Frost/freeze damage report: will plants recover?

Temperatures over most of Illinois dropped to the upper 20s or low 30s on Saturday morning, May 9. This resulted in damage or even death to emerged and emerging corn and soybeans. The extent of damage was closely tied to when fields were planted.

Corn planted during the warm part of April—the first week—was up and growing (slowly) by May 1, with limited leaf area. In some fields, emerged stands were already subpar, especially in the wettest parts of the state, whether or not water stood in the field. According to NASS, 68 percent of the corn crop was planted by May 10, and 23 percent had emerged. With 8 percent planted by April 19 and 37 percent planted by April 26, we can estimate that corn planted by about April 23 had emerged by May 10. About 110 to 120 GDD accumulated between those dates in central Illinois. To match a similar GDD accumulation, that date would have been later in southern Illinois and earlier in northern Illinois.

Low temperatures on May 9 caused damage to emerged corn plants, and possibly to plants that had not yet emerged. With different low temperatures across regions, and with damage ranging from minor leaf loss to death, the only way to know if seedlings that are still alive will survive is to see if they produce new, green tissue after a few days with warmer temperatures. By early next week, we’ll know.

What we won’t know is whether or not damaged plants that produce minimal green leaf area, and that grow or regrow very slowly (these two are connected) will turn into thriving, productive plants. My suggestion is to take a “realistically pessimistic” approach to this, and to include in stand counts only those plants that look ready to make normal growth by May 17 or 18. With what we hope will be warm, drier weather coming after that, replanted fields should get off to a fast start, which will help to restore yield potential.

Soybeans planted by the end of the first week of April emerged early enough to have produced some leaf area by May 9. These mostly escaped serious injury, although in low-lying areas, where temperatures might have been even lower than those recorded, damage including leaf loss could be more serious.

Two percent of Illinois soybeans were planted by April 19, and 16 percent were planted during the week that ended April 26. Ten percent had emerged by May 10, so we can estimate that those planted before April 22-23 (the same as for corn) had emerged by the frost date. Temperatures just beneath the soil surface may have dropped low enough to cause some damage to soybean seedlings that had not yet emerged. Soil crusting in some fields might also have decreased emergence.

Soybeans planted between about April 10 and April 22 had enough time and temperature to emerge, or to be in the process of emerging, by May 9. These suffered the most damage. Reports are that soybean damage ranges widely, and is severe enough in some fields that replanting will be necessary. We will need to wait a few more days to assess the potential for regrowth, but we should be able to make realistic counts of (productive) stand by early next week. That will enable us to decide whether to replant or to supplement the early-planted stand with more seeds.

I’ll close with two take-away points:

  1. Be realistic when going to the field to assess stands. With few if any starts to a season like we’ve had this year, we don’t have much guidance on what to expect. But we can’t take for granted that marginal stands will—as they often have in the past—produce full yields this time. Having plants suffer early in the season is not known to increase their ability to yield well by the end of the season.

2. Use the replant guidelines posted to Farmdoc earlier today to decide whether your fields need to be replanted. That article is similar to the May 6 Bulletin article, but includes a suggestion to adjust the first planting date based on the slow start for this year’s crops.

Replanting corn and soybeans

Both corn and soybean planting progressed at about normal speed into May, with 56 percent of the Illinois corn crop and 31 percent of the Illinois soybean crop planted by May 3. Unfortunately, the period of warmer, drier weather we have been hoping for has not yet materialized.

Over the ten days through May 5, about two-thirds of Illinois has gotten from 3 to more than 6 inches of rainfall. Temperatures have not cooperated very well, either. Although there have been a few warm days, only about 160 growing degree days have accumulated here in Champaign since April 9, and we’re predicted to get only about 35 GDDs over the next week, with a chance of frost on May 9.

Corn requires about 115 GDDs from planting to emergence, so corn planted before April 21 should be emerging this week. Eight percent of corn and two percent of soybeans were planted by April 19, and those same percentages had emerged by May 3. If a field was planted more than 120 GDDs ago and plants don’t seem to be emerging, dig up some seeds and see if they’re close to emerging, or if they might have stopped developing at some point. Seeds that have spent time under water or in saturated soils may be at risk. Cool soils may have prolonged their life long enough so they’ll struggle up eventually, but such fields may not be very pretty.

It’s unlikely, but not impossible, that frost later this week might be severe enough to kill emerged plants. The growing point of corn is protected in the soil, and few fields have made enough growth to be damaged much by light frost. Wet soils cool more slowly than dry soils, so offer a little better protection. Dark-colored soils also radiate better, and so offer more protection from radiational cooling that can kill leaf area that’s exposed to the sky (that is, more horizontal) on a clear night. In a planting date study at Urbana in 2005, corn planted on March 30 was at the 2-leaf stage when the low temperatures dropped to 32 degrees or less (the lowest was 28) for four nights in a row the first week of May. A substantial number of plants were killed (on a more or less random pattern down the row), but enough survived to compare yields from that and later plantings at 30,000 plants per acre. It was a very dry year with low yields, but the same population planted on April 19 yielded 115 bushels per acre compared to only 47 bushels from the March 30 planting. So we’ll only know the real effects after the period of low temperatures, and maybe not until the end of the season.

Soybeans are vulnerable to frost injury for only a day or so as they’re coming through the soil. Wet soil holds heat better and also cracks open less during soybean emergence, which lessens the chance of damage. Cool, wet soils are a bigger problem for both crops than a night of light frost.

If more than 140 or GGDs have accumulated since planting, it’s time to evaluate the stand and to decide if it warrants replanting. Hardly ever are stands decreased uniformly over an entire field, so while it’s helpful to count stands in several places across the field, it’s also necessary to get an idea of how much of the field consists of patches with no plants at all. A drone may be helpful in doing this. If the stand is good in places and missing in other places, calculating a (weighted) average stand doesn’t help, other than to suggest how much of the field might need to be planted again.

Corn replanting

Back in the 2000s we generated data to update a replant decision tool for corn, by planting at different dates and establishing a range of plant populations within each planting. That tool is available in the Illinois Agronomy Handbook. But I do not believe that the data used to produce that tool is valid for current hybrids, which lose yield more slowly as planting is delayed, and that also show less yield loss as plant population decreases in comparison to earlier hybrids.

We found little interaction between planting date and plant population in the earlier sets of data used to formulate replant guidelines: that is, population responses didn’t change much as planting date changed. We think that’s still the case, which means that we can combine planting date and plant population data from different trials without producing big inaccuracies. Based on that, I used our most recent planting date and plant population response data to generate Table 1 below.

Table 1. Corn yields (percent of maximum) for different combinations of planting date and plant population. Derived from the results of 39 planting date and 44 plant population trials in Illinois.

If lower stands are mostly from random skips down the row rather than from larger spots without plants, use the table above to help decide whether or not replanting will pay for itself. Use the original planting date and existing stand to estimate expected percentage of maximum yield, then move to a later planting date range and expected plant stand from replanting to determine expected yield if the field is replanted. As an example, if a field planted on April 15 has a stand of 26,000, we would expect it to yield 92% of maximum. If we can replant on May 15 at 35,000 plants, we would expect 95% of maximum yield, for an increase of 3 percent, or 6 bushels if we expect the field to yield 200 bushels per acre. Note also that unless replanting can be done (in this case) before the last week of May, there’s little chance that replanting will result in more yield than keeping the current stand.

