It’s crunch time for corn.

Grain traders are anxiously waiting USDA’s weekly crop progress report late Monday as it will tell the story of how U.S. corn was planted this season.

As of May 24, American farmers still had 15 million corn acres of a projected 85 million yet to plant as heavy spring rains in the eastern Corn Belt delayed planting by at least 10 days to two weeks in many areas. Midwest acres seeded after mid-May usually lose more than a bushel a day on yield.

“Analysts are wondering how many of the remaining 15 million acres will be planted to soybeans rather than corn,” Mike Woolverton, Kansas State University grain economist, said in his weekly newsletter. 

“The consensus guess is one million acres if it immediately stops raining in the eastern Corn Belt and Mid-south; up to two million acres if it stays wet,” he said. 

Midwest farmers are also bumping up against planting deadlines in their crop insurance plans. In Illinois — the No. 2 corn producer following Iowa and among the eastern Corn Belt states struggling the most — farmers have until June 5 to decide whether or not they are going to plant corn or cash-in on their insurance. 

Soybeans are also lagging. 

There were some 13 million acres of U.S. soybeans that should be in the ground that were not as of May 24, analysts said. The biggest concern was the amount of unseeded acres in the southern Midwest — an area that typically produces the first beans of season to relieve old-crop tightness. The good news is that soybeans are a quicker maturing crop so farmers still have some breathing room. Insurance plant dates range from June 15 to June 20 in the Midwest.

But the wettest region of U.S. crop belt is surely the Northern Plains, an area plagued by spring flooding and cool temperatures. North Dakota is particularly wet, which is also the top producer of the high-protein wheat that millers and exporters will pay high premiums for.

“The underlying threat to production from loss of acreage is real. The market is roughly penciling in a half million to three-quarter million acres of spring wheat in the U.S. as lost,” said Tim Emslie, analyst for Country Hedging in Minneapolis. 

The bottom line is that these late planting dates will mean tighter supplies of U.S. crops in the coming year, a worry as the United States is the largest food exporter.

Traders expect corn seeding at 93 percent, versus the 5-year average of 98 percent by June 1, and soybeans at 65-75 percent, compared to typical pace of roughly 80 percent. Monday will also be the government’s first word on the condition ratings of the just-seeded 2009 corn crop.