Last week's heat wave across the central and southern United States stressed the developing corn and soybean crops, causing the U.S. Department of Agriculture to cut its ratings for both on Monday.

USDA rated 63 percent of the U.S. corn crop good to excellent, versus 66 percent a week ago. For soybeans, 64 percent of crop was rated good to excellent, down 1 percentage point from a week ago.

Analysts had been expecting a 1 to 2 percent decline in the ratings for both and corn and soybeans and will be focused on corn in the coming week as it is in the midst of its critical pollination growth phase.

But corn and soybean crops are in much better shape than a year ago when the United States, the largest food exporter, suffered its worst drought since the 1930s. Crop yields dropped in 2012 and U.S. farmers hope for bumper crops this year to regain export sales that were lost after the drought.

The big surprise among grain analysts was corn deteriorated east of the Mississippi River as well as west where soils are parched.

"I expected some additional deterioration in the dry areas of the western states but we had a four-point drop in Illinois, a two-point drop in Indiana, and a two-point drop in Iowa. Those were a bit unexpected," said Shawn McCambridge, a senior grains analyst for Jefferies Bache.

Among the states under the most stress was Iowa, the top corn, soy producer. USDA said 55 percent of Iowa corn was in good to excellent condition, compared to 78 percent for Indiana, 65 percent for Illinois, 66 percent for Nebraska.

Top soil moisture in Iowa was rated 57 percent short to very short, up from 35 percent the week before, according to the USDA Iowa field office. It was the third week in a row with below-average rainfall, it said.

"I'm not surprised," said Roger Elmore, Extension agronomist for Iowa State University. "We need substantial rains. The crop is using a third to four-tenths of an inch of water per day - we are mining the subsoil right now."

Rains are needed this week in Iowa along with other top corn states including Nebraska, Kansas and Minnesota as corn pollinates. USDA reported that 57 percent of crop has yet to pollinate as of Sunday.

"Another week of limited precipitation and seasonably hot temperatures stressed dryland crops with irrigation active statewide," USDA's National Agricultural Statistics Service office in Nebraska reported. "Pockets of rain did occur early and late in the week, but no general rainfall was received as crops neared peak moisture demands."

In Kansas, 71 percent of the top soil moisture was short to very short, according to the NASS office in Topeka.

For Illinois, the downgrade in corn as well as soybeans was due to drought stress, said Emerson Nafziger, University of Illinois Extension agronomist.

Nafziger said Illinois corn was in the midst of pollinating but the late planted corn would likely not complete pollination until the first week in August.

"The most stressed crop was planted late and hasn't tasseled yet. The root systems really aren't very good on the late planted corn," he added. "Temperatures are moderating which will help."

In Ohio, warmer weather helped farmers push along on wheat harvest - now nearly even with the five-year pace - as well as give corn a boost. USDA rated 77 percent of the corn crop good to excellent, up from 74 percent a week ago.

"Corn has recovered well after flooding and winds from last week, and condition looks good as some is even beginning to dough. Soybeans are blooming but there are reports that in some fields the crop is looking stressed due to water damage from heavy rains in the previous couple weeks."