Depending on replanting costs, it might or might not make sense to replant for 6 bushels per acre; this is where judgement comes into play. If the stand that’s there is uneven and it looks like some plants might not survive to thrive, or if there are many spots (more than one row wide) without any plants, then it’s entirely possible that the original stand will yield less than 92 percent. Crop insurance and seed company policy on replant seed also come into play. And some prefer to replant “ratty-looking” fields just because they don’t want to look at them all season and wonder if they should have replanted. Keep in mind that replanted corn will usually have higher grain moisture at harvest, which should be counted in the replant cost.

Are there any adjustments we should make if we decide to replant? If all of the N has been applied, the replanted crop may not need any more, given that we haven’t had warm soils that enhance losses. But where there’s been a lot of rain since N was applied, nitrate-N has been moved down into the soil, and it might pay to apply some N with the planter to make sure there is some there after the crop emerges, especially if soils are still cool (meaning low rates of mineralization) at the time of replanting. There’s no reason to change plant population or hybrid maturity from what was planted originally, although actual hybrid may have to change depending on seed supplies.

It can be difficult and frustrating to “repair-plant” partial stands, but if nearly all the missing plants are in spots rather than scattered down each row, and you can find a 4- or 6-row planter to minimize the area that ends up with twice too many plants, that can save costs. In any corn replanting operation, leaving existing plants to grow along with the plants from replanting is often disastrous: hybrids tolerate high populations, but not double what they should be. Using trash movers to dislodge existing plants might work OK in some cases. But in many cases, especially for whole fields, existing plants should be sprayed out with herbicide so they don’t compete with later-planted plants.

Replanting soybean

Evaluating soybeans stands is a little more “complicated” than evaluating corn stands, although the same problem of uniformity of (remaining) stand occurs in both crops. It does make it easier when conditions are such that few soybean plants emerge, and given that we consider 80% stand establishment to be acceptable in soybean, we expect, and more often see, stand reductions in soybean that are more uniform than those we see with corn. An adage that we might apply to soybean is, “When plants are easy to count without bending over, there aren’t enough of them.”

One concern is that those who advocate for lower seeding rates and early planting may encourage stands to be kept even when they will result in a yield loss. We have over the years learned that stands that may look inadequate when plants are small usually fill in nicely and produce high yields. This has taught us to be patient when growth starts slow and not to let emotions guide replant decisions. Still, staying with stands that are too low to produce maximum yields should not be done just to “prove a point.”

Instead of laboriously counting the number of plants in a hoop or 3 ft of row, it might be faster and accurate enough to use a scale of 0 to 4, with each number the approximate number of plants per square foot. On this scale: 1 (43,560 plants per acre) would be too low; 2 (about 87K/acre) would be probably be acceptable if plants are healthy; 3 (131K) is a full stand; and 4 (174K) is more than enough. One plant per square foot is a plant every 4.8 inches in 30-inch rows, and every 9.6 inches in 15-inch rows. With a little practice, it should be possible to get close-enough stand counts with a relatively quick glance at the ground. Getting quick counts in more places is usually preferable to getting counts that are more accurate in fewer spots.

I took the same approach to estimating expected outcomes from replanting soybeans (Table 2) as I described above for corn. One caveat is that we do not have much data from recent studies that included both planting date and seeding rate, so our assumption that soybeans respond similarly (on a percentage basis) to seeding rate at different planting dates is not well supported. Joshua Vonk did some studies that included both seeding rate and planting date in 2010 and 2011 and found no interaction between these two factors across sites. But as others have found, he didn’t see much response to seeding rate, so not finding such an interaction was expected. Those studies also didn’t include planting quite as early or quite as late as the table includes, and so the indication that 50,000 plants will yield only 10 percent less than 130,000 plants when planted in mid-June isn’t rock solid.

Table 2. Soybean yields (percent of maximum) for different combinations of planting date and plant population. Derived from the results of 29 planting date and 25 seeding rate trials in Illinois.

According to Table 2, as much yield will be lost from planting after May 20 as from losing nearly half the stand from April-planted soybeans. Experience and judgement will play a role in this decision, as will the cost of replant seed and planting. Unlike corn, there’s no need to destroy existing soybean plants when replanting. In a study we did some years ago, supplementing low plant stands with a reduced amount of seed or destroying the low stand and planting a full rate of seed produced the same yield.

Planting corn and soybeans in 2020

March rainfall in Illinois ranged from normal to a couple of inches above normal, but the last week of March and first week of April have been relatively dry, and field operations are getting underway. The April 6 NASS report indicates that there were 3.1 days suitable for fieldwork in Illinois during the week ending on April 5, but no planting was recorded. As is often the case in early April, soils are wet over most of the state.

The 4-inch soil temperatures at 10 AM have been close to 50 degrees in southern Illinois, and over the past week they have increased from the low 40s to the mid-40s in central and northern Illinois. The forecast is for a return to cooler weather later this week, and possibly to wetter conditions as well. Such “yo-yoing” is normal for April, and it often brings up questions about what to do when the weather forecast is for conditions to deteriorate as planting approaches. Do we plant or do we wait?

There is no question that the ideal is for seed of both corn and soybean to be planted into soils that are relatively dry, and that are warm (and warming) enough to allow germination and emergence to get started quickly, and plants to grow steadily after emergence. The most recent example of the benefits of this was in 2018, when planting was delayed until May, then May weather was very warm, and the crops “never looked back” on their way to new yield records. In 2017, early corn planting was followed by a week of cool, wet weather, which led to a lot of replanting. The replanted crop often yielded more than the first crop, almost certainly because it had warmer conditions under which to germinate and begin to grow.

Having soils stay dry after early planting into cool soils is much better than having them turn wet: the germination process is very slow at low temperatures, so seeds will bide their time until soils warm up, and dry soils are a safer place to do that. If it turns wet, seeds will last longer in cool soils than in warm ones, both because low temperatures delay the germination process (and the demand for oxygen), and because colder water contains more oxygen than warmer water. Still, seeds that spend a week or more in wet soils at temperatures in the low 40s are subject to “imbibitional chilling injury” that can mean abnormal growth and poor emergence even if seeds survive. This is considered more of a problem in corn than in soybean, in part because more soybean seeds than corn seeds tend to die under such conditions and so don’t show those symptoms.

Planting date

Now that we’ve passed the first week of April, plantings of this year’s corn or soybean crops from now on can’t be considered “very early”, but the message from some agronomists about the need to plant soybeans as early as March continues, and more producers are choosing to begin planting soybeans before they begin planting corn. With planting date responses for the two crops essentially identical on a percentage basis, which crop to start with is more or less a tossup. The deciding factor in that case should often be which fields are ready first. Fields where soybeans grew last year will often be in good shape to plant earlier than those where corn grew, and that may mean planting some corn first. It certainly makes little sense to plant soybeans when it’s too wet just to plant them earlier than corn.

