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Planting-time Considerations

Emerson Nafziger

Department of Crop Sciences
University of Illinois

March 23, 2021
Recommended citation format: Nafziger, E. "Planting-time Considerations." Department of Crop Sciences, University of Illinois, March 23, 2021. Permalink

March rainfall in Illinois has ranged from as much as an inch below normal in the northern third of the state, to close to normal in central Illinois, to as much as 2 to 3 inches above normal in the southern third of Illinois. This, along with some rain this week, has dampened the hope for a March start to planting in most areas. That gives us time to think about issues related to planting and early-season management.

Is it getting late to start planting, especially for soybean?

March is a cool and wet month on average, and it’s rare that much planting gets done in March. One exception was 2012, when March temperatures were record-high and planting began in mid-March. Little rain fell for months after that, so planting early didn’t help yields very much. In fact, later-planted soybeans were able to respond to late rains in some places, and yielded more than early-planted soybeans. Frost in April 2012 damaged March-planted corn, and some of it had to be replanted. The idea that soybeans yield most when planted in March has captured a lot of attention in the past few years, but there is still no solid evidence that March-planted soybeans will yield consistently more than those planted in April. There is more risk planting in March than in April, and little if any expected reward to offset that risk.

Should we start planting soybean or corn first?

The forecast is for warmer and a little drier conditions moving into April. If that holds, it makes little difference which crop gets planted first—the decision can simply be based on which fields are ready first. If it stays warm without heavy rainfall after planting, both crops will get off to a good start and benefit equally. Prolonged cool temperatures following very early planting of both corn and soybeans can limit yield potential, although this is rare. If the forecast changes to cooler and wetter, neither crop will have much advantage: corn emerges a little better than soybeans under such conditions, but this is offset by the need to have a higher percentage of corn seeds emerge. Planting date responses measured as percentage of maximum yield are similar for both crops, and, as we have seen in recent years, even late-planted crops can yield well if the growing season is favorable. Even though planting early helps crops get off to a good start if the weather cooperates, planting date has less influence on yields than the weather during the warm months of the growing season.

How do we know when to replant?

By far the biggest threat to stand establishment in both crops is heavy rainfall after planting that prevents seeds from getting oxygen. The risk of this is not closely related to planting time—the likelihood is about the same in mid-May as it is in mid-April. Cool soils have more oxygen and may allow seeds to survive longer, but also tend to worsen seedling diseases. Warm soils mean more demand for oxygen, and may hasten death of seeds or seedlings in saturated soil. Full crop insurance coverage begins at later dates for soybean than for corn, but replanting soybean usually does not require killing off the emerged stand like it might for corn, and replant seed policy may be more generous for soybean, so replanting costs may be less for soybean. In theory, deciding when to replant is as simple as counting plants and replanting if numbers are below a certain level. In practice, many fields get replanted if it’s still relatively early, plants are unevenly distributed, and plants look like they may not fully recover. If soybean stands end up as uniformly-spaced, intact plants with more than 2 plants per square foot, replanting may not increase yield very much. In the more likely case that stands are poor in some places and not others, supplementing the existing stand with a partial seed rate might be appropriate. Corn stand problems from wet soils might require replanting low spots only, but if a lot of stand is missing in most areas of the field, a full replant following destruction of the existing stand may be necessary. Frost after emergence might kill enough plants to require replanting, but plants with leaf damage only tend to recover to produce full yields.

Does automated planting-depth or down-pressure adjustment improve stands and yields?

For fields that have produced excellent stands and high yields without these features, probably not. In Illinois, it is rare that loam-textured or heavier soils are dry enough at planting to limit germination of corn or soybean seed planted at normal depths (1.25 to 2 inches). Planting deep enough to place seeds into sensed moisture—a feature of some systems—sounds promising, but the advantage of doing this compared to uniform depth placement is not clear, especially if the topsoil has moisture, which is often the case. Most planting depth studies show shown that planting 3 inches deep lowers yield, and if an automated system can and does place seed that deep, it could cause harm. Non-uniform emergence is a problem mostly when it’s plant-to-plant down the row; if a part of the field with light soil has to wait for rain to emerge, that’s less of a problem because plants still emerge at the same time as their neighbors. The ability to change planting depth on-the-go based on things such as soil type or conditions has some appeal for variable fields, but will often mean having to guess about what might happen after planting more than knowing what current soil conditions are.

