Growers are generally advised to get corn seed in the ground by mid-May, a bit later in areas further north, to optimize yield potential, agronomists said. Yields can drop by a bushel an acre or more per day if planted later, they said.

Less than one percent of corn has been planted in the top producing states in the Midwest as of last Sunday, which was slightly less than average, according to the US Agriculture Department. Agronomists said wet weather has likely prevented any planting progress since then.

"The rains that we're getting today that are fairly statewide are sure putting a damper on activity for another week probably," said Bob Nielsen, extension agronomist at Purdue University in Indiana.

"But the fact that we don't have much in the ground right now is not necessarily a precursor to gloom," he said, adding that historically, most corn in the key growing areas of the Midwest is planted after April 20.

The optimal planting window in Iowa, the top corn producing state, extends from about April 15 to May 10, according to Roger Elmore, extension agronomist at Iowa State University.

"If things do change in the next day or two and we go back to a normal spring, the majority of corn would still go in before that date. If we've backed up our planting schedule by about a week, we'd still have more than 80 percent of our corn planted by then," he said.

However, saturated soils throughout the Corn Belt and lower than optimal soil temperatures may keep farmers on the sidelines longer than a week, agronomists said.

Driving planting equipment in soggy fields increases soil compaction, which could lead to poor root placement. Farmers also cannot apply some fertilizers if soils are too wet.

"(Soil moisture) is the highest it's been in the past 12 years, maybe longer than that," said Iowa State University agricultural meteorologist Elwynn Taylor.

As of last Sunday, 36 percent of fields in Illinois and Iowa had surplus topsoil moisture, compared with about 20 percent last year, according to USDA. Half of Indiana's fields had surplus topsoil moisture, versus 41 percent last year.

Cold ground can slow germination of freshly planted seeds.

Agronomists reported soil temperatures around the Corn Belt in the low- to mid-40s degrees Fahrenheit following a recent cold snap, below the typical planting target of 50 degrees . It may take a week or more of warm, dry weather to raise those temperatures to more optimal levels, they said.

Wednesday's storm was expected to dump 5 to 10 inches of snow on the northwest Corn Belt, according to Mike Palmerino, forecaster with DTN Meteorlogix weather service. The eastern Midwest could receive up to an inch of precipitation, he said.

Below normal temperatures this week will slow the drying process, although conditions were expected to clear and turn warmer by early next week in the western Midwest. The east will continue to see a rainy pattern, Palmerino said.