case that soybean leaves tend to give up their N (that is, break down proteins and export the N to the rest of the plant) as stress continues, in some cases even dropping leaves like they normally do at the end of the season. There are two reason why loss of leaf color is a concern. One is that soybean plants store significant amounts of N in their leaves as podsetting starts, and how much N is stored this way is a major factor how many pods set and how well seeds fill. The other concern is that paler leaves means lower current photosynthesis, and so slower growth. The “dark green blanket” of plants and leaves that we saw in nearly every field in mid-July in 2018 is not present in many fields this year, and this makes it hard to be optimistic about this year’s crop.

As with late-planted corn, some have proposed adding N or using foliar fungicides as a way to lower the stress on soybean plants and to increase yields this year. But when growth is limited by the amount of water available, either because soils are dry or because roots don’t have access to the water in the soil, then adding other inputs is unlikely to add much yield. In the soybean planting date studies we conducted since 2012, we included a treatment comparing foliar fungicide (usually with an insecticide as well) with no fungicide at each planting date. Over six trials in southern Illinois, foliar fungicide produced an average yield increase of 4.1 bushels per acre when planting was in mid-May, and of 3.7 bushels per acre when planting was in mid-June. Across 17 trials in central and northern Illinois, foliar fungicide increased yield by 2.5 bushels per acre when planting was in late April, and by 1.7 bushels per acre when planting was in early June. We have not done similar studies using fertilizer N, but have found such limited response to using N on soybeans that it’s unlikely that it will ever pay on soybeans, especially on soybeans that undergo stress that limits early growth.

As I’ve often stated, the crop canopy will “tell the tale” of this year’s corn and soybean crops better than any other thing we can look at. If in mid-August we see fields with dark green leaves, we can have hope for good—if not great—yields.

Southern rust in Illinois- it’s complicated

This week we started picking up Southern rust in the southern Illinois.  Thusfar, reports indicate that disease severity is low.  However, the recent hurricane remnant and warm forecasts may mean that we may see the disease progress somewhat in the coming days and weeks.

When people in Illinois hear the words southern rust, it brings back memories of a few years ago when the disease moved in and environmental conditions favored disease development for a prolonged period of time.  Many fields suffered losses as a result of the disease.  This year the situation is complicated and different from a few years ago.  First, we are dealing with extreme differences in planting dates throughout the state.  One field may have tasseled a week or two ago and the field across the road might just now be reaching V10.  Second, yield potential in late planted fields is likely to be substantially lower than typical, meaning that there is less yield to protect and less money to cover potential application costs.  Third, commodity prices, although they may have increased slightly in recent days, are low, making it hard to justify the cost of applying a fungicide unless necessary and the potential to recover costs is high.


Southern rust.  Image A. Sisson.


Common rust vs Southern rust on corn. Image C. Bradley.

Let’s take a minute and first go over southern rust, then move back into what factors you should consider before making a fungicide application to manage this disease in 2019.

Southern rust is caused by the obligate fungal pathogen Puccinia polysoraPuccinia polysora produces fuzzy, raised structures called pustules on leaves and stalks of corn.  Pustules contain thousands of small orange spores.  When you rub these pustules between your fingers, the spores may leave a dusty orange coat on your fingers, hence the reason it is called a “rust.”  Pustules of Southern rust are orange to light tan, and often small and circular.  Pustules are mostly found on the upper leaf surface, which can help distinguish it from the less damaging common rust.

Spores from pustules can be dispersed miles on air currents, allowing the disease to spread rapidly.  Under hot humid conditions, spores of the fungus can infect suceptible corn, and symptoms can be observed within 3-4 days.  Within 7-10 days, spores are produced and can be dispersed.  The cycle of spore-infect-spore can continue as long as conditions are conducive and corn plants are green.  Conditions that favor disease development include hot temperatures (morning low of 75°F and daytime high of 93°F) and at least 4 hr of consecutive leaf wetness.  Outside of these conditions disease progress can occur, but at a slower rate.  Our colleagues to the South state that Southern rust can continue chugging along at 110 degrees.  That’s pretty impressive.


Southern rust does not overwinter in Illinois and blows into the region from warmer regions.  In years where it develops to a significant degree early in southern regions, it can move into Illinois during critical stages in crop growth.  In general, we see the disease move in most years in late July or early August.  This means that in years when plantings are delayed, the disease can arrive on time but plants may be at greater risk for yield loss because the earlier  infections occur the more yield can be impacted.  Experience from our Southern colleagues indicates that stalk integrity isn’t likely to be affected unless you see significant infections during the vegetative stages of crop development.

Example lifecycle of Southern rust. Note that it blows in from warmer, southern regions.

Now that we are on the same page about this disease, what about management?  As I mentioned previously things this season are complicated.  Let’s start by considering management decisions.  Below is a table modeled after one produced by fellow plant pathologist and Jason Statham look-alike Travis Faske at the University of Arkansas, depicting the likelihood that a fungicide application for Southern rust will provide a benefit:

Growth Stage Southern rust present Forecast favors S rust (75-93F) min 4 hrs leaf wetness Benefit of fungicide?
Vegetative yes yes yes
VT/R1 yes yes yes
R3 yes yes yes
R4 yes yes Unlikely
R5 yes yes No
Maturity yes yes No


Experience from the South indicates that trying to hold off an application until VT/R1 if possible is going to give you the highest likelihood of coming out even or ahead of this disease.  If you apply during the vegetative stages, realize that that means that you might need to come back again and make a second application.  You now have likely doubled your application costs.

Now let’s consider some other aspects of controlling this disease in our late planted crops.  If you were unable to switch to a shorter day hybrid, and decide apply a fungicide for managing S. rust at say, R3, your plant will be protected from disease, and retain greenness later into the season.  Depending on your location in the state, this means you might need to consider frost, and what that means for your yield potential and crop harvestability.

Before making an application, consider these points and also run the numbers.  Remember that fungicides DO NOT INCREASE YIELD POTENTIAL.  They do not increase yield.  They protect potential yield by mitigating losses due to fungal disease.

That being said, you can calculate the amount of protected yield required to pay for a fungicide application by using the formula Yield protected (bu/A) = application cost ($/A) crop price ($/bu).  After calculating your required protected yield, you can then determine if potential yield, frost, and other factors will make it worthwhile to spray.


Continue to scout.  For updates on Southern rust and it’s presence in Illinois and surrounding states click here


Cover crops on prevented-planting acres: an update

There have been a number of developments since my June 17 article on managing prevented-planting (PP) fields. A major change was the granting of permission to harvest cover crops planted on PP acres after September 1, instead of November 1. In addition, harvesting after September 1 can now be done with a forage harvester—as silage—rather than only by grazing or making hay.

In simple terms that means that cover crops on PP acres can be managed as forage crops, as long as their management complies to some extent with their use as a cover crop. For corn, that means using narrow rows or higher seeding rates than would have been used for corn grain. Seed source was covered in the earlier article. Dr. Joe Lauer at the University of Wisconsin also has an informative article on this here. He suggests using a seeding rate of at least 35,000, compared to 70,000 that I mentioned in the earlier article. In narrow rows, 70,000 would probably not lower dry matter yields, and it would produce cover more quickly. But planting 40,000 to 50,000 seeds would not compromise dry matter production compared to higher seeding rates, and if seed cost is an issue, planting 40,000 to 50,000 seeds in narrow rows should work both as a cover crop and as a forage source.

We are past the late planting period for corn, so planting corn as a cover crop on PP corn acres should be allowable, but check with your crop insurance agent to make sure. Corn isn’t the only candidate cover crop for PP acres; if forage harvest is a goal, then forage sorghum or sorghum-sudangrass hybrids will probably produce as much or more dry matter than corn, especially if we get some hot, dry weather. One of the big issues is use of nitrogen fertilizer on a PP cover crop to be harvested as a forage. Nitrogen is not needed for corn (or another grass forage species) to be effective as a cover crop, but using some N will increase forage yields substantially.

For the many fields where harvest of forage as a livestock feed is not an option, the information in the earlier article still holds. With shortages and large price increases for seed of more traditional cover crops, seed costs are a major factor in cover crops for PP acres. High seed prices aren’t the only issue: Illinois Crop Improvement Association recently report that a sample of bin-run oats contained a lot of weed seeds, including seed of noxious weeds. At the ICIA site you can also find an item from the American Seed Trade Association regarding use of PVP-protected seed from a farmer’s bin to plant a cover crop. This may be possible for PVP-protected varieties, but it could be used only by the farmer who produced it. Due to agreements under which seed was purchased, most corn and soybeans now in bins can’t be used as seed for cover on PP acres. Seed companies can presumably allow such use if they so choose.

In the letter that we issued regarding cover crops for PP acres, we stated (on p. 2) that soybeans could not be planted on PP soybean fields. That is by RMA rule, which says that the PP crop cannot be planted on those same acres within the late planting period. The late planting period ended the last week of June for corn in Illinois, but for soybeans the late planting period extends 25 days past the final planting date, which was June 15 in northern Illinois (see the May 7, 2019 Farmdoc article for a map) and June 20 in the rest of Illinois. So the late planting period ends on July 10 for northern Illinois and on July 15 for the rest of the state. Soybeans planted and managed as cover crops after those dates should be allowable even on PP soybean fields, but check with your insurance agent to make sure. Planting soybean after July 1 as cover for PP corn acres should also be allowable, since the late planting period has ended for corn.

If soybeans are an allowable cover crop and seed is available at a reasonable cost, is planting them a good idea? In narrow rows and a seeding rate of 140,000 or more, they will produce cover quickly, though not quite as quickly as warm-season grain/forage crops. They will fix nitrogen: we found over a two-year study that high-yielding soybeans (average across four sites was 72 bushels per acre) had about 290 lb N per acre in the above-ground dry matter at stage R5, at the beginning of seedfill. Yield was correlated with plant N content, but plant N content projected for a 30-bushel soybean crop was still about 240 lb N per acre. If the canopy has a good green color by September and plants have pod numbers that suggest the potential for a 25- or 30-bushel yield, plants likely contain more than 200 lb of N per acre. How much of that would be available for a corn crop in 2020 if the crop is destroyed and left on the surface this fall is not known.

Could soybeans be chopped as silage or harvested as hay after September 1? I see no indication to the contrary, but this (and corn silage harvest) would leave the soil with essentially no cover going into the fall, and erosion potential would be high. It might be advisable to follow such a harvest with a small grain cover crop to help limit soil loss.

I’ve had several inquiries regarding replanting corn this late. I checked the GDD calculator at the MRCC, and the signals from it are not promising. A 90-day hybrid (needing 2,159 GDD) planted on July 5 in central Illinois is predicted to pollinate on August 24 and to accumulate only 2,000 GDD by October 23; it is not expected to reach black layer by the end of December. Chances are better in southern Illinois, where the same maturity hybrid planted on July 5 would be expected to reach black layer on October 16. This is still risky, though, with any short periods of stress likely to decrease yields considerably, and wet grain and stalk quality issues to be expected. Short-season corn is also in very short supply this year, and if the only corn available is that planned originally, it has a near-zero chance of reaching maturity before frost.