I have mentioned before the possibility that soybeans planted very early—in March or early April—might occasionally yield less than those planted in late April or early May. I dug up some data from a study that we did back in 2001-2003 in which we started planting as soon as we could (without planting in mud) using different seeding rates and varieties with different maturities. Figure 1 shows yields from this study, with planting date averaged over sites. Yields were not as high as we’d expect today, but the earliest planting yielded the least of all the planting dates. This was not due to low stands, with the exception of the Urbana site in 2001, when it froze (temperatures in the upper 20s) just as the crop was emerging, and about half of the plants from the first planting date were killed. We had five sites in southern Illinois, where average yields were even lower, but the earliest planting there (average of April 15) yielded less than either the early May or late May planting.

Figure 1. Soybean planting date responses over nine trials in central/northern Illinois, 2001-2003.

While changes in seed quality, spring weather, and perhaps genetics have lowered the threat of such losses from very early planting, we can’t rule out the possibility that planting soybeans in March or early April may not always maximize yield. That’s not necessarily because of stand loss from frost or wet soils. Frost can typically kill soybean plants only in a one- or two-day window as the plants are breaking through the soil surface. Frost that occurs after the first two leaves unroll can kill the growing point, but then buds will break and form (usually two) new stems. Most low stands in soybeans follow heavy rainfall soon after planting, and chances of that happening are not closely tied to when the crop is planted. Instead, the evidence is that low temperature stress during early growth may limit node and seed number per plant, therefore limiting yield potential. The fact that the earliest planting in northern Illinois responded so much to seeding rate reflects the fact that these plants did not have as many seeds as those planted later.

One of the incentives to plant soybeans very early is that some seed companies provide free replant seed. I do not know if “free” includes the cost of seed treatments (for replant seed) that are commonly applied to soybean seed at the point of sale. Soybean seed meant for early planting is often treated with several plant protectants, including ILeVO® for decreasing the incidence of SDS. That disease is generally considered more likely to be a problem when soybeans are planted into cold soils.

The debate among agronomists regarding the merits of planting soybeans in March or early April—before the start of corn planting—is still alive, but focusing on “corn versus soybean” as if it’s a contest mostly misses the point. Both corn and soybean benefit from early planting most of the time, and both face similar risks when conditions deteriorate after we plant early. We shouldn’t decide when to start planting or which crop gets priority based on how “tough” each crop is or on trying to prove someone wrong. The goal instead is to minimize risk and to maximize yield potential. The 2019 growing season was such that that penalty from late planting was relatively less for soybeans than for corn. That doesn’t mean that corn should get first planting priority this year. Both crops should get priority, with actual planting order determined by factors such as logistics, how fast fields dry, and crop insurance.

Recent research on how both corn and soybeans respond to planting date in Illinois is summarized below in Figure 2. I’ve shown both lines on the same figure before, but here I’m including the actual data for both crops along with the curves in order to show how variability changes as planting is delayed. While we did not try to plant soybeans before mid-April in this study, note that hardly any of the April soybean plantings produced less than maximum yields in these trials. With mid-April plantings yielding the same as late-April plantings, it seems unlikely that yields from planting in March would have been higher than those from planting in April.

Figure 2. Corn and soybean planting date responses in Illinois trials. Each trial included four planting dates, and yields were converted to percent of the maximum yield in that trial.

Unlike soybean, the earliest planting dates for corn did not consistently produce the highest yields in the trials shown in Figure 2. This was not due to poor stands or frost damage, but was the result of growing conditions later in the season, and was more common when yields levels were lower. It’s difficult to untangle what happened in each of these, but in a few cases the early-planted crop experienced cool temperatures in May that might have lowered yield potential. The growing season was relatively dry in some of these sites as well, and small differences in rainfall timing could have favored the crop that was planted a little later. We added an additional planting date in mid-March in the very dry spring of 2012, and lost about half of the stand to frost during the second week of April.

Planting depth

Recent developments in automated depth and down-pressure controls on planters have brought new attention to the issues of planting depth and seed placement. While research done over a few sites often identifies a “best” depth, such results don’t very well predict what the best depth will be in a given field the next time. We can guess the best planting depth about as well as we can guess the weather, although the depth decision is easier in some soils than in others. Most studies include planting both too shallow and too deep, with a few depths in the middle, and results typically show, to no one’s surprise, that it is better to avoid planting too shallow or too deep.

An additional feature available on some planters is a sensor for soil moisture coupled with the ability to vary planting depth based on where in the soil there’s enough moisture to get germination started. This has potential for dry areas where soil moisture frequently is low during the planting season. But I think we need to be cautious with this in the eastern Corn Belt, where soils are heavier and where heavy rainfall after planting and before emergence is a much serious threat to stand establishment than dry soil at planting. Planting deeper means that emergence almost always takes longer, and that means more chances of having problems related to wet soils and surface compaction (crusting) as soils dry out after they get wet. In practice, I think this means that planting 3 inches deep or deeper in most Illinois soils (sandy soil is an exception), even if that’s where soil moisture is adequate, has a better chance of lowering stand counts than it does of increasing them. Most corn seed has the ability to emerge from 3 inches deep if soil conditions are good, but when soil conditions deteriorate after planting, those three inches can turn onto an obstacle course for seedlings. That can compromise stands and stand uniformity, both of which are needed for getting the highest yields.

Today’s planters do a good job of pressing soil against seeds for the sides and above, resulting in good seed-soil contact without compacting the soil above the seed. Good seed-soil contact forms a conduit by which water can move through the soil into the seed as germination begins. That effectively enlarges the soil volume from which seeds can draw water, which means that even soils with lower moisture content often have enough water to allow germination, especially in silt loam and silty clay loam soils without clods. Clods form when soil that was tilled when it was wet dries out. With less tillage and less time between tillage and planting today, soils often do not to dry out very much before planting. As a result, uneven stands due to uneven soil moisture is relatively rare in most Illinois fields. Those who can’t remember when they last saw uneven stands due to uneven soil moisture at planting—that is, times when some seeds had to wait for rain before they emerged—might have reason to question the advisability of having soil moisture determine how deep seeds are planted.

So where, between too shallow (let’s say one inch) and too deep (3 inches in most soils) should we plant?  Soybeans planted in the first half of April with soil temperatures (2 inches deep measured at 7 or 8 AM) less than 50 should probably be planted 1.25 to 1.5 inches deep, and corn at least 1.5 inches deep. When planting into warmer soils later in April or in May, 1.5 inches is good for soybeans and 1.75 inches for corn. Manually changing planting depth on a 24-row planter is good exercise, but may not always be worth the time it takes. As long as we’re planting between 1.5 and 2 inches deep, it’s not clear that trying to fine-tune depth based on current and future soil conditions has much potential to improve stands.

Especially when planters move at speeds of 6 mph or faster and when the soil surface is not very smooth, some seeds end up shallower and some deeper than the nominal setting. Equipment and seed companies have looked at the effect of planting depth on stands and yields, and have in some cases managed to produce large yield differences by employing “mistake” settings. Measuring the uniformity of seeding depth by digging up seeds is difficult, but high-speed cameras can estimate depth as seeds drop and settle in place. One study done by digging up corn roots at maturity reported a standard deviation of about an eighth of an inch, which would mean that about 5 percent of seeds would be at least a quarter of an inch shallower or deeper than the average. That’s probably acceptable at normal planing depths. More weight and more uniform down-pressure have improved planting depth uniformity, and if 75 percent or more of plants emerge over a period of about 15 growing degree days (24 hours at average temperature, longer than that if it’s cool) and the rest within one more day, it’s unlikely that any yield has been lost due to non-uniformity of planting depth.

Uniformity of distance between seeds is good enough to maximize yield potential in most fields, and needs no further mention. Despite what yield contest winners say they do, there is no reason for most people to plant slower than they do now. If the monitor says enough seeds are being dropped, and either the monitor or previous experience (by seeing how stands look after emergence) say they’re spaced uniformly enough, they probably are.

Seeding rate

Most people have decreased the number of soybean seeds dropped per acre over the past decade or so, but seed quality has also improved, and so the number of plants needed to maximize yield has probably not decreased as much as the seeding rate. We know that seeding rate responses are highly variable: in a series of 25 seeding rate studies in Illinois between 2015 and 2018, we found that the stand (not seed) numbers needed to maximum dollar return to seed ranged from 50 to 200 thousand, and there was no correlation between yield and plant stand needed to produce that yield. That means that the best way to set seeding rates is to average over seeding rate trials to get a best-guess prediction.

Averaged over the 25 responses, the plant stand needed to maximize the net return to seed was about 107,000 plants. At 80% stand establishment, that would require planting 134,000 seeds per acre. While that seems like a reasonable seeding rate, the “best” seeding rate was higher than that in about half of the trials and less than that in the others. Responses were fairly flat in most of the trials, though, which says that moving around within a range of 125,000 to 145,000 seeds per acre won’t miss the mark by much. If you expect emergence to be higher than 80%, seeding rates can be decreased. If you’ve gotten good yields planting only 100,000 or 110,000 seeds in the past, feel free to do that again. Keep in mind, though, that yield responses to seeding rate may not be very visible. So while 100,000 seeds might produce a good yield of 75 bushels, using 130,000 seeds might increase that by 2 bushels, which won’t look like much but would increase profits by $12-13 per acre.

The response of corn to plant population is much more consistent that for soybeans. Figure 3 below shows the response to corn plant population over 44 trials in Illinois between 2012 and 2018. Each trial included four to six hybrids, with planted populations ranging from 18,000 to 50,000 per acre. Final stand closely matched seeding rate, so they’re used interchangeably. The average yield at the 100% (of maximum) yield level was 237 bushels per acre. We used a wide range of seeding rates in order to produce visible responses, even though we know that this range extends far outside the range that producers might consider. Yields at 48-50,000 plants were lower than those at 34-36,000. So what we chose as the high end of the range ending up “bending” the curve, which changed where it reaches a maximum. The curve fitted to yields from the populations up to and including 42,000 shows that the maximum yield was produced at 36,900 plants per acre, and the optimum population—where the last seeds added were paid for by the increase in yield—was 33,400 plants per acre.

Figure 3. Corn plant population response over 44 trials in Illinois, 2012-2018.

It’s also worth noting that, although we find best returns from plant populations in the 32,000 to 35,000 per acre, having them a few thousand higher or lower is not going to change yields or net returns by very much. Yield level doesn’t make much difference: yields in 2012 were about 50 bushels lower than in the highest-yielding years of this study, but the population response was about the same as in other years. Going up to 40,000 isn’t very likely to increase yields, but it won’t increase costs much, either, so it won’t do much harm in productive soils. Marlin Jeschke of Pioneer recently reported that harvest populations for non-irrigated entries in the NCGA Corn Yield Contest over the past five years was 36,700, so it’s clear that current hybrids don’t require unusually high populations to produce high yields.

If planting is delayed in 2020

Should management of corn or soybean change if planting is delayed in 2020 like it was in 2019? We’re certainly hoping that any delays are not on the scale that we saw in 2019, but we did not see many signs last year of things we should change if planting is late in 2020. That may have been because of good weather and good yields even after the late planting. About the only thing we might want to consider if corn planting is delayed into June is to move to earlier–maturing hybrids in the northern part of Illinois. Hybrid strip trials planted in that region in early June last year showed lower yield for hybrids later than 107-108 days RM. We did not see this with late-planted soybeans there, nor for either corn or soybeans in central and southern Illinois.

Managing Nitrogen for Corn in 2020

As was the case a year ago, there have been limited opportunities to apply nitrogen fertilizer since last fall. Rainfall in Illinois through the first three weeks of March has been at or above average, and temperatures have been a few degrees above normal. Soils remain wet, and there is little in the current weather pattern to indicate that a drying period is on its way soon. Potential drying rates will increase as temperatures rise, though, and we will hope that rainfall remains at or below normal to allow soils to dry as we move into April.

N rate

Despite difficult conditions in 2019, Dan Schaefer of the IFCA and John Pike in southern Illinois, with funding from the Illinois Nutrient Research & Education Council (NREC), were able to conduct on-farm N rate trials that showed that responses in most regions, even with late planting, were similar to those found in recent years. Yields were generally not as high as in 2018, but in central and northern Illinois, the fact that responses were similar to those already in the database meant that adding the data from the 2019 trials didn’t change the guideline N rates (MRTN values) by very much for this part of the state.

The 2019 data in southern Illinois, however, continued the trend we saw in 2017 and 2018, in which higher yields required higher N rates to reach those yields. Such a correlation between optimum N rate and yield across trials does not exist in higher organic-matter soils in central and northern Illinois. We think this is because weather conditions (warm temperatures and plentiful moisture) that lead to high yields (and high N uptake) also increase the amount of N supplied by mineralization of soil organic matter, leaving the amount to be supplied by fertilizer unchanged, at least on average. In contrast, soils in southern Illinois have less organic nitrogen to mineralize, so high yield levels there make the crop more dependent on N from fertilizer.

This correlation between N rate and yield in southern Illinois supports the idea that we consider adding more fertilizer N to corn growing in lower organic-matter (<2% OM) soils in southern Illinois if the crop has high yield potential. I suggest using the MRTN rate for yields up to 190-200 bushels per acre, and for yield potentials above that (determined based on crop condition when corn is 2 to 4 ft tall), use a total of 1 lb of N for each bushel of expected yield. That may often mean applying N with high-clearance equipment, either as broadcast urea or as UAN dribbled near the row. Dribbled N often distributes more uniformly, and leaving UAN on the surface near the row moves it closer to the root system and may improve uptake.

Use the N rate calculator to calculate best (MRTN) N rates for corn in Illinois. We updated the database in early March, adding the 2019 data and removing some of the older data. Using a corn price of $3.50 per bushel and the current ammonia price of about $500 per ton ($0.30 per lb of N) produces the MRTN values and ranges shown on Table 1 below. Low and high ends of the range are those N rates at which the return to N ($ per acre) are $1.00 less than at the MRTN. MRTN values are also shown for N prices of $0.40 and $0.50 per lb, keeping the corn price at $3.50 per bushel. N prices for UAN and urea are currently around $0.43 per lb of N.

Remember that the MRTN rate (and ranges) generated by the N rate calculator includes all of the N applied to the field, not just to the main application. This means counting into the total any N applied with MAP or DAP in late fall or spring, any N applied with herbicide or with the planter. If N from several different sources is used, base the rate of the last application (adding in all previous amounts) on the price of fertilizer N that is used for the last N application.

Table 1. Current MRTN (guideline N rates) for corn in Illinois, after adding the data from the 2019 growing season.

N timing

In about 90 percent of on-farm trials comparing N rates applied as ammonia in both the fall (with N-Serve) and the spring, fall- and spring-applied N have produced virtually identical responses to N rate, at the same yield levels.  Across 16 trials, including several in which spring-applied N performed better, and several in which fall-applied N performed a little better, the optimum N rate averaged about 12 lb higher—181 versus 169 lb/acre—and the yield at the optimum N rate 1 bushel less—235 versus 236 bushels per acre—for fall-applied N compared to spring-applied N. That meant an advantage of $9 per acre in return to N for spring-applied N, but since getting that added return would have required knowing when and by how much to decrease N rates for spring-applied N, it would have been difficult to realize this benefit. The average optimum N rate for fall-applied N was almost identical to the MRTN for central Illinois (Table 1): using the MRTN would meant using more than the optimum N rate in more of the spring-applied N trials, and so would have meant less advantage for spring-applied N across these trials.

One of the main lessons we’ve learned from our N timing and N form studies in recent years is that, in order to maximize yield potential, corn plants need to have a substantial amount of N available in the soil near the row after plants emerge and before their nodal (main) root system starts to develop. In one study in 2019, the crop was planted in late April but fertilizer rates couldn’t be applied until early June due to wet May weather. As a result, N responses rose in a straight line up to the maximum N rate used (250 lb of N), and did not reach a maximum. We also saw several instances in which cover crop rye was not controlled early, and probably because the rye roots had stripped the N from the upper soil, corn yield suffered even when high rates of N were applied after the crop emerged.

We don’t know exactly how much N needs to be present during early corn growth, but we believe that this N needs to be in the soil near the plants when the nodal roots begin to appear—at about growth stage V2. To have 40 to 50 ppm available N in the upper soil at V2 means incorporating 40-50 lb N in the top 3.5 inches of soil, and having most of the N stay there. If we incorporate N into a zone 7.5 inches wide by 3.5 inches deep centered on the row, only 10 to 12 lb N per acre will produce 40 to 50 ppm, if the N uniformly distributed and if it stays there for at least 3 weeks (300 GDD) after planting. That amount (but not much more than that) could be applied in-furrow, but any downward movement of that N would take it out of the rooting zone of small plants. Applying 30 to 50 lb N in a 2 x 2 placement, or dropping liquid or dry fertilizer over the row to provide 30 or 40 lb of N per acre would better assure having N when it’s needed early, if there’s equipment to do that. In-furrow placement of 10-12 lb of N as UAN is better than nothing. Seedling damage from such applications is rare, but split-tube placement with a seed firmer will protect a little better against this.

Even if planting is delayed and takes precedence over N application, some N really does need to be applied into or atop the row before the crop emerges: it is too risky to wait for several weeks to get the first N applied, especially if even that N is not placed near the row. If it stays wet this spring, some producers and retailers might need to get creative in order to get this done. Delayed planting means warmer soils at planting, and warmer soils mean more mineralization. This will boost the soil N supply some, but especially if rain moves some of the mineralized N down, there may still not be enough to maximize yield potential of the crop.

Splitting N

In one set of results from different forms and times of application of 150 lb N per acre, we found that a split with 100 lb at planting and 50 lb applied in-season generally yielded a little more than applying all of the N between the rows at planting. Applying 50 lb N as broadcast UAN at planting (to mimic the use of UAN as herbicide carrier at or after planting) then 100 lb as UAN injected at stage V5 did not yield as well, probably because there wasn’t enough N near the root system when it was needed, before sidedress. Most of the treatments with 100 lb N injected at planting followed by 50 lb as sidedressing worked about equally well. Waiting until sidedress time to apply all of the N was not an effective way to apply N, and placing UAN on the soil surface also produced lower yields, even when urease inhibitor was included. All of these point to the importance of having enough N in the soil early enough to maximize yield potential during early growth, and of applying all of the N in a way that results in less loss.

We also found in these studies that splitting N—with some at or before planting and the rest as sidedress—often produces yields no higher than applying the same rate (with appropriate placement) early. That does not mean we shouldn’t split-apply N, but we should do it more for logistical purposes than as a way to get higher yields with the same (or lower) rate of N, at least on productive soils. We have found no advantage to keeping back 50 lb N to dribble in-row at tassel, nor have we found an advantage to applying N several time (spoon-feeding) during the season. Very wet June weather, such as we had in 2015, in some cases meant a response to adding additional N. But getting N applied under such conditions is not easy, and every trip to apply N brings the added cost of application as well as the risk of not having the N get to the roots for uptake in time for the plant to respond.


Despite the fact that inhibitors sold as N fertilizer additives have been around for decades, there remains a considerable amount of confusion about these products, including what they do, and when and how they should be used. Nitrification inhibitors slow the activity of bacteria in the soil that convert ammonium to nitrate. Both ammonium and nitrate can be taken up by plants, but the ammonium form is attracted to negative charges on clay and organic matter, and so stays in the soil, while nitrate is negatively charged, so moves readily with water as it moves down through the soil. So slowing the conversion of ammonium to nitrate (nitrification) is a way to keep more N in the soil and available to the crop under high-loss (wet) conditions. Chemicals sold as nitrification inhibitors include nitrapyrin (products include N-Serve® and Instinct II® by Corteva); pronitridine, a newer product developed and sold as Centuro® by Koch Ag; and dicyandiamide (DCD), a nitrification inhibitor sold by a number of companies under different trade names.

We normally add a nitrification inhibitor with anhydrous ammonia applied in the fall. The later we apply ammonia in the spring the less likely it is that a nitrification inhibitor will be needed to protect the N. As a biological process, nitrification is slow when soil temperatures are in the 50s (through early, mid- and late April in southern, central, and northern Illinois), and begins to speed up once soil temperatures reach 60 and above, which usually occurs in late April in southern Illinois and mid-May in northern Illinois. If we add in the effect of the NH3 itself in suppressing microbial activity, it’s unlikely that applications of ammonia made after mid-April in southern Illinois or after early April in northern Illinois will need the further delay in nitrification provided by nitrification inhibitor. There are exceptions to this: May can be warm and wet, with rapid conversion to nitrate, in which case a nitrification inhibitor can be helpful. But if the crop is planted early and grows fast in May, uptake starts early as well. And if the weather is relatively dry, N is unlikely to move in the soil even if it’s all nitrate. This makes it difficult to know at the time of application whether we should add a nitrification inhibitor, and we should play the odds based on current conditions and expected planting time to help make this decision.

Because cool soils are slow to dry, early spring (preplant) applications of ammonia are usually done when soils are wetter than ideal. Ammonia application on wet soils means more soil compaction, and with the diameter of the ammonia band very small when application is into wet soil, its concentration in the band is high. If the soil dries out considerably after application (a rarity if it’s wet into April), NH3 can begin to leave the band where it’s been dissolved and to move up in the soil through the knife track, where it could damage seeds or roots. Using RTK to apply the band 6 to 8 inches away from where the row will be planted can eliminate such damage. Tilling after ammonia application can also help disperse the band and will usually lower or eliminate the risk of ammonia injury on seedlings. Deeper placement can also help prevent damage, but will leave the N farther from the roots.

