Watching corn emerge is at least as exciting as watching paint dry, and the economic consequences are almost always greater. At few times during the season is the question about the future potential of the corn crop as clearly drawn as it is at emergence: Low stands simply do not yield up to potential, and even complete stands that have uneven plant size early in the season will not produce maximum yields.
It takes about 120 air temperature growing degree-days after planting for corn to emerge. This number is fairly variable, from perhaps 80 to 170 GDDs, depending on how well soil temperature tracks air temperature. Corn that we planted at Urbana on March 24 emerged on April 15, during which period GDDs totaled about 160, depending on how many you counted of the 21 GDDs on April 15 itself. There were zero GDDs recorded on about half of the days from planting to emergence, interrupted by two warm periods, when GDDs accumulated. Clearly, the seed was experiencing ups and downs in temperature, and probably starts and stops to the germination process as well.
Stands from the March 24 planting are good where we used uncoated seed and not very good where we used Intellicoat polymer-coated seed. This was not because of the intended effect on coated seed: For some reason our resident ground squirrels (and possibly birds) much preferred the coated seed to the uncoated, and they ate more than half of the coated seeds before emergence. We think that they identify seed by smell (there is only one small hole opened in the soil to each seed) and that perhaps they can smell the polymer coating. In any case, this is a problem at the research center that would be much less likely to occur in production fields, except where there are rodent problems in some fields.
Corn planting is well under way, with the official report showing that 21% of the state's corn crop had been planted by April 20. Planting conditions continue to be very good in most areas, though cool soil temperatures still are holding back the rush in the northern part of the state. This week will see another jump in progress, though, and we might well reach the 50% by May 1 that has marked seasons with early-corn planting in recent years.
Although good soil conditions at planting usually minimize emergence problems, such problems can still crop up. Herbicides that can interrupt seedling growth and result in plants leafing out underground are not as commonly used at high rates as they once were, and we do not see this problem as often as we once did. Cloddy soils will also let light penetrate into the soil, and once light hits the tip of the growing seedling, as it does normally just after emergence, the tip of the coleoptile stops growing and the leaf inside it breaks out. If this happens before emergence, such leaves usually fail to emerge. Soil crusting can also lead to eventual leafing out underground. Some seed treatments have been implicated as doing this, but there isn't much evidence to support that. People who test seed in the lab also report that some lots produce "abnormal" seedlings, sometimes with twisting or splitting of the coleoptile. That seems rare in the field, at least when emergence conditions are good and emergence is rapid.
Regardless of the cause, plants that leaf out underground, or otherwise have the emergence process seriously retarded, often do not survive. If they do survive, they may be so far behind their healthy neighbors that they fail to compete for water and light, and hence produce little or no yield. Ideally, corn plants in a field will all emerge within about 48 to 72 hours. If something delays emergence of some plants, especially in comparison to the plants next to them, they will start behind and may never catch up. In other words, if there are emergence delays, it's much better if the whole field or large parts of the field are delayed uniformly, compared to uneven size of adjoining plants.--Emerson Nafziger