Northwestern Illinois Agricultural Research and Demonstration Center Agronomy Day, July 17th

MONMOUTH, Ill. – The Northwestern Illinois Agricultural Research and Demonstration Center will host a Field Day on Wednesday, July 17. Join University of Illinois Department of Crop Sciences faculty, researchers, students, and Extension specialists as they address issues pertinent to the 2019 growing season.

The program will begin promptly on Wednesday, July 17 at 8:00 a.m. and is open to the public at no cost. Refreshments and BBQ sandwiches will be available at the end of the tour.

Weather permitting, presentations will take place outside next to research plots. Participants will board buses to tour portions of the farm.

Field Day topics include:

  • Sizing Up the 2019 Corn and Soybean Crops – Dr. Emerson Nafziger
  • Insect management in corn and soybean – Dr. Nick Seiter
  • Updates in Applied Disease Management in Illinois – Dr. Nathan Kleczewski
  • 2019 Weed Management Update – Doug Maxwell and Charlie Mitsdarfer
  • Overview of Industrial Hemp Production in Illinois – Phillip Alberti

Certified Crop Advisors may earn two continuing education units. (1.0 – CM, 1.5 – IPM)

The Northwestern Illinois Agricultural Research and Demonstration Center is a 320-acre facility, established in 1980. The Center is located 1 mile North and 4 miles West of Monmouth at 321 210th Avenue. Each year, more than 40 different projects are conducted by campus-based project leaders and the center superintendent.

Source: Greg Steckel, 309-734-7459,

Date: June 27, 2019

Managing Prevented-Planting Fields

With a lot of acres of corn and soybeans still unplanted as we move into the second half of June, prevented planting (PP) is unfortunately going to be a major part of the story of the 2019 cropping season in Illinois. Here we’ll look at goals and options for managing acres on which the intended crop—corn or soybean—does not get planted.

The main goals of managing PP acres will be: 1) providing a vegetative cover in order to keep the soil in place and to prevent “fallow syndrome”; 2) to prevent or manage weeds so they don’t reseed the field; and 3) to take up nitrogen, including that from any N-containing fertilizer (including DAP/MAP), and any N that will be released from soil organic matter during the growing season. We also need to find ways to keep costs down, given that the PP insurance payments leave little room for adding expenses to these acres. This may not be the best time to invest in expensive cover crop seeding mixes. With high demand this year, such seed—and seed of some less exotic cover crops as well—will be expensive, and some may not be available.

We have not seen “fallow syndrome” very often in Illinois, but there was some in 1994 in fields that were flooded for most of the season in 1993 and did not produce crops or even weeds that year. The symptoms include stunting and purpling that indicate phosphorus deficiency. Plants growing in fields host a type of beneficial fungus (VA mycorrhiza) that assists in the uptake of P; these fungi seem to die off when there aren’t any plants, and they come back slowly the next year. We don’t expect to see this in every field, and it’s more likely to show up where water stood for a long period of time this year. The best prevention is to have plants present sometime during this season to help maintain these fungi. Just about any plant with roots will work, including weeds, but a cover crop species we choose to plant will be preferable to weeds.

Having plants present to take up N is more to keep the N from leaving the field this year than it is to make it available for next year’s crop; it’s not clear how much N captured in crop biomass this season will become available to next year’s crop. But mineralization takes place in every field once soils are aerated, regardless of whether the previous crop was corn or soybean. Grasses with deep roots are the best way extract N from deeper in the soil, and to keep this N out of tile drainage water.

We won’t try to reiterate here the complex rules regarding PP certification, but will only deal with managing these fields to provide cover. It appears that any species will work as cover, as long as the rules regarding what’s done with the cover after the season are followed. That means no harvest of grain (or silage) at all, and harvest by grazing or by making hay only after November 1. Every decision on what to plant should be tested with your crop insurance agent beforehand.

PP corn

Where corn was the intended crop in 2019 and soybean is planned for 2020, using a small grain as a cover crop this summer is an option. Winterhardy cereal rye and wheat won’t form heads until after a period of temperatures in the 30s, so probably not until next spring. They should emerge and provide quick cover, but these are cool-season crops, and when they remain low-growing and don’t send up stems with heads, they likely won’t stay very healthy or grow vigorously through a normal summer season.

Spring oats or spring wheat might do a little better than winterhardy wheat or rye. These tend not to tiller much at high temperature, but they will set seed. It can’t be harvested as grain; check the rules on whether it can simply be left to have the seed shatter out in the fall once it’s ripe. That may reseed the cover crop, but these plants won’t survive the winter. None of these are likely to grow roots as deep as when they grow in cool weather, but they should provide decent cover. With the 2019 oats crop in Illinois planted late and not exactly thriving, it will be difficult to find seed locally. Spring wheat seed will have to come from states north and west of Illinois.

Grain from a bin or an elevator, including from this year’s harvest, might work as seed for small grains, since this is not a “crop” in the usual sense. With wet weather this spring, we anticipate that some harvested grain will have diseased kernels that lower its market price, which may provide an incentive for using it as cover crop seed. Test germination, and if germination is low, increase the seeding rate to plant about at least 15 viable seeds per square foot, using a drill. While drilling will usually produce better stands and require less seed, broadcasting 20-25 live seeds per square foot might work. Shallow tillage with a vertical-tillage implement before or after broadcast seeding will probably improve stands.

Sorghum-sudangrass hybrids and forage sorghum produce a lot of residue and are good at taking up soil N. These species grow well in high temperatures, and they tolerate dry soils. If they won’t be grazed (after November 1), it’s probably better to limit their growth to lower the amount of residue present next spring. Lack of adequate N will limit growth in most fields, and delaying planting until mid-July or so can also help. If there is still a lot of growth, plants can be mowed in September so the residue can start to break down this fall. Some sorghum-sudangrass hybrids are male-sterile, and these species don’t produce much seed in any case. There is no danger of having plants of these species overwinter.

In fields that haven’t had herbicides applied that would prevent their growth, species such as radish, turnip, rapeseed, buckwheat, and forage grasses and legumes could be used on PP corn acres. None of these will be as effective as a well-rooted grass crop at taking up N, and those that grow slowly after emergence will generally not provide good cover early, and they won’t compete with weeds very well. Their seed tends to be expensive, and those with very small seed (such as clovers) can be difficult to establish in mid-summer without specialized equipment.

It may be possible to plant corn on PP corn acres, as long as care is taken not to produce corn grain. Ways to assure this include planting it later than July 15, drilling or planting it in rows no more than 15 inches apart, and planting at least 70,000-80,000 seeds (roughly a bushel) per acre. Lack of N will also help keep seeds from forming or filling, as will very late (September) pollination, which should mean failure of the crop to mature. Some seed companies may offer treated seed that they won’t be keeping over at a price low enough to make this an option. It may also be possible to take seed out of a bin of non-GMO corn grain to use for this. Make sure such seed will germinate, and check to make sure the planter is dropping enough seeds. By the time frost kills them, corn plants should not have formed seed that is mature enough to germinate the next spring. If grain begins to form and seeds begin to fill despite these measures, the corn can be mowed with a stalk chopper to prevent formation of viable seeds.

Soybean PP

Management of PP soybean acres has the same goals as those for PP corn acres, but management changes some if these fields will go back to corn again in 2020. Undisturbed corn stalks have by now broken down to some extent, but they still provide some cover, and keeping some of the stalk material on the soil surface will help preserve moisture and to keep soil in place as a cover crop gets started. The presence of high-C residue from the previous corn crop means that there will be less net mineralization in these acres because some mineralized N will be tied up as microbes break down residues. Even so, good root growth from a cover crop will help to take up N and to keep it from leaving the field.

It is possible to use the growing season that remains in 2019 to produce a leguminous crop that can fix N to supplement the N supply for next year’s corn crop. Such a crop should provide good early growth in order to take up N present as the over crop is getting established. Clovers are small-seeded forage legumes that can work, although seed costs might be high and these species may be incompatible with any herbicides that were applied before planting was prevented. Planting them into corn residue will also be challenging, although no-till drilling may work if seed can be placed well. Broadcasting into corn stalks without tillage is not likely to result in good stands. Red clover is more widely available than more exotic clovers, but supplies of all of these might be limited this year. Sweet clover has larger seed and will grow aggressively once it’s established. It will usually provide more dry matter by spring, and will also be more difficult to control before planting the next crop, compared to other clovers. Hairy vetch also grows vigorously, but its seed is expensive and it may not overwinter very well; this species will work in southern Illinois but is probably not a good choice in central and northern Illinois.

Another legume that can provide fairly rapid cover and that is widely available is soybean. As with corn used as a cover crop, soybean should be planted late, in narrow rows and at a high seeding rate (80 to 90 lb of seed per acre, if germination is at least 80%), to provide fast cover and to keep seed production to a minimum. It is not clear that GMO soybean seed can be used to plant for any purpose except commercial grain production. In cases where treated soybean seed cannot be returned to the dealer, the seed company might be asked if use as cover crop seed this year is allowable. There is no other good use for this seed, and it will probably not remain viable if stored until next year.

Using bin-run non-GMO soybeans as cover crop seed for this should be possible; check with your seed dealer to make sure. Non-GMO soybeans are typically marketed as such, and so are likely to be limited in supply now, unless producers have them in their own bin. Later-maturing varieties would make more vegetative growth and be less likely to set and fill viable seeds than normal-maturing ones, but that would add the expense of finding and transporting such seed. All told, soybeans may not be as obvious a choice as they appear to be at first glance, especially if leftover seed can’t be used for this purpose.

Soybeans used as cover should not be allowed to set and fill viable seed. That’s both to avoid complications from planting a crop following prevented planting of the same crop, and also because the maturing crop may have more residue than desired. Mowing plants off at about stage R5 (beginning seedfill) should work to control growth and prevent seed formation while still allowing capture of some fixed N. A crimper-roller might also work. Soybean plants this size can be difficult to control with herbicides, and mechanical control that leaves the residue on or near the soil surface is probably a better option.

A small grain such as wheat or oats can also be used as a cover for PP soybean acres, although that means foregoing the fixation of nitrogen. These will probably be quite N-deficient when planted into corn stalks, and while this will limit the amount of cover they produce, they should make enough growth to provide fair cover by late fall. If winter wheat or rye is used, they should be terminated in the early spring so they don’t interfere with early growth of the corn crop that follows.

If P and K fertilizers were applied in preparation for this year’s crop that didn’t get planted, their availability for next year’s crop should not be affected as long as the soil stays in place. If MAP or DAP will be applied this fall, a green cover crop present at the time of application should take up some of the N in these P-fertilizer materials, and to preserve it from loss if application is made while soils are still warm. If P and K couldn’t be applied for this year’s crop, PP provides an opportunity to sample soils if needed, and to get these nutrients applied this fall. Late planting will mean late harvest of corn and soybeans this year, which will allow for timely fall work on PP acres.

Rain, late planting, and nitrogen

One of the most pressing questions as planting continues into June after a very wet May is whether or not the high rainfall amounts over the past month have affected the amount of nitrogen fertilizer needed for the corn crop this year. This is a complicated question, related both to concern about how much early-applied N might be lost and to decreased yield potential from late planting that might lower the need for N. The recent price increase in corn also provides an incentive to make sure the crop gets enough N.

Dr. John Sawyer of Iowa State University posted an article this week in which he described their finding that the crop is likely to respond to additional N if total rainfall from April through June exceeds 16 inches in most of Iowa, and if March-June rainfall exceeds 18 inches in southeastern Iowa. The MRTN rate from the N rate calculator in Iowa is 140 lb N per acre, less than the 180 lb N or so in southern and central Illinois, and 170 in northern Illinois, for corn following soybean.

The current map shows that rainfall from April 1 through June 4 this year has totaled 10 to 12 inches in east-central Illinois; 12 to 14 inches in most of the rest of central and southern Illinois; 14 to 16 inches in much of northern Illinois; and more than 16 inches in a small area near the Quad Cities. If June rainfall is close to average, totals for April through June will exceed 16 inches over most of the state.

It’s not clear how well this way of predicting the need for additional N applies to corn in Illinois, especially with so much of the crop planted late this year. The on-farm N responses produced by Dan Schaefer of IFCA (with funding from NREC) may give us a clue about this. The following table shows percentages of central and northern Illinois (where most of the trials included here were located) that received more than 16 inches of rain between April and June each of the last four years, and percentages of the on-farm N trials that showed the need for more than 180 lb N.

April-June rainfall and N rates from on-farm N rate trials (corn following soybean) in Illinois, 2014-2018. Percent of area with more than 16 inches of rainfall was estimated from maps generated by the Cli-Mate program within the Midwest Regional Climate Center.

While these numbers show that higher rainfall during the spring is associated with the need for more N, the fact that the numbers in 2014 and 2018 don’t follow this trend indicates that this is complicated. For one thing, when it rains might be as important as how much it rains: April and May rainfall were at or below normal in 2014 and most other years (2017 was an exception), while June rainfall was well above normal in 2014, 2015 and, except in western Illinois, in 2018—all years with more responses to higher amounts of N. Yield levels were also higher in 2018 than in other years, which likely increased N demand. Because June rainfall isn’t very predictable at the beginning of June, we will probably need to wait into at least the middle of this month to assess the need for more N based on rainfall amounts.

When the N was, or will be, applied is a factor in trying to pin down N rate this year. It’s not a stretch to guess that some of the N applied last fall or early this spring has moved (as nitrate) below the rooting zone, and some of that is likely to have left the field through tiles. In places where water has stood, some has also been lost to denitrification. So in well-tiled fields in the “wet triangle”—from about Quincy to Chicago then west to Carroll County—it is likely that some early-applied N has been lost to this year’s crop, and may need to be replaced. If the crop got (or will get) some N at planting in that area, that will probably maintain yield potential until we can better guess at final amounts of N.

As I wrote here last week, when and if rainfall pauses long enough to allow soils dry to dry out will be a critical factor is how soon the crop gets access to N already in the soil. Corn planted on May 16 here (with N applied as UAN before planting) reached stage V4 on June 4 (at 403 GDD after planting, very close to the 395 GDD to V4 given in the Illinois Agronomy Handbook, Chapter 2), and is beginning to take on the healthy, green color we’ve been hoping for. So as roots grow out into the soil, they’re bringing N into the plant with water. If there isn’t much rain over the next week, we can expect a rapid improvement in the color of the crop. That’s helped along by good mineralization rates, and by warm, dry weather to stop the downward movement of N.

Where N was applied in May followed by wet weather, and the crop has been planted only in the last week, will that N be enough N? We should wait to see: if we’re in a period of moderating rainfall now, and the crop develops good color by the time it has 2 or 3 leaves, we may not need more. If it turns wet again and the crop stays yellow or pale green for a week or more after emergence, we might consider that as a sign that the crop isn’t getting the N that it needs. If it dries out (again) and the crop greens up, the roots are likely back to taking up N again, and the need for more N will be averted.

Soil tests for N?

Some have considered using a soil test for nitrate or perhaps a tissue test to see if there’s enough N for the crop. Tissue tests can be expected change quickly over time this soon after planting, and they are probably no better than simply observing the crop to see how green it is. Soil N testing that we’ve done (funded by NREC) over the past four years shows that, following application of 200 lb N in the fall (spring-applied ammonia is similar), nitrate levels in the top foot of soil were about 30 ppm in mid-May (stage VE to V2); about 30 ppm in early June (stage V5-V6); and a little less than 20 ppm by mid-June (stage V8-V10) averaged across three sites and four years. In all cases this amount of N was enough for maximum corn yield, so we can’t say with precision what the minimum soil N amount should be.

Plants take up less than 15 lb of N per acre by stage V5-V6, but take up as much as 75 or 80 lb per acre between stages V5-V6 and V10. So it makes sense that soil N levels decline as plants take up N; 1 ppm N in the top foot of soil is roughly 4 lb N per acre. So if soil N drops by 10 ppm (from 30 to 20) as the crop takes up 75 lb of N between V5 and V10, only about half of the N taken up is accounted for by the (net) drop in soil N; the rest comes from mineralization. Because there are these two sources of N, as well as some N perhaps moving up from deeper in the soil as plants take up water, the amount of nitrate we find in the soil during vegetative growth is not a very good indicator of the N supply to the plant, especially in June when soils are warm.

Using the crop to indicate N status

Growing plants will often indicate soil N availability better than does the amount of N in the soil. So we can watch the crop to see when it begins to take on a better green color. One difficulty with this is that it can, during early growth, be hard to tell how dark green the plant should be: we can’t very well know if a pale color means that there’s N in the soil that’s not available to the plant or if there simply isn’t enough N. Those doing canopy color sensing address this problem by putting “high-N reference strips” at one or more places in a field, with the idea that crop color in that strip can only be limited by factors not related to N level. Canopy color in the rest of the field is then referenced to that in the high-N strip to give a relative value.

