One farmer who reported in was down to about 350 acres of all crops left to plant before the end of April. Obviously he can read the thermometer. He knows it's cool but he's hedging that it's better to get the crop in while soils were in good condition than to wait on temperatures to rise.

Bob Nielsen, Purdue University Extension corn specialist, noted in the third edition of Purdue's Weekly Pest and Crop Newsletter that is obviously one bet many farmers may opt for. He points out, however, the reality of how slow corn could be to emerge when planted into cool soils.

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Slow journey: It will take corn longer to reach this stage if planted in late April this year than in more normal springs, including last year.

Corn needs 115 to 120 growing degree days to emerge after planting, he says. Growing degree days are a measure of how much warmth there is for growth over a given time period. Some companies talk about maturity of corn hybrids in growing degree days, or at least know how many GDDs it takes for certain hybrids to reach physiological maturity.

Here's the math. Under normal temperatures in the early days of May, it would take six to seven days to accumulate that number of GDDS and for corn to emerge, Nielsen says. However, this year GDDs have accumulated at a much slower rate so far – GDDs are based on air temperature, not soil temperature.

In central and northern Indiana just a few days ago, GDDS were accumulating from zero to five per day. If that cool weather stretch should continue, based on the math alone of how many GDDs it takes for corn to germinate and emerge after planting, it could take three weeks or more for corn to emerge.

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That doesn't mean you won't get a good stand, but it does mean that seeds and seedlings will be exposed to possible insect and disease pests below ground for a longer time than normal. Scouting could become extra important this year.