U.S. corn and soybean futures eased on Friday on profit-taking as the ongoing harvest of big crops in South America continued to weigh on the market.
Wheat futures also were lower, with concerns about additional supplies flooding the export market pