Grain prices also struggled due to a broadly stronger U.S. dollar, as dollar-priced commodities become more expensive to investors holding other currencies when the greenback gains.
The dollar index, which tracks the performance of the greenback against a basket of six other major currencies, was up 0.2% to trade at 82.80, the strongest level since June 4.
Demand for the dollar continued to be underpinned after Fed Chair Ben Bernanke said last Wednesday that the central bank could begin slowing asset purchases by the end of 2013 and wind them down completely by the middle of 2014 if the economy picks up as the central bank expects.
On the Chicago Mercantile Exchange, corn futures for September delivery traded at USD5.8200 a bushel, down 1.6% on the day.
The September contract fell by as much as 2% earlier in the day to hit a session low of USD5.7988 a bushel, the weakest level since June 25, 2012.
Market players continued to focus on U.S. weather conditions and crop prospects.
According to the USDA, 92% of the U.S. corn crop was collected as of last week, below the five-year average of 97% for this time of year. 100% of the corn crop was collected during the same week a year earlier.
Meanwhile, soybeans futures for August delivery traded at USD14.0688 a bushel, down 0.45% on the day. The August contract declined by as much as 0.7% earlier in the session to hit a daily low of USD14.0275 a bushel, the weakest level since May 15.
The USDA said that nearly 85% of the U.S. soy crop was planted as of last week. Nearly 98% of the U.S. soy crop was planted in the same week a year earlier while the five-year average for this time of year is 91%.
The agency also said that 66% of the U.S. soybean crop emerged as of last week, compared to the five-year average of 80% for this time of year and below the 94% recorded in the same week a year earlier.
Elsewhere on the CBOT, wheat for September delivery traded at USD6.9863 a bushel, down 0.9% on the day. The September contract fell by as much as 1% earlier in the day to hit a session low of USD6.9788 a bushel.
According to the USDA, approximately 11% of the U.S. winter wheat crop was harvested as of last week, compared to the five-year average of 25% for this time of year and below the 51% recorded in the same week a year earlier.
Corn is the biggest U.S. crop, followed by soybeans, government figures show. Wheat was fourth, behind hay.