Analysts expect the U.S. Department of Agriculture to cut its forecasts for the country's corn and soybean harvests when it issues new estimates Wednesday, according to a poll by Dow Jones Newswires.

The USDA will release updated crop forecasts in a monthly supply-and-demand report due Wednesday at 8:30 a.m. EDT.

Analysts on average said they expect the agency to lower its forecast for the national average corn yield this year by 2.3%, to 120.6 bushels an acre from 123.4 bushels an acre, due to the country's worst drought in decades, the Dow Jones survey found. Estimates by 19 analysts ranged from yields of 117.6 bushels to 123.4 bushels an acre.

Analysts on average expect the USDA to cut its forecast for total corn production by 3.5%, to 10.403 billion bushels from 10.779 billion.

They also expect the government to pare its projection for domestic corn inventories at the end of the 2012-13 marketing year by 4.9%, to 618 million bushels from the 650 million forecast in its report last month.

This late in the season, the USDA's new corn-yield estimate is likely to be close to the final yield found when farmers have finished harvesting, said John Kleist, an analyst with trading firm ebottrading.com.

However, high corn prices may have reduced demand for the grain more than accounted for in previous USDA forecasts, so the agency may cut its demand forecasts for exports, the ethanol industry and animal-feed usage, resulting in a higher corn-inventories forecast than analysts expect, Mr. Kleist said.

For soybeans, analysts on average expect the USDA to trim its yield forecast by 1.7%, to 35.5 bushels an acre from 36.1 bushels an acre. Estimates in the poll ranged from 32 bushels to 36.7 bushels an acre--including two estimates that the government will actually raise its yield forecast.

Analysts generally expect a lower yield forecast due to the drought, but others said August rains in parts of the Midwest helped some areas to recover potential.

"For my producers in the Midwest, the August rains were a lifesaver," Mr. Kleist said. If the USDA cuts its soybean-yield forecast by half a bushel an acre, "I think we'll gain it back" when the USDA updates its forecasts again next month, he said.

The main wild card for soybean prices is exports to China, as Chinese demand has remained strong despite prices setting records this summer, Mr. Kleist said.

Analysts expect a total soy-production forecast of 2.638 billion bushels, down 2.0% from 2.692 billion last month. They expect the USDA to forecast inventories at the end of the 2012-13 marketing year of 106 million bushels, down 7.8% from 115 million last month.

Wheat traders will watch the USDA's corn forecasts, as the two grains can both be used in animal feed and their prices often influence each other.

Analysts on average expect the government to raise its projection for domestic wheat inventories at the end of the 2012-13 marketing year by 1.6%, to 709 million bushels from a forecast last month of 698 million bushels.