The USDA’s November forecasts of the size of the 2012 U.S. corn and soybean crops were larger than expected, particularly for soybeans.

As a result, the general downtrend in soybean prices since mid-September has accelerated, with January futures now at the lowest level since June 29.

Corn prices have moved into the lower half of the trading range that has been in place since mid-September and December futures are at the lowest level since Sept. 28.

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“So far, prices seem to be following the classic pattern associated with small crops — peaking early in the marketing year and then declining as the year progresses,” said University of Illinois agricultural economist Darrel Good.

According to Good, the futures market reflects expectations that prices will continue to decline, especially into the 2013-14 marketing year.

“The expected rebound in South American soybean production, Argentine corn production, and U.S. corn and soybean production in 2013 all contribute to the expectation of lower prices,” he said. “If those crops are as large as generally expected, prices will be even lower than currently reflected in the futures market.”

Good said the USDA is forecasting record South American production of both crops.

“If planted acreage of corn in the United States in 2013 is at the same level as in 2012 and the U.S. average yield is near a trend value of 162.5 bushels, the crop would total 14.6 billion bushels, about 1.5 billion larger than the record crop and record consumption of the 2009-10 marketing year, Good said.

Similarly, he reported, if soybean acreage is maintained at the 2012 level and the average yield is near the trend value of 43.8 bushels, the 2013 crop would reach 3.34 billion bushels, near the record levels of 2009 and 2010.

A combination of record, or near-record South American and U.S. crops in 2013 would likely push prices down to or below the long-term averages of about $4.75 for corn and $11.00 for soybeans.

“While the expectation for lower corn and soybean prices in 2013 is reasonable based on historical patterns and prospects for large crops, the timing and speed of the return to more ‘normal’ prices will be influenced by a large number of factors,” Good said.