I’ll suggest here using this approach in fields recently planted, especially in fields where it’s not clear if enough N remains available to keep the crop green and growing well. A simple way to do this is to drop some urea alongside or on top of the row in a small area in a field, then to watch the crop there relative to the crop that doesn’t have the extra N, to see if there is a difference in color. What rate we use for this isn’t critical, but a half-cup of urea (46-0-0) applied to 20 ft. of row is roughly 90 lb of N per acre—that should be plenty. To do this, find a place past the endrows (or a few rows into the side of the field) that will be easy to see later, and dribble a half cup of urea down 20 ft (8 steps is close enough) of each of 4 rows next to each other, to form a little “high-N patch” (HNP) for comparison. Plant a flag in the center of the patch, or flags on the corners. If a field has both light- and dark-colored soils, it might be interesting to place an HNP in each soil.

Then, simply look at the HNP every few days, especially a day or two after rain moves the urea into the soil, to see if the corn there turns greener than the corn outside the patch. If it doesn’t, then the crop is likely not limited by the amount of N available to it. If it does turn darker green, then continue to watch it for a week or so longer. If the rest of the crop greens up so that the HNP is no longer visible, then no more N should be needed, providing that enough N was applied.

But if the corn in the HNP stays greener than corn outside for a week or more, that is a signal that applying more N would likely improve yield prospects. How much more N should be used in a supplemental application? For the trials in the table above that showed the need for more than 180 lb N, the average amount by which they exceeded 180 lb N was between 25 and 35 lb N except in 2015, when it was 51 lb N per acre. So unless June turns out to be wetter than normal, 30 to 40 lb of additional N should be enough. Apply in such a way that gets N to the plants quickly—dropping solution N (UAN) near the row is probably the best way to do this. Broadcast urea would be next-best, and might be better if it can be applied by air when field conditions don’t allow ground applications.

Where corn is planted but most or all of the N still needs to be applied, the MRTN rate, applied as UAN, urea, ammonia, or a combination should be enough N for the crop, unless June ends up being very wet. A lot of rain in June often causes root damage, and that plus the lack of soil oxygen under very wet (including flooded) soils lowers the ability of the roots to take up N. Because the crop looks deficient whether there’s not enough N or the roots can’t take it up, it can be difficult to know whether adding more N will boost yields. We’ll wait to see what June brings.

A final note: now that we have a crop that’s mostly been planted late and into less-than-ideal soil conditions, it will be better if rainfall and temperatures during the rest of the 2019 growing season remain in normal ranges, and above-normal rainfall in July and August would be even better. There won’t be very many days this year between our wishing it would stop raining and our hoping that it starts again.

Dealing with very late planting

Despite the fact that the “active” weather pattern gave no signs of changing over the past month, few of us thought we’d see so little planting progress by now. But here we are, with only 35% of the Illinois corn crop and 14% of the soybean crop planted by May 26. With more rain this week, we will have less than half the corn and less than a fourth of the soybeans planted before June 1 in Illinois.

I’ll use the Q & A format to address some issues still outstanding as we try to get this crop planted.

Q: How bad is it?

A: The entire state of Illinois received above-normal rainfall in May, ranging from 1 to 2 inches above normal in the southeastern edge of the state to as much as 8 inches above normal (12+ inches total) in northwestern Illinois, from the Quad Cities to the south and east. That area had some planting days early—some of the corn at our Monmouth research center was planted in April—but that has been hammered by inches of rain. As bad as things are in Illinois, some areas west and southwest of Illinois are even worse, with some rainfall amounts more than 12 inches above normal for May.

Q: How soon can we expect progress?

A: We have a short break from rain in places the last few days of May, and some places that thunderstorms bypassed this week might dry up enough to plant by the weekend. The forecast isn’t promising a change to a drier pattern, though, and with soils as wet as they are now, widespread planting won’t start very soon and even after it begins, planting progress won’t set any speed records.

Q: Won’t driving on soils that are still wet 4-6 inches deep compact the soil so badly that yields will suffer?

A: We’ve consistently said that “mudding in” corn or soybeans in April does more harm than good. But that’s because planting in early April tends to produce yields no higher than planting in late April or early May; there’s no reward for aggressive planting that early. Once we get to late May, things change. We don’t want to get stuck in the field or to plant where the seedbed soil is too wet to crumble, but the need to get crops planted means that it makes sense to start even though we know that heavy equipment will (as always) cause compaction. Compaction from heavy equipment moving down the field (not directly on top of the row) typically does not lower yields appreciably in many of our more productive soils. While the formation of a physical barrier (if soils dry out after compaction) is a problem, pressing soil particles together as air is expelled can increase capillarity some, helping water to move to the roots from deeper in the soil. The most visible signs of compaction—plants stunted and with leaves showing drought stress symptoms—is typically on endrows, where equipment moves at right angles to rows and thus limits rooting ability more than it improves capillarity.

Q: Should we “open up” soils by tilling them when they’re wet so that the surface dries out faster?

A: This is not an uncommon practice in the claypan soils in southern Illinois. The capillary rise of water from deeper layers to the surface is broken up by surface tillage, and that means slower water movement to the surface and faster drying of the surface soil. Of course, that also means that deeper layers stay wetter. The claypan is already a compacted zone, so driving on it may not do much damage. Doing surface tillage in already-tilled soil or in deeper soils may make some cosmetic improvement—the surface will look drier making us believe it’s in better shape to plant—but is unlikely to do much real good.

Q: How do I decide whether to take prevented planting or to plant the crop even if it’s late?

