CHICAGO (Dow Jones)--Chicago Board of Trade corn futures are expected to open lower Tuesday following overnight losses amid ongoing expectations of a strong crop.

Corn is called 2 to 4 cents lower. In overnight trade, September corn was down 2 1/2 cents to $3.23 3/4 per bushel and December corn was down 3 1/2 cents to $3.26 1/4.

Outside markets weighed overnight, but are more mixed Tuesday morning, traders said. But the market has little to rally on, as the trade remains fixated on a large, healthy crop.

The U.S. Department of Agriculture said in Monday's crop progress report that 69% of U.S. corn is in good-to-excellent shape, down 1 percentage point from last week. Although within trade expectations, the rating at this point in the season is historically high, and is usually falling more sharply this time of year.

While the crop looks good, bulls have pointed to its late development as a concern.

The USDA said 32% of the crop was in the dent stage, down from 42% last year and the five-year average of 60%.

"All things being equal, record yields are still possible, and historically high yields are almost certain; but one or two nights of frost anytime in the next two weeks shall be troublesome indeed," Dennis Gartman wrote in a Tuesday's Gartman Letter..

But WSI AgTrader said in a forecast there is "no threat in the next two weeks and likely beyond with lack of cold air southern Canada."

There were no deliveries reported against the September futures contract. Analysts say there is ample corn available, but that in the short-term holders of the grain are reluctant to give it up.

The corn bulls' next upside price objective is to push December prices above solid technical resistance at $3.50 a bushel, a technical analyst said. The next downside price objective for the bears is to push and close prices below solid technical support at the August low of $3.11 1/2 a bushel.

First resistance for December corn is seen at Monday's high of $3.32 and then at last week's high of $3.37 1/2, the technical analyst said. First support is seen at $3.25 and then at Monday's low of $3.20 1/4.

In export news, the USDA said Tuesday that Mexico bought 120,000 metric tons of U.S. corn for the 2009-10 marketing year.

In international news, China's government sold 1.92 million metric tons of corn during its weekly auctions Tuesday, the highest volume sold since the auctions started July 21, as drought raised concerns about a fall in output.

The volume represented 77% of the 2.5 million tons it planned to sell. The average price for the corn sold was CNY1,597/ton. Prices ranged from CNY1,565-CNY1,612/ton in four northeastern and northern provinces.

(Zheng Xiaolu contributed to this report.)

-By Ian Berry, Dow Jones Newswires; 312-341-5778; ian.berry@dowjones.com