Down-pressure adjustment capability assists in getting proper planting depth, and also in controlling how much “row compaction” there is. It may have value for no-till planting into dry soils, but perhaps less value for planting into tilled seedbeds. The most important planting objectives include getting seeds placed at proper depth, reasonable spacing uniformity, and good seed-soil contact to help water move into the seed for germination. Most modern, well-maintained planters do this very well in most soils, without add-ons.

Are late-emerging plants weeds?

Early planting with uniform stands tend to produce the highest yields, and having all plants emerge at the same time is a worthwhile goal. But calling late-emerging (or late-developing) plants “weeds” may be an oversimplification: a late-emerging plant that yields some is better than having no plant there at all. A number of studies have shown that the more leaf stages behind its neighbors a plant is, the lower its yield. It’s possible in such cases that whatever caused the delay in emergence or in development—uniform emergence does not guarantee uniform plants—continued to affect the plant long after emergence. So what looks like delayed emergence might be something like disease or insect injury that continues to affect the plant. At the same time, a plant that lags behind its neighbors early tends to fall farther behind as its neighbors “steal” its sunlight, water, and nutrients. Soybeans show a lot of plant-to-plant variability, and they show more stand loss at higher than at lower emerged stands. It’s difficult to know the extent to which these might cost yield. They are used to support the idea that lowering seeding rates might increase soybean yields, but we have not found that to be the case in our seeding rate studies, where optimum plant stands range widely, but high populations usually have little effect on yield.

Have optimum seeding rates changed?

Corn plant populations have not continued to increase as much as many of us once thought they would. In large part that’s hybrids have added more yield per plant, without needing more plants. In 34,000 to 38,000 plants per acre often maximizes yield. Planting 42 to 45 thousand seeds usually does not lower yields compared to planting 36 to 40 thousand, but the added cost of seed is not covered by the average increase in yield. Variable-rate seeding, with more plants in more productive areas of a field, makes sense, but it needs to be done without lowering the population in any part of the field to less than uniform planting would use. Returns from VR seeding will normally be modest, in fields with normal uniformity.

Soybean seeding rates have changed in recent decades, to lower rather than higher rates. That has decreased the number of fields planted at 175 to 200 thousand seeds, which had helped assure enough plants when seed emergence was less than it is now, and with less-capable planting equipment. Aiming for final stands of 110 to 120 thousand, using seeding rates adjusted for conditions and equipment, is a sound approach. It is difficult to find data to support variable-rate seeding for soybeans, and we do not find much relationship, either positive or negative, between yield and optimum seeding rate. Rather than productivity level, it might make sense to vary soybean seeding rate based on field conditions, with the goal on getting adequate stands in all parts of the field.

Cover crop management

Letting cereal rye cover crop grow to take up more N before termination conflicts with getting soybeans planted early. It may work to leave the cereal rye grow for a while after planting soybeans green, but that can be tricky—cereal rye roots actively take up water and nutrients, and while they can help dry out wet soils, they can also take up water and possibly N that soybean plants may need for early growth. Despite claims that corn can also be planted green into cereal rye at or before the time the rye is terminated, doing this can result in reduced early growth of corn plants and lower yields. Part of this effect is from competition for N, and applying N near the row with the planter can help alleviate this, but may not eliminate it. It also helps to have corn seeds in a zone without active rye roots, either by killing the rye some weeks before planting, or by strip-till to move the roots out of the planting strip. The safest practice is to kill rye far enough ahead of planting so there is no green tissue by the time corn is planted.


Starter fertilizer with N, N + P, or N + S can help assure access to nutrients that corn plants need as they start to grow. This is helpful especially if planting is into cool soil, where N mineralization is slow. Any of the different ways to apply N with the planter can work. Placement into the seed furrow directly with no more than 4 to 5 gallons of UAN solution in normally safe. More can be applied if it’s away from the seed, but applying 15 to 30 lb N (5 to 10 gallons of 28-0-0) with the planter should be enough. We don’t know if starter fertilizer helps soybean plants get off to a better start, but it probably does not do so in most soils in Illinois. We have seen some large yield increases for broadcasting stabilized urea on the surface at planting in light-textured soils, and in these cases we also saw a visible response to N. Anyone with soybeans planted in such soil (loam or sandy loam) might want to spread some urea (at about 2 pounds of urea per 1,000 square feet) in a small area after planting to see if it produces a visible response.

Novel products

A large number of new products have appeared on the market, including ones that contain microbes (mostly bacteria) said to increase plant growth or to fix nitrogen for corn. Some seed already comes with microbial treatments, and the seed label will show what those are. We have little information on how well such products perform, but because they seldom if ever will produce a visible response in the field, doubts arise about some of the claims made for them.


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