The other type of inhibitor sold for adding to N fertilizer is urease inhibitor. Inhibitors that do this include NBPT (sold under different brand names), and mixtures of NBPT with duromide (ANVOL® from Koch Ag) and with NPPT (Limus® from BASF). Thiosulfate, which is also used as a sulfur source, is thought by some to inhibit urease, although lab studies tend to show that it’s less effective. As the name implies, urease inhibitors are effective only when added to urea or to other urea-containing fertilizers such as UAN solution. They do not slow the conversion from ammonium to nitrate; they only slow the breakdown of urea into ammonia and carbon dioxide. If this breakdown happens on or near the soil surface, ammonia can go off as ammonia gas into the air.

Ammonia is extremely soluble in water, so if urea breaks down in moist soil, the ammonia released will dissolve immediately, and hardly any of it will escape into the air. The urease enzyme that speeds up this breakdown is very common in soils, so if urea or UAN is broadcast on the soil and there is no rain for a week or more, a lot of ammonia can be lost into the air. Broadcast UAN, because it spreads the N in a thin layer over the soil surface, exposes more of the urea to urease activity. But only half the N in UAN is in the urea form—the other half is nitrate and ammonium, which aren’t affected by urease. UAN does contains some free ammonia in solution, and some of this may volatilize as the solution dries. Dry urea, once it dissolves in soil water, is all subject to urease, but urea granules that fall into cracks in the soil surface may gain some protection.

Rainfall moves urea into the soil and also wets the soil and dissolves ammonia, greatly decreasing the loss of ammonia. This means uncertainty regarding whether or not to use urease inhibitors. If urea or UAN is incorporated into the soil at or soon after planting or at sidedress time, there is no need to add a urease inhibitor, since ammonia rarely escapes from soil. Dribbling or surface-banding UAN exposes it a little less to urease and moves some into the soil a short distance. UAN dribbled on the surface near the row is a little less exposed to sunlight and wind, and water coming down the plant stems from light rain or dew can help move the N into the soil. Still, surface-applied UAN can never be considered completely safe from volatilization loss, so an inhibitor might be useful if the forecast is for warm conditions without rain for a week or more after surface-banding near the row.

With warm surface soil temperatures, nitrification will begin soon after the urea is dissolved and in the soil (as ammonium). SuperU® (from Koch), which has both urease and nitrification inhibitors, has performed well in trials when broadcast on the surface, and has yielded more than broadcast urea with the urease inhibitor Agrotain (NBPT). Assuming that both products inhibited urease equally, the difference must have been due to more rapid conversion of ammonium to nitrate, and movement of some of the N out of the rooting zone.

Novel products sold to increase microbial N fixation

There has been a recent upswing in advertising and products that are said to provide the microbes or to stimulate existing soil microbes that fix atmospheric nitrogen and make it available to the corn crop. Microbial N fixation is the way that soybeans get most of the N they need, but such fixation in legumes involves the pant producing nodules that are attached to the roots below the soil surface, and in which anaerobic (low-oxygen) conditions exist to aid in the fixation process. We’ve known for a long time that there are some “free-living” (not in nodules) bacteria in soils that can fix N for the atmosphere, but measured fixation rates by such microbes tend to be very low – on the order of a few pounds of N per acre. That’s in part because fixing atmospheric N requires a great deal of energy, and a soybean plant can pump sugars into nodules a lot faster than sugars leak out of (corn) roots to feed this process in microbes that live near the roots. There was once hope that corn plants could be genetically modified to produce nodules and house bacteria that could fix much of their own N, but the machinery the plant needs to form nodules and to transport fixed N in the plant is so complex that this seems to be unlikely, or at least a long ways off.

There are two types of these products, mostly developed and marketed by startup companies backed by venture capital. One type is a preparation of the microbes (bacteria) that fix N; these are usually applied in-furrow, with the idea that they’ll multiply and grow near the root to eventually get enough sugars from the roots to fix N that the plant can take up. The idea is that corn plants and such bacteria form a mutually beneficial (symbiotic) relationship, with the corn providing sugars and other growth substances and the bacteria giving back N. It’s not entirely clear that bacteria can act as little “N pumps” like this, and if they can, it’s not clear how such a symbiosis would benefit either the plant or the microbe.

The other type of product being marketed is a chemical product that is said to stimulate the growth and function of bacteria that go on to fix N for the corn plant. It appears that some of these can be applied as foliar sprays, presumably with the idea that they can be released by the roots into the soil, or that they stimulate the plant to release something on its own that in turn stimulates growth of bacteria that fix N.

Claims on websites for these products might say that they make the plant (and roots) grow faster, and often show photos to that effect. Some mention how much N fixation might be expected from using the product. I have not done any work with any of these, but will just observe that pinning down rates of N fixation by microbes when rates are low (25 lb N per acre per season seems to be a somewhat typical amount) is really difficult, and any such numbers should be viewed with caution. One way that some such studies have been done in the past is to use a relatively high rate (say 200 lb N per acre) and then a lower rate, say 160 or 175 lb N per acre, along with the product, and if the yields are about the same, to conclude that product provided the difference.

I’d suggest a wait-and-see approach to products like this. Some companies are asking producers to conduct on-farm trials, and if it’s possible to do a set of paired strips, assigning with and without treatments randomly within each pair, that might provide some information. Split-field trials are a lot less satisfactory, since the two halves of a field never yield exactly the same, and field variability is likely to be greater than any treatment effect. But most companies will control the data from such trials, and in most cases products “win” when such results are put up on websites.

Managing N this spring

One lesson we learned from the 2019 growing season is that we can get nitrogen applied even when conditions are not very good. That doesn’t mean that N was used to its best advantage in every field: there were examples of fields where N was not applied early enough to maximize yield. But with proper attention to applying the right rate at the right time, using a form that will protect against loss, Illinois farmers have the ability and flexibility to get N management done right, even when spring conditions are challenging.

While it’s wet over most of Illinois now, and the weather forecast doesn’t look very promising that it will turn warm and start to dry very soon, we can begin to plan our N management strategy based on principles discussed above. Instead of developing elaborate scenarios of what might happen this spring and how to respond, I’ll list here a number of things to keep in mind as we go forward:

