Planting date for corn and soybeans in Illinois

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.

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.