DES MOINES, Iowa -- As spring rains soaked the central United States and helped conquer the historic drought, a new problem has sprouted: The fields have turned to mud.The weekly drought monitor report, released Thursday by the National Drought Mitigation Center in Lincoln, Neb., said the heavy rains that also caused some flooding in the last week brought drought relief to the upper Midwest, the western Corn Belt and central portions of the Plains.The dry spell hasn't snapped everywhere, though. It remains solidly in place in parts of California, Colorado, Kansas, New Mexico, Oklahoma and Texas, according to the drought monitor. It intensified across western Texas, southeastern New Mexico and the Oklahoma Panhandle.Where it has rained, farmers may be thankful that the land is no longer parched, but it's too wet to plant in corn country, and freezing weather and lingering snow have ruined the winter wheat crop."Right now, we're wishing it would dry up so we can get in the field," said Jerry Main, 74, who plants corn and soybeans on about 500 acres in the southeast part of Iowa. He's measured more than 9 inches of rain since April 18 -- and farmers in his area prefer to plant corn by May 10 at the latest.Cold is a factor, tooAside from being too wet to plant, it has been too cold for seed to germinate. Main said temperatures dipped to 27 on Tuesday and to 32 on Wednesday, a chill that has been widespread across the Midwest."We need some heat; it's been down in the upper 30s at night," said Darren Walter, 41, who farms near Grand Ridge, Ill. And farmers in southwest Kansas, Oklahoma and Texas have lost a significant portion of their wheat crop because of unusually late freezes and have begun knocking it down to feed it to livestock.But just as better crop technology helped U.S. farmers harvest one of their biggest corn crops ever amid the worst drought in decades, it's likely to save them from a late, wet planting season. There are corn varieties that mature faster, nearly 30 days in some cases, but the shorter the time to maturity, the lower the yield.Kevin Malchine, who farms 2,100 acres in Wisconsin, said he did better than expected last year thanks to drought-resistant corn -- harvesting 80 percent more than in 1988, the last time there was a comparable drought."We took a hit, but it was much better than I would have thought, and that's just due to the genetics of today," he said.Sandy Ludeman's 2,500-acre farm in Tracy, Minn., about 50 miles east of the South Dakota border, is covered with snow. A year ago, he had finished planting corn. This year, he'll be lucky if he can start in two weeks.Ludeman says he'll consider switching from his typical 105-day corn to 95-day corn if planting runs late."I guess I'm not abnormally concerned about it," he said.Declaring an end to drought requires looking at how much moisture an area has received, how much soaked into the ground and the impact on agriculture, said Richard Heim, a climate scientist at the National Climatic Data Center who helps draft the drought monitor."It takes a while for that moisture to percolate down especially if it's been dry for a long time," he said.