A: Go to Farmdoc and read the useful articles posted there in the past week or so. One issue that isn’t yet there in detail (but may be coming soon) is what our yield expectations should be if the crop isn’t planted until after its last date for full crop insurance coverage: June 5 for corn; June 15 for soybean in the northern third of Illinois; and June 20 for soybeans in the rest of the state. We don’t have recent data on corn planted after early June, but in a recent paper from Iowa State [Baum et al., Agronomy Journal 111:303-313 (2018)], researchers reported yield losses of about 25% (55 bushels) by June 10, 40% (88 bushels) by June 20, and 61% (133 bushels) by June 30. So planting corn on the last insurable date (June 25) would be expected to produce about half the normal yield. I suspect it could do better than that, but for now that’s our best guess.

Based on experiences with doublecropped soybeans in the southern half of the state, we expect yield declines to be less steep for soybean than for corn after early June, reaching 50% only by early July in south-central and southern Illinois. There is a large effect of latitude with late-planted soybeans, though, and yields of soybeans planted in late June in central Illinois are very much affected by the growing season weather, and so can be expected to vary widely. Soybeans planted after mid- or late June in northern Illinois may not mature early enough to avoid frost.

Q: Should I change corn hybrids to ones with earlier maturity?

A: Maybe. Many people in northern Illinois have probably done this by now, or at least have lined up seed to do so if planting stretches out much longer. The Corn Growing Degree Day decision support tool can help with this question, but can’t answer it very precisely. That tool allows one to choose any Corn Belt county, enter the planting date and hybrid maturity, and generate a graph that shows projected GDD accumulations through the season, including the date on which you can expect that hybrid, planted on that date in that county, to mature.

As an example, a 108-day RM hybrid (which the tool estimates will need 2,600 GDD from planting to maturity) planted on June 1 in Woodford County, IL is projected to mature on October 10. Because October 10 is a 12 days before 50% chance of frost there, I would not hesitate to plant this hybrid if I were able to do so by June 5. That same hybrid, though, if planted on June 1 in DeKalb County, does not project maturity until the beginning of December, or about 6 weeks after the 50% frost date. Changing to a 100-day hybrid there would move projected maturity to October 9.

One important adjustment missing from this tool is the fact that planting corn late usually lowers the GDD needed to get a hybrid from planting to maturity. In an article on his website, Dr. Bob Nielsen at Purdue includes a calculator that uses data he and Dr. Peter Thomison at Ohio State University collected to adjust the GDD requirement downward based on how late planting actually is. This is not a trivial adjustment: planting a hybrid on June 1 lowers the GDD requirement by more than 200 GDD. So a hybrid that needs 2,650 GDD to mature if planted on May 1 will require an estimated 2,439 GDD if planted on June 1. The revised GDD number can be manually entered into the GDD tool instead of days RM for the hybrid.

The downward adjustment in GDD with late planting is not a chiseled-in-stone number; in fact, if growing season temperatures are on the cool side, planting late may not lower the GDD requirement at all. We saw this in 2009, when some corn in northern Illinois did not mature by November. There are two main reasons why late planting might lower GDD requirement. One is that the crop develops under higher temperatures, and the high-temperature “cutoff” of 86 degrees may actually be higher than 86; that is, corn may grow faster at 90 degrees than at 86 degrees. The other reason is that late-planted corn tends to have more limited root growth, which adds to any stress from periods of hot, dry weather. Late-planted corn also finishes in September or October, when days are shorter and temperatures are often lower than ideal during late grainfilling.

The outcome of all this is that reduction in GDD requirements when corn is planted late is almost always accompanied by a reduction in yield. Still, if the crop ends up needing fewer GDD to mature, it’s better that this come with a longer-season hybrid than with a shorter-season hybrid, which might mature earlier but at even more cost to yield. Adding to this is the fact that 100-day RM or shorter hybrids were not really developed to be planted in Illinois, and they may not bear up very well under Illinois conditions when planted late.

Because soybean flowering responds to photoperiod, varieties of typical maturities will flower at about the same time if planted in the second half of June. So there is little need to change soybean varieties to ones with earlier maturity, except that north of I-80 in Illinois, varieties no later than MG 2.8 or 2.9 should be used if planting is delayed past mid-June. If July temperatures are normal this might prove to be unnecessary, but cool weather (especially nights) in July could delay flower to late July, and that could substantially delay maturity and push seedfilling into less favorable conditions.

Q: What do we do about nitrogen fertilizer for corn?

A: This is one of the really tough challenges, with such a wet May following a wet fall with limited N application and limited ability to apply N this spring. As with other things, we’re in uncharted waters here: we have done a lot of N research over the past five years, but have had no conditions like this, and so we need to do a lot of guessing, of which only some will be “well-educated.”

If a producer was “lucky” enough to get be able to apply N last fall, how much is still in the soil to be available for this year’s crop, once it’s planted? We can be sure that nearly all of the N applied last fall or before May this spring is in the nitrate form by now, whether or not N-Serve was used. Nitrate moves with water, and with so much rainfall and no chance for the soil to dry to halt movement, there’s a good chance that most of the N has moved down into the soil, well below where the crop needs it to be once growth begins. While some of this N has likely reached the lines by now, we would expect that most of it is still in the soil, and capable of moving back up towards the roots zone when (if) water stops moving down. Where water has stood in fields for a week or more, we can expect that, at least by the time the soils there dry out, as much as half of it may be gone, lost to leaching and to denitrification. Some of the most persistent water holes won’t likely be planted, which will make the need for additional N there moot.

On the positive side, with soil temperatures in the 60s and 70s now, mineralization has kicked in, and this will be an important source of N as crop growth gets underway. That takes some of the urgency away from having to apply N right at planting. Even with the soil N supply kicking in now, it will be important to get fertilizer N on by the time growth gets underway, in order to avoid deficiency. Broadcast UAN used as herbicide carrier or N applied with the planter will boost the short-term N supply and postpone the need to apply in-season N. It will help a great deal if soils dry out at some point in order to stop the downward movement of N and to allow the roots to grow and take up N, and the plants to take on the darker green color that says they are not lacking in N. This won’t happen until the water coming into the plant carries with it more N than it is carrying today.

