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Planting Corn and Soybeans in 2020



by Emerson Nafziger, ILLINOIS Extension Agronomist

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 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.

Adjusting Nitrogen for this Corn Crop

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Despite the wet weather many think may be causing nitrogen fertilizer to get away from corn plants, it is still far too early to make that decision.

While it seems likely some nitrogen fertilizer has moved out of the upper soil as a result of rainfall this year University of Illinois Agronomist Emerson Nafziger says if soils dry out, the torrential rains stop, the sun shines, and the weather gets warmer things should be all good, “The crop is going to tell us this. If by the middle of June some of the crop has really greened up nicely and some has not, then we might need to think about those that haven’t and determine if enough nitrogen is missing to cause this to take place. My suspicion is we will not see very much of that at all. If we are warm and dry and with sunshine for a week, I think the crop is going to look good in almost every field.”

My suspicion is we will not see very much of that at all. If we are warm and dry and with sunshine for a week, I think the crop is going to look good in almost every field. - Emerson Nafziger

One indication the topsoil hasn’t been stripped clean of nitrogen is the good recovery of green leaf color. Nafziger says, as soils dry out, root systems start to expand and the color will change. He explains the corn crop at this point looks like it does not because of lack of N, but due to cool temperatures and abundant rainfall. While it is premature to revise nitrogen management based on what has happened so far, Nafziger cautions it cannot be ruled out, “I would be very reluctant now to make a decision that we need to go put more nitrogen on, especially if we’ve already put the full amount on. If we still need to side-dress and we add 10, or 15, or 20 pounds I don’t have a problem with that. But I think it is premature to decide so much of the nitrogen is gone that we put out there that we need to go back and plan to put more on at this point.”

The good news is there is still time to make such decisions. The corn crop takes up barely one pound of N per acre for every inch of growth it makes up to about knee-high.

Nitrogen deficiency develops over time, and Nafziger says it is almost always more related to current soil moisture than to the amount nitrogen in the soil. So, if fields aren’t extra wet or extra dry over the next month, this season could still turn out to be much more typical than many now expect.

Yellow Corn Needs Some Heat

Farmer don’t worry too much about a few very young yellow corn plants in their fields. They do get concerned when every plant is yellow. The problem, in this case, isn’t the wet weather says University of Illinois Agronomist Emerson Nafziger.

Corn & Soybean Planting Date Recommendations



by Emerson Nafziger, University of Illinois
see The Bulletin article

Relatively dry weather in recent weeks throughout much of Illinois and an early start to fieldwork might provide the unusual opportunity this year of letting us choose corn and soybean planting dates instead of having to wait until it’s dry enough.

There are reports that some corn and possibly some soybeans were planted as early as February this year. The main motivation for such plantings is often the excitement that comes (or doesn’t) from having the crop survive “against all odds.” While that may be satisfying, it doesn’t offer much profit potential. If the crop survives it hardly ever produces yields as high as those from planting at the normal time, and planting very early affects insurability and can also increase the cost of replant seed.

In the warm, dry March of 2012, we planted one date of our planting date study at Urbana on March 16. The crop emerged uniformly and grew well until frost on April 11–12 killed the tops of the plants to the ground. About 75% of the plants survived and grew back, though, and to our surprise this planting also yielded about 75% as much as the April plantings. Most corn planted in mid-March in 2012 (about 5% of the state’s corn was planted by April 1 that year) had to be replanted.

Most people avoid taking insurance coverage risks by planting before earliest allowable planting dates under the federal crop insurance program. Those dates for corn are April 10, April 5, and April 1 for northern, central, and southern Illinois, and for soybean are April 24, April 20, and April 15 in northern, central, and southern Illinois.

Having the earliest insurable dates for soybean about two weeks later than for corn reflects what until recently we considered to be the greater danger from planting soybeans very early compared to planting corn very early. In fact, with better seed handling and treating today, soybean seed produces acceptable stands with mid-April planting about as often as corn does.

Contrary to what many believe, soybean is no more vulnerable to frost than corn after emergence. The only time we’ve seen soybean seedlings killed by frost is when it gets near freezing at the time the hypocotyl hook is exposed to the cold sky, before the cotyledons are pulled from the soil. This period of vulnerability typically lasts no more than a day or two; after the hypocotyl straightens and the cotyledons open, soybean plants are fairly cold-hardy. While corn plants have been considered safe from frost until the growing point is near the soil surface, we have seen corn plants killed by low temperatures (often below 30 degrees) even if they have only two or three leaves exposed.

The primary cause of stand loss in both crops is having heavy rainfall soon after planting. Stand loss from wet soils before or during germination is greater for corn when soil temperatures are low. For soybean, having warm soil under wet conditions speeds up the germination process and mean that seedlings run out of oxygen before emergence. But chances of having heavy rainfall soon after planting are not higher with early planting, and stand problems due to wet soils are as common with May planting as with April planting.

Between 2007 and 2016, we ran planting date studies for corn at a total of 22 Illinois site-years, and between 2010 and 2016, at a total of 26 site-years for soybean. There were four planting dates in each trial, ranging from early April through late May for corn and mid-April through early June for soybean. Data are expressed as percentage of the yield at the highest-yielding date within each site-year.

As shown in Figure 1, planting date responses expressed as percent of maximum yield within each site-year are surprisingly similar for corn and soybean across recent trials. Both crops showed near-maximum yields when planted in mid-April to early May, and yields dropped to 95, 91, and 86% as planting was delayed to May 10, May 20, and May 30, respectively.
Figure 1. Planting date responses over 22 corn and 26 soybean site-years in Illinois.
What should we take from the fact that yields of both crops declined at about the same percentage rates as planting was delayed through May? The main message is that we need to give similar priority both crops in terms of getting them planted on time. For those with more than one planter, that may mean planting both crops simultaneously, as fields get ready to plant. Our long-held idea of planting corn first them starting to plant soybean requires rethinking and possible adjustment. At the same time, the penalty for late planting of corn is a little lower once we get to late May and into June compared to that for soybean, so in fields that stay wet longer, soybeans may still be a slightly better choice.

We also see from the data in Figure 1 that neither crop is likely to yield more when planted in early April than when planted in mid- or late April. If fields for both crops are ready to plant in central Illinois on April 6, there are two reasons to plant corn first: 1) it’s insurable; and 2) corn seed is somewhat better able to emerge at high percentage when planted early than is soybean seed.

On the other hand, we generally expect about 85% of soybean seeds and 95% of corn seed to establish plants, so corn can be a little more vulnerable to less-than-desired stands if conditions turn bad after planting. In neither crop, however, would dropping desired stands by 5 percentage points cost much yield.

Finally, we should take care not to be overly influenced by what happened in 2016, a season when growers reported much higher yields from early- compared to late-planted soybeans. Statewide, over the past 20 years or so, the average date by which we get 50% of the crop planted is about May 1 for corn and May 22 for soybean. It would be good if we could move both of those dates up some, and even better if we could move the two dates closer together. Still, with years like 2012 when planting was very early but lack of rain lowered yields by a lot, there’s little relationship between average statewide planting date and average statewide yield.

Most planting delays are due to wet soils, and so are more or less beyond our control. Mudding in either crop, especially in April, is usually a mistake, given the slow rate at which yields for both crops fall as planting is delayed into May, and given the prevent-plant provisions of crop insurance in effect. We should be diligent at starting to plant when all (not just soil) conditions are right, but there’s little reason to panic when planting isn’t as early as we’d like