CHICAGO: US soybean futures fell for a third straight session on Tuesday on slowing export demand and as forecasts for more rain in South American crop areas tempered concerns about tightening supplies.

Corn and wheat followed soybeans lower, pressured by improved South American crop prospects and a firmer US dollar.

Grain prices have scaled to multi-month or multi-year highs in recent weeks on strong demand, notably from China, and tightening supplies. Dry weather had also threatened Brazilian and Argentine crops that are due to begin replenishing global supplies in the coming months.

"The market had a job to do, and that was ration supplies. For the moment, the perception is that we've accomplished that job," said Ted Seifried, chief ag market strategist at Zaner Group.

"We will continue to be somewhat weak unless there is a dramatic change in South American weather back to a drier trend," he said.

Widespread showers in recent days are expected to benefit Brazilian crops, while rain in Argentina has aided planting.

More rain is expected in central and southern Brazil and northern Argentina in the six-to-10-day window, and in the 11-to-15-day period in center-west and northeast Brazil, according to Commodity Weather Group.

Chicago Board of Trade January soybeans ended down 12-3/4 cents at $11.45-3/4 a bushel. March corn fell 4-1/4 cents to $4.19-3/4 a bushel and CBOT March wheat dropped 7 cents to $5.70-1/2 a bushel.

Traders are turning their attention to the monthly US Department of Agriculture (USDA) world supply and demand report on Thursday, which is expected to show smaller South American crops and tighter corn and soybean supplies.

Wheat futures remain capped by forecasts of bumper production in Australia and Canada, which have tempered talk of tightening global availability after a recent wave of importer purchases.