U.S. corn futures are expected to start lower Tuesday on better-than expected condition ratings for the crop.

Traders predict corn for December delivery, the most-actively traded contract, will drop 2 cents to 3 cents a bushel at the start of trading at the Chicago Board of Trade. In overnight electronic trading, the contract slipped 1 1/2 cents, or 0.2%, to $6.84 1/4 a bushel.

Prices came under pressure after the U.S. Department of Agriculture, in a weekly report issued after the close of trading Monday, kept its good-to-excellent rating for the overall corn crop unchanged from last week. The steady outlook was a surprise as prices had climbed Monday in anticipation of a decline in ratings.

Yet, the market should not collapse as declining ratings for individual corn-producing states show the crop has deteriorated in key growing areas. The good-to-excellent rating dropped six percentage points from last week in Illinois to 53% and five percentage points in Iowa to 75%.

"Monday's crop-progress report presented a curious picture: While nationwide ratings were unchanged, state-by-state totals told a different story," said Bryce Knorr, analyst for Farm Futures, an agricultural publication.

Traders are keeping a close eye on condition ratings amid uncertainty about the impact of hot weather on the crop last month. Traders are waiting for the USDA to update its output forecasts next week to see whether the government indicates the high temperatures caused significant damage.

The unchanged condition rating Monday could decrease the odds of the USDA lowering its yield forecast in the Aug. 11 crop report, according to MF Global. The company said traders were "dialing in lower corn yields than USDA's July forecast, but crop conditions this week and next will have significant bearing on whether the estimate is changed from last month."

Some analysts are already projecting the USDA will lower its outlook. Susquehanna International Group, a trading firm, expects the government to trim its yield forecast by 1 bushel to 2 bushels an acre from its July estimate of 158.7 bushels an acre.

Traders are waiting for other private firms to issue forecasts, as well. Brokerage firm FCStone is set to issue corn and soy output estimates around 4 p.m. EDT Tuesday, while analytical firm Informa Economics is slated to release estimates later this week.