The most active contract for December corn fell 6.75 cents weekly, or 1.8 percent, to 3.67 dollars per bushel. December wheat delivery went down 2.5 cents, or 0.48 percent weekly, to 5.1475 dollars per bushel. November soybeans suffered a 10.75-cent decline, or 1.24 percent, to 8.5675 dollars per bushel.
Long lasting rainfalls in the U.S. Midwest had led to harvest delays, which in turn supported the prices of corn and soybeans in the past two weeks. However, as dry weather gradually came back and prevailed in the area, pressure started to build up upon their futures.
On Thursday, The U.S. Department of Agriculture (USDA) released its weekly export sales report for the period of Oct. 5-11, 2018, showing corn and soybean exports were far less than trade estimates, with corn sales hitting an early marketing year low.
U.S. exporters reported sales of 382,500 metric tons of corn, compared with the trade's expectations of between 800,000-1,250,000 metric tons.
Soybean sales only reached 293,600 metric tons, versus the trade's expectations of between 600,000-1,000,000 metric tons.
Oliver Sloup, with Blueline Future, described the export sales as "absolutely pathetic" for both corn and soybeans.
The bearish U.S. ethanol data gave additional pressure on corn, its raw material.
Official data pegged the ethanol production through the week ending Oct. 12 at 297 million gallons, down 9 million from previous week. Meanwhile, ethanol inventories totaled a near-record 1,014 million gallons, up 5 million on the prior week.
As for CBOT soybeans, the futures rallied on Monday and spent the rest of the week correcting the early gains, said analysts with AgResource, a Chicago-based agricultural research firm.
Harvest delays and even losses and crop quality concerns triggered funds short-covering for the initial rally. But disappointing export sales dragged down the prices.
Export sale cancellations to unknown destinations were noted in the USDA weekly sales report, followed by additional cancellations at the end of the week. Cancellations to China are not a surprise given the 25 percent tariff and more can be expected in the year ahead, said market watchers.
CBOT wheat futures saw a dull week, while the world wheat trade simply lacked enthusiasm. USDA's weekly export sales report showed that a total of 476,000 metric tons of U.S. wheat was exported, fulfilling the trade's expectations of between 250,000-600,000 metric tons.