Soybean and corn fields across central Illinois look headed for impressive harvests in the coming weeks, but the crops could yield slightly less than the record-large hauls seen in the last few years, scouts on an annual crop tour said on Wednesday.

Big yield gains are needed in Illinois and in Indiana for the U.S. Department of Agriculture (USDA) to realize its forecast for the largest U.S. corn and soy crops in history, topping last year's record soy harvest and 2014's record corn crop.

Scouts surveyed Indiana's crops on Tuesday and were in Illinois on Wednesday, as part of the four-day Pro Farmer Midwest Crop Tour for the USDA.

"It's good, solid corn here from what I've seen so far, but it's just not as uniformly good as it was in 2014," said Brian Grete, leader of the tour's eastern leg and editor of the Pro Farmer newsletter. "In 2014, Illinois corn crop was the best individual state crop I've ever seen."

After 11 stops in Illinois on two routes through Iroquois, Ford, Livingston, McLean, Tazewell, Peoria, Knox, Warren and Mercer counties, corn yields were estimated at 183.8 bushels per acre (bpa). That was up from 171.5 bpa last year and the three-year tour average of 179.6 bpa, but below the USDA forecast of 200 bpa.

Illinois soybean crop potential was strong, with high pod counts and good soil moisture, scouts said.

The number of soybean pods in a 3-by-3-foot square averaged 1,458.2 pods along the two routes, up from 1,205.7 pods last year and the three-year tour average in these areas of 1,221.6 pods.

The tour does not project soybean yields but instead calculates the number of pods per square yard to gauge yield potential.

USDA predicted the Illinois crop would yield 57 bushels per acre, up from 56 bpa last year.

Fields were muddy following rains early on Wednesday. The precipitation could provide a finishing touch to crops, filling corn kernels and soybean pods ahead of harvest.

However, there were some doubts that yields would be as good in far southern Illinois, a less-fertile area of the state which the tour does not survey. Growing conditions there were more ideal in the state's record-crop years, Grete said.

The tour's eastern leg began in Columbus, Ohio, on Monday and the western leg in Sioux Falls, South Dakota. The groups converge in Rochester, Minnesota, on Thursday, and Pro Farmer will issue its U.S. crop forecasts on Friday.