By April 16, 10 percent of the Illinois corn crop and 4 percent of soybeans had been planted. Rainfall across Illinois is below normal so far in April, with an unprecedented 10 (of 20) days with no rainfall recorded anywhere in the state. Topsoil moisture ranges from slightly above to slightly below normal across Illinois; there are no areas of really wet or of really dry soils. Some rain fell over the last 24 hours, but little rain is forecast for the next week. We expect to see planting continue over the coming days.
April temperatures have shown typical fluctuations, including highs in the 80s and lows in the 30s over the past week alone. Temperatures have been warmer than normal: over the first 10 days of April, GDD accumulations averaged 21 above normal, totaling about 60 in northern Illinois and 100 in southern Illinois. Between April 11 and April 20, GDD accumulation averaged 37 above normal, with 105 in the north, about 120 in central, and 130 in southern Illinois. The forecast is for cooler temperatures over the coming week, and it may be a while before we see a return to GDD accumulations as high as those over the last 10 days.
Temperature drives emergence
Heavy rainfall after planting is the single largest concern when we plant early. It appears that the 2023 crop will largely escape that problem, although a lot of acres remain to be planted and the forecast could change before emergence is complete. Without flooding, temperature becomes the main driver of emergence. Unless there’s frost, the effect of temperature is more on the time it takes plants to emerge than on the percentage of seeds that become established plants.
We normally estimate that about 110-120 GDD needs to accumulate after planting before the crop emerges. If we want to split hairs, we can count the GDDs that accumulate on the day of planting if we plant early in the morning, not count them if we plant late in the day, and count half if we plant in mid-day. We can use that number for both corn and soybeans, although considerable variation has been found depending on conditions at planting—for example, seeds planted into warm soil may emerge in fewer (air) GDDs. Still, GDD accumulations based on soil temperature don’t always predict emergence more accurately than those based on air temperature. That may be because daily fluctuations in temperature, or how quickly temperatures change over the course of a day, may affect the timing of emergence.
If we use 115 GDD as the accumulation required to reach emergence, crops planted by April 8 to 10 should be emerging, or about to emerge. At lower temperatures over the next several days, emergence will take longer; crops planted this week will probably take longer to emerge than those planted before mid-April. Seed lying in cool soil is generally safe as long as the soil isn’t wet as well, so unless the forecast turns out to be inaccurate, stand numbers aren’t threatened at this point. But having temperatures stay warm after planting has often led to high yields, and if we could push a button to make that happen, we’d do so every year.
Uniformity of emergence has captured a lot of attention in recent years, with many opinions offered regarding how uniformly crops need to emerge in order to maximize yield potential. While we all like to see rapid and perfectly uniform emergence, claims about yield loss due to non-uniformity of emergence are not easily proven. In addition, there is no remedy for failure to emerge uniformly—no one credibly suggests destroying and replanting a stand only because it emerged unevenly. The emphasis on uniform emergence is usually applied to corn, in part because we can see individual corn plants so easily, and also because we more often see soybeans emerge less uniformly. Soybean seedlings need to pull their cotyledons above ground for emergence, and that presents more barriers than corn seedlings encounter. Deep planting of both crops can contribute to non-uniformity as well.
One common claim is that plants that fail to emerge within a short time interval—as short as 12 or 24 hours—after the first plants in the field emerge are destined to yield less than early-emerging plants, or even to yield nothing. Some add that a delay of 48 hours or more could turn a late-emerging plant into a ”weed.” In the case of corn, that would mean a plant with no ear or a small ear, that competes enough with its neighbors to cause yield loss greater than if it (the “weed”) were missing altogether. In a study in which we damaged young plants to the point where they produced no ear, the plants on either side yielded the same as when there was no plant in the middle at all. That is, the earless plant was unproductive, but it did not act as a weed to lower the yield of remaining plants. This means that walking fields to cut out late-emerging plants is unlikely to increase yield.
Everyone who walks fields knows that some plants take longer to emerge than others, although this delay can be short if it’s warm and soil conditions are uniform. We can imagine a lot of possible reasons why plants might emerge late: the seed wasn’t oriented the right way; seed ended up placed a little deeper than its neighbors; it got dinged a little by fertilizer or herbicide; an insect or disease attacked it; or it wasn’t a sound or genetically identical seed. Refuge-in-a-bag seed has a mixture of genetics, but these are carefully selected to look and behave the same in the field, and we have no indication that this contributes to non-uniform emergence.
Temperature and uniformity of emergence
While the actual cause of non-uniformity of emergence remains elusive, we know for certain that low soil temperature increases the time difference separating the first plants to emerge from the last ones to emerge. The question to ask is whether a difference of 48 hours in emergence of two plants in cool soil is worse (in terms of yield of the late-emerging plant) than a 12-hour difference when soils are warm? Let’s imagine that the two plants were, for whatever reason, destined to emerge 15 GDDs apart. At the Champaign airport, about 10 GDDs have accumulated per day so far in April; this has ranged from zero on three different dates (April 2, 7, and 17) to 20 GDD on April 5. Gaining the additional 15 GDD would have taken only about a day if the crop was planted on one of the first five days of April, but would have taken 3 or 4 days if the crop was planted between April 6 and April 10. It might help to calm us if, instead of writing off the prospects for plants that emerge two or three days late, we changed the “standard allowable lapse” of emergence, from 24 or 48 hours to 15 or 20 GDDs.
Uniformity of emergence and yield
The other question is whether or not the prediction that a late-emerging plant is doomed to life as a weed or low-yielder is sound. While we like uniform emergence, it’s a standard of perfection that we never reach, no matter how good the seed, equipment, soil conditions, or seed placement. We do know that corn ear size is much more variable than emergence time, and we have never succeeded in tying ear size very closely to emergence time, to space between plants, to leaf orientation, or to anything else that we can measure or see, with possible exceptions for direct damage to individual plants from insect, disease, herbicide, hail, or lodging. Even those types of damage can have less effect than we expect, especially if conditions are good following the damage.
One of the problems in trying to measure emergence timing effects on yield is that whatever factor was responsible for delayed emergence may or may not continue to affect that plant into the season. Whatever the case, plants that emerge modestly later often show no effect of this by the time they reach mid-vegetative stages (V8 or V10). Conversely, we can often find unexpectedly small ears on plants that look perfectly healthy. In a study several years ago, we found that, at normal plant populations, stem diameter at V9 was correlated to grain weight per plant; what we didn’t learn was what caused stem diameter to vary among plants. We found much better uniformity in plant size and per-plant yields at low plant populations, so we know that non-uniformity results from plant-to-plant competition. While planting 20,000 seeds per acre would largely cure the problem of non-uniformity in plant and ear size, it’s not a solution that makes agronomic or economic sense.
We know that individual plants yield more when they have less competition from their neighbors, and that non-productive plants hardly ever become weeds. We certainly know that fields that do not emerge very uniformly can produce very high yields, and despite a lot of attempts, no one has demonstrated convincingly that the mixture of per-plant yields (ear sizes) we see in every field is a certain indicator of lost yield potential. Corn or soybean plants that are crowded together compete hard, and usually end up producing high yield on a per-area, if not per-plant, basis. We wouldn’t want it any other way.