Any corn planted will likely benefit from some additional N, regardless of what’s been applied up to now. With so much water moving into and through the soil, we can’t count on full availability to the crop even of N applied a month ago in preparation for planting that didn’t happen then. How much N we should apply once the corn crop is planted, including how much to credit already-applied N, are tough questions.

If we applied 180 lb N in the fall following soybean harvest, let’s guess that 150 of it is still in the soil, and we’ll further guess (hope) that rainfall will slacken enough that, at some point before tasseling, soils will dry enough so that roots can take up some of this 150 lb of N. In such a case, we should still consider applying 10 gallons of UAN (about 30 lb N) at or after planting; if not before emergence, then dribbled near the row soon after emergence. The idea is to sustain the crop as it establishes itself, with additional N coming from mineralization until the roots begin to reach and take up the N from the fall application.

If less than the full amount of N planned for application was applied in the fall or early spring, then we might similarly discount that amount—by a fourth to a third—when determining how much more to apply. If corn was planted in mid-May with only 25 or 30 lb N applied then, we should apply the rest of the N as soon as we can reasonably do so in order to ensure a good N supply as growth gets underway. How we apply this N may not matter much, as long as we get it on in a way that keeps it relatively safe from immediate loss.

Although many may be shying away from anhydrous ammonia due to the need to apply before soils dry out, remember that ammonia injection provides better retention of applied N than does any other form or method of application. The addition of stabilizer to ammonia for in-season application is not recommended. If ammonia can be applied before the crop is more than a few inches tall, injecting it closer to the row than 15 inches can help improve access by the roots to the N. Once the crop has more than 4 leaves, application should be moved to the row middle to avoid damage to the roots.

If the main source of fertilizer N will be UAN, it should be applied using method other than surface broadcast. That means shallow injection; broadcast followed by incorporation by tillage; or dribbled, streamed, or surface-banded (all mean more or less the same thing) on the soil surface. The idea with streaming UAN is to concentrate the application so that some of the UAN moves down into the soil, where any ammonia released from urea (by urease enzyme) is captured by dissolving in soil water. Even so, applying a band of UAN in the surface leaves some of it without much protection from loss. Using a urease inhibitor may limit volatilization some, but when surface soils are warm, this won’t prevent all loss. Injection is a safer method, and dribbling UAN near the row, especially once the crop has 4 or more leaves, can get the N into the crop more quickly than if the UAN is midway between the rows. If rainfall continues, the concern will shift from urea volatilization loss to movement of the N deeper in the soil.

Broadcast urea can also be a good source of N, with the advantages of rapid application and the ability to broadcast by ground or by air onto the emerged crop without damage. A disadvantage is the potential for volatilization loss, as we discussed above. Urease inhibitors like Agrotain can be used to limit such loss, but rain within a few days effectively limits losses. Controlled-release forms should be used with caution, as we are interested in having the N from urea get into the soil and to the root quickly once the crop starts to grow.

If we get lucky and rainfall returns to more normal patterns in June, we should be OK using the MRTN rates for N, which in round numbers are about 180 lb N/acre for corn following soybean in southern and central Illinois, and about 170 lb N/acre in northern Illinois, with ammonia at $600 per ton and corn at $4.00 per bushel. Rates for corn following corn are 200 to 210 lb N per acre. If June is wet like May, then we may want to go back with another 30 to 50 lb N in the two weeks before tasseling. I don’t know of a good way to be sure whether this will be needed or not, but as a general guideline, having the crop take on a dark green color in mid-vegetative stages would tend to indicate that it isn’t needed, while getting a lot of rain during June but still having good crop growth and apparent yield potential might be a signal to go ahead.

Q: Should we change anything regarding other nutrients?

A: When we plant into warm soils, we tend not to see, and crop plants don’t experience, temporary nutrient deficiency symptoms related to having cool soils early. Any planned applications of P and K this spring that didn’t get done should be delated until fall.

Q: Should we adjust seeding rates or make other agronomic changes when planting this late?

A: Probably not for corn. For soybeans, there is a sense that narrow rows and higher seeding rates will help compensate for smaller plants when planting is late. That hasn’t been fully supported by research, but it certainly makes sense to avoid low stands and the potential for incomplete canopies during seedfill. Dr. Shaun Casteel at Purdue suggest increasing the seeding rate more the later the crop is planted. Our research has shown that we should probably have 115,000 to 120,000 plants already, and the important things is to make sure stands are at least that high.

Q: What about switching from corn to soybeans?

A: This is less an agronomic question than a matter of crop insurance, how much has already been invested in fields that were to be planted to corn, and current crop prices. For those with an optimistic mindset, corn prices relative to soybean prices, the fact that corn will not be planted as intended in some places (such as flooded river bottoms), and the fact that in some places in recent years (Ohio is one such place) corn planted in early June has yielded well will mean sticking with corn. It’s not an automatic decision to switch to soybeans when planting is so late like it might have been a few decades ago.

Q: Is there any possibility that we could end up with good yields despite the start?

A: Trendline yields for corn and soybean in Illinois in 2019 are about 190 and 67 bushels per acre, respectively. It’s too much even for an optimist to hope that actual yields will reach those levels, but if we get the best weather possible—no temperatures above normal (and not too many days much below normal, to get the crop to maturity)—and no lack of soil moisture on a single day during the growing season, we might hope to see yields in the vicinity of 90% of those trendline yields. Even that’s an audacious hope, but we haven’t tested today’s hybrids and varieties under such conditions in the past five years, and I’m hoping we’ll be surprised at what they can do in 2019.