  • Use the N rate calculator as the start to determining how much N to apply. Note the “profitable range” that extends on either side of the MRTN. For most fields the total N rate should be within this range, and results of hundreds of trials over the years in Illinois tell us that we can expect the return to N (increase in yield and gross income minus N cost) to be maximized at the MRTN.
  • While we have said in the past that we might consider moving N rates out of (above) the range given by the calculator, we have found a consistent advantage to doing only when it’s been very wet in June. Root damage from too much soil water and/or loss of N may in such cases mean that the crop can benefit from additional N, but only soils dry some to improve root function, and if N can be applied by or before the time of pollination.
  • In southern Illinois, apply rates within the MRTN range, and wait until V5 or V6 to decide whether yield potential is above 190 to 200 bushels per acre; if it is, consider adding some N later in vegetative growth to bring the total rate up to 1 lb N for each bushel of expected yield.
  • Rainfall from last October 1 through March 23 has ranged from a little below normal to normal in the northern half of Illinois, and from 3 to 6 inches above normal in the southern half of the state. There were a few spikes in temperature and rainfall over the winter, but we don’t think that more fall-applied N has been lost than usual; we can count on its being present for the 2020 crop.
  • If we get a break in the weather that allows ammonia to be applied before late April, we should consider taking advantage of that. Ammonia is currently cheaper, and is safer to apply, than any other form of N. We should take care to avoid applying it in such a way that planter units can drop into the application band, but otherwise the chances for seedling damage from ammonia are low.
  • If wet soils delay both planting and the application of N, it will pay to find a way to get some N (at least 40 to 50 lb N per acre; more may be better if it’s not concentrated close to the row) applied so that it is available to the nodal roots as they start to develop at about stage V2.
  • If cereal rye is present in fields where corn will be planted, try to spray it to kill it several weeks before planting. The large the rye is when killed, the more critical it is to kill it early. If the rye makes substantial (more than 8 inches) of growth before it’s killed, pay additional attention to getting N close to the row at planting in order to replenish when the rye removed from the soil.
  • If you plant corn where there was no crop in 2019 and where weeds were controlled by tillage or herbicide, the 2020 crop might benefit from planter-applied phosphorus in order to prevent “fallow syndrome.” If there’s a flush of spring weed growth, or if MAP or DAP is broadcast this spring, there will be less (or no) need for placing P close to the row.

Nutrient movement off frozen and snow-covered soil

Snow has now fallen throughout much of Illinois, and temperatures have dropped going into the last weeks in 2019. With the recent Illinois Nutrient Loss Reduction Strategy biennial report highlighting P and N levels in Illinois waterways, this is a good time to review the application of nutrients on frozen and/or snow-covered soils.

Last spring, after a short and often-muddy fall fertilizer season, a considerable amount of fertilizer—mostly P in the form of DAP or MAP and K as KCl—was applied during the first week of March when the soil surface was frozen. Between March 3 and March 8, 2019, minimum air temperature averaged less than 15 degrees F, and maximum temperature averaged less than 30 degrees over most of Illinois. This was one of the few times last winter when soils were frozen and there was little or no snow; and many took the opportunity to apply P and K.

Unfortunately, an inch or more of rain fell over most of Illinois south of I-80 followed fertilizer applications on frozen soils in early March 2019. Most of this rain fell during the day on March 9, and as evidenced by the rapid rise in streams and rivers, a great deal of this rainfall ran off the (then-frozen) soil surface.

As part of the ongoing water monitoring that IFCA conducts within their Keep it 4R Crop Program, several samples of water moving out of fields after this rainfall event were collected. In fields where P and K had not been applied recently, average levels of chloride, nitrate-N, ammonium-N, and phosphate-P were 2.3, 0.7, 0.1, and 0.3 ppm, respectively. We expect to see such low values, since these nutrients don’t tend to remain on or near the soil surface during the winter.

Samples from fields where P (MAP) and K were applied on frozen soils in early March had an average of 39.0 ppm chloride, 1.2 ppm nitrate-N, 8.3 ppm ammonium-N, and 19.1 ppm phosphate-P. These fertilizers don’t contain nitrate, so the low nitrate-N number was expected. Both MAP and KCl are highly soluble in water, though, and the elevated chloride (K wasn’t measured), ammonium, and phosphate levels are a clear indication that the rain dissolved fertilizer nutrients and moved them off the field. Samples from a few other fields showed increased sulfate levels, probably coming from ammonium sulfate spread before it rained.

When there is an event like this, higher concentrations come off initially, and concentration declines with time. So we can’t know how much really came off these fields. But as a conservative estimate (because samples were taken some hours after the rainfall began), let’s imagine that the samples had average concentrations, and that an inch of water per acre moved off these fields. An acre-inch of water weighs about 226,600 lb, so if an inch runs off with 19.1 ppm of phosphate-P, about 4.3 pounds of P, or 9.9 lb of P2O5, moved off each acre. KCl is about 47.6% chloride, so 39 ppm chloride would convert to movement of about 9.7 lb K or 11.7 lb K2O per acre.

Another reason for concern about loss of P from events like this is that dissolved P at 20 ppm is 400 times the level (0.05 ppm) in surface waters that many consider to be the point where water starts to become impaired—where P supports the growth of undesirable algae. The major loss from an event like this economic—some of the fertilizer paid for is simply gone from the field. Even more nutrients have likely moved off higher elevations to lower elevations, producing a new “variable-rate” application—but one whose pattern is not likely to line up with existing soil nutrient levels.

Even though events like this are relatively rare, movement of nutrients off fields in this case was both predictable (the rain was forecast) and preventable. It’s understandable that retailers want to apply nutrients when the (frozen) soil will support application, and many producers are in favor of doing this before it gets muddy again. As it turned out, there were not many days in the spring of 2019 when soil conditions were dry enough to allow for surface application of P and K. There are, however, few fields in Illinois with P and K levels low enough to have decreased yields had fertilizer application had been delayed to after planting, or even to the fall, after harvest. Delaying application to the fall would mean adjusting the rate to account for removal by another crop, but would have meant little or none of the loss and redistribution that can result from application to frozen soils.

What about soils that are snow-covered but may not be frozen? Soluble nutrients like MAP/DAP and potash can melt their way down to the soil surface and possibly into the soil, but a “slow melt” with little movement of water off the field is an ideal that we seldom see. With nutrients dissolved in the snow, any water moving off the field as snowmelt will carry a lot of nutrients with it. Deeper snow helps wide (floater) tires stay out of the mud, but it also makes it more likely that rain will fall before the snow finishes melting, and when that happens, a lot of water can leave the field, carrying nutrients with it.

Wet grain, test weight, and late corn harvest

According to NASS, 20 percent—some 2 million acres—of the 2019 Illinois corn crop was still in the field on November 17. Following unprecedented delays in planting, the warm weather in September helped move the crop towards maturity, and frost did not come earlier than normal. So most of the corn in Illinois was at or close to maturity by mid-late October, but temperatures have been below to much-below normal over most of the past month, and this has delayed drydown of the crop. Most of the corn still standing in the field is in the northern third of the state.

One concern with late maturity and harvest of corn is low test weight, which in some cases can mean price dockage at the elevator. Test weight is “bulk density”, or the weight of corn grain per unit of volume, expressed as pounds per bushel (one bushel is 1.24 cubic ft.) The “standard” test weight for corn is 56 pounds per bushel, and this weight (not a volume of 1.24 cubic feet) is the standard unit for marketing corn grain. Test weight is easily measured, but is not easily understood: it’s a complex characteristic, affected by kernel shape, kernel density, and the slipperiness of seedcoats, which affects how well kernels slide past one another. These are all affected both by genetics and by how the crop grows, matures, and is dried for storage.

There are two main reasons for low test weight. The first is the premature end to kernel filling that can result from poor growing conditions, disease of leaves or ears, severe drought, or frost that comes early or that occurs before late-planted corn is mature. Starch deposition in kernels starts at the crown of the kernel and moving towards the base, and when the movement of sugars into the kernel stops before kernels are full-sized, the base of the kernel may be shrunken. The starch deposited under poor conditions can also be less densely packed on the endosperm. The result can be kernels that don’t weigh as much as usual and that don’t fit together very well, both of which can lower test weight. Such kernels have less starch and a lower starch-to-seedcoat ratio, but they might have higher protein and oil, since these are deposited before starch deposition ends.

The market typically rewards sound, dense kernels, so shrunken kernels may be docked in price, with the amount of dockage tied to test weight. Test weight acts as a proxy for harder-to-measure things such as starch content or kernel density. Kernels that don’t fill completely may also contains sugars that didn’t get converted to starch, and these can darken during heated-air drying, causing additional dockage due to kernel damage. The last time we saw a substantial amount of this was on 2009, when cool weather prolonged the season, and test weights were as low as the mid-40s in some fields. We don’t believe there was much of this in 2019, but some fields planted very late with normal-maturity hybrids might not have finished filling before freezing.

The other main reason for low test weights is having high grain moisture when test weight is measured. This is complicated—if all other kernel characteristics stay the same as kernels dry, loss of water weight might lower test weight. But loss of water from kernel starch usually causes starch granules to pack together more tightly, which increases kernel density and test weight. Dry kernels tend to slide past one another more easily, so they pack a little better, which can also raise test weight. As kernel moisture drops into the low teens, it might even be possible for kernels to lose test weight as water is lost but nothing else changes much.

We did a small experiment in 2017 to see how test weight responded as kernels dried down. We selected ears from several different trials, with different hybrids and management factors, including planting date, with the goal of starting with grain at about 30% moisture. Three grain samples were collected from each source, and grain was allowed to dry down in the open air in the lab. At the start and then every few days, samples were stirred, grain moisture was taken using a GAC tester, and test weights were taken using an official (funnel) test weight apparatus. Weights of one-pint volumes of grain were taken using an electronic balance. Testing continued for two to three weeks, and ended once grain moisture dropped to the low teens. Changes in test weight as grain moisture dropped are shown for five grain sources in Figure 1 below.

Figure 1. Changes in grain moisture and test weight measured on grain samples air-dried indoors.

While responses of test weight to grain drying varied some among sources of samples, trends were fairly consistent. From about 30% to about 27-28% moisture, test weight tended to drop by a pound or two, presumably as kernel lost moisture (2 percentage points of moisture weighs about 1 pound per bushel) but little else changed. From 25 down to about 15% moisture, test weights tended to rise more or less as a straight line. The increase in test weight as moisture dropped from 25 to 15% ranged from 5.3 to 7.7 pounds per bushel, and averaged 6.1 pounds per bushel. As moisture dropped below 15%, the rate of increase in test weight slowed, but stopped as moisture dropped to 10 to 11%.

I don’t know if corn grain delivered at, say, 28% moisture with a test weight (measured on wet grain) of 53 pounds per bushel gets docked in price, but if the grain is bright and sound, there is likely to be flexibility in assigning dockage amounts, especially if high-test-weight, dried grain is being delivered as well. Our results show that drying the grain (very gently, without heated air or mechanical disturbance) to 15% moisture should increase test weight of such grain by at least 5 pounds per bushel. Mechanical handling, such as moving grain through augers, rubs kernels together, which might increase test weight a little more.

To put this into perspective, a 56,000-lb truckload (1,000 “wet” bushels) of corn that gets dried from 25% down to 15% moisture loses 1-75/85 = 11.8% of its weight as water, so ends up as 882 bushels of “dry” corn. There is also some mechanical weight loss during drying—for example the “wings” and dust that scatter from driers—but that’s typically only a fraction of a percent, and we’ll ignore it here. If the test weight at 25% moisture is 53 lb/bu, that amount of grain would occupy 1,310 cubic feet, or 1,056 bushels, of bin space. If after drying the test weight is 57 lb/bushel, this grain would occupy only 1,075 cubic feet, or 867 bushels of bin volume. The fact that weight drops by 11.8% while the grain volume drops by 18% shows that about two-thirds of the decrease in volume is due to weight loss and about one-third is due to increase in test weight.

The price of corn and cost of drying may cause some to allow the corn to stay in the field for more days or weeks before they harvest it. If the crop is still standing well after some wind events we had over the past six weeks, and if ears remain firmly attached, then there may be little danger that harvest losses will increase very much or very quickly. Temperatures are expected to be somewhat higher over the next week or so than they have been in recent weeks, and this will help. But daytime high temperatures in the 40s are not going to speed up drydown by very much, and it could easily take a week to lose a point of moisture, although sunshine and some wind might increase this rate. We’ve all heard of corn staying out until February or March and being harvested safely with good quality. While that might mean no direct storage costs, we need to count as “storage” costs any grain that ends up on the ground and not in the bin once we do harvest it. It’s not an easy decision, but we need to stay alert and be ready to harvest as soon as that looks less risky than leaving the crop in the field.

A request for 2019 yields

On October 17, 2019, the UI College ACES put out a news release that described an effort to gather yields from a lot of Illinois corn and soybean fields in 2019. We’re doing this because of the unique opportunity we have to try to get a handle on how planting date affected yields in 2019, so we know better what to expect if and when planting is this late again.

Although late planting is nothing new in Illinois, never before have so many acres been planted as late as in 2019. I describe it in the release as a “giant, unplanned, and involuntary experiment conducted by Illinois farmers.” No one wanted or expected this, but with thousands of fields planted late, we can use planting dates and yields—if we get them from enough fields—to estimate how much effect late planting had on yields. We can also see if changing to an earlier-maturing hybrid for late planting increased yield. It takes a large number of fields for this because variability among fields is so great that having only a few dozen yields from, say, the third week of June won’t give us a very sound estimate of yield.

We already have a fair amount of data from planned experiments in which we planted the same hybrid or variety in the same field over several dates. But we have not planted corn past early June or soybeans past mid-June in most of these trials, and so had little to go on when more than half of the corn and some 80% of the soybeans couldn’t be planted until after June 1 this year.

To have a chance to make this work, we need yields along with planting date, hybrid/variety maturity rating, and yield, from a lot of fields, representing a range of planting dates from early (April) to very late (July). Getting ten or more of these from individual producers from their different fields would be great. Getting a hundred or more from a seed dealer, agronomist, or other retailer who work with numbers of farmers would be even better. We’d like to get these by the end of December, or any time before memory of this year’s harvest begins to fade.

One easy way to submit information for this is online, using the anonymous form found here. With only eight things to fill in (including county and crop), we think it will take only a minute or so to do this for a field, once planting and harvest dates are on hand. You can also fill out a form with information from a number of fields on the form found here: You can return this form as an email attachment, as a file or scan, or you can print and send it by U.S. Mail.

We’ve found a few cases in which information already submitted has “variable” as the field yield, sometimes with a low-to-high range. We don’t have a way to use such data. I suggest you leave out fields like this, or put in yield from a yield map (or a written estimate during harvest) from a uniform part of the field that you think represents the field.

Thanks in advance for helping to make this work. We have no funding for this work, but it’s important enough that we’re willing to put in the effort to make it work. We hope never to have a spring as wet as this last one, but if we do, having this information will give us a much better idea of how to deal with it and of what to expect.