Shirt-sleeve weather this week across America's central grain belt tempted farmers to start planting corn early but expensive seeds and worries about insurance covering any sudden cold snap have kept crop planters out of fields.

"We've got a lot invested in this crop. We want to be careful," said central Illinois farmer Tim Seifert, who doesn't want to take the chance of seeing young corn seedlings hurt by an April frost.

"At $400 a bag for seed and the chance that insurance may or may not cover a loss if the crop gets hurt, it's not worth the risk to plant too early," he added.

But Seifert, who farms near Springfield, has been out all week applying nitrogen fertilizer to his future corn fields, taking full advantage of 80-degree F days (27 C), clear skies and dry soils after a snowless winter.

Soil temperatures are ideal for planting corn, 50 degrees and higher across the top corn states of Iowa and Illinois. But he usually does not plant before April 5, when his crop insurance policy activates.

Farmers like to plant corn as early as possible, as yields are generally higher in central Illinois if corn is seeded by mid May. Yields can slip as much as a bushel an acre per day after that. Also, farmers know that the outlook for a tight corn supply before the autumn harvest this year will mean early harvested corn will bring a hefty premium from processors and exporters competing for scarce supplies.

"There is a lot of nitrogen fertilizer going on but hopefully they aren't thinking about planting for a month or so," Roger Elmore, a corn extension agronomist at Iowa State University, said in an interview on Friday.

"Iowa farmers should wait until the week of April 9th in the southern part of the state and April 15th for the northern half to plant. The chance of frost and snow between now and the middle of April are pretty high," said Elmore.

That said, he added, daffodils have already sprouted in Ames, Iowa, where ISU is located.

It is not surprising that the Midwest spring season appears to be coming a month early. It follows one of the warmest winters in U.S. history. Forecasters say that high temperatures will stay in the 70s to 80s through the weekend and into next week.

"Most areas are running 25 to 35 degrees above normal on these warmest days, with this warm trend continuing on through the balance of the month," said Joel Widenor, agricultural meteorologist for the Commodity Weather Group based in Bethesda, Maryland.

"That will mean the central and southern Midwest will see an early start and early emergence of corn. So it will be important that we avoid any cold shots during early to mid April," said Widenor.

Currently, Widenor said, "there is no clear signal of cold threats but it's something to watch."

Illinois farmer Leon Corzine said: "We generally plant on the early side but never this early. If the weather stays like this we'd plant in about 10 days for sure."

Corzine, who farms 100 miles northeast of St. Louis near Assumption, Illinois, and a distant cousin of former MF Global top executive Jon Corzine, was tilling a field on Friday, dealing with an early crop of weeds due to the warm winter. (Reporting by Christine Stebbins; Editing by Peter Bohan and Bob Burgdorfer)