CHICAGO, June 23 (Reuters) - Chicago Board of Trade corn futures turned lower for a third straight session on Tuesday, as weaker crude oil prices and a stronger dollar continued to weigh on prices, market analysts said.

• Corn futures often follow crude oil, as it is commonly used as feedstock for biofuel. O/R

• CBOT July corn CN26 settled 1-3/4 cents lower at $4.09-3/4 a bushel.

• Still, fresh news of export demand helped keep a floor under corn prices, traders said.

• On Tuesday, the U.S. Department of Agriculture said in its daily reporting system that exporters sold 100,000 metric tons of U.S. corn to Mexico in a mix of 30,000 tons of old-crop corn and 70,000 tons of new-crop corn.

• Market analysts said that some of Tuesday's moves also were reflective of speculative selling and investors adjusting to reset their positions ahead of first notice day, as traders push to exit expiring futures contracts and roll their positions forward.

• Late on Monday, the USDA left its good-to-excellent ratings of U.S. corn crops at 68%, unchanged from last week and in line with trade expectations.

• Abundant rainfall and moderate temperatures in the U.S. Midwest continue to weigh on corn prices, though traders say wet conditions and excess rainfall may start to impede growth.

• Mike Zuzolo, president of Global Commodity Analytics, said the grain and oilseed markets are caught between a potentially bullish weather story and a potentially bearish macroeconomic story. Grain traders are weighing the possibility of weather-related production risks from a strong El Niño against headwinds from a stronger U.S. dollar and expectations of interest rate increases surrounding the new Federal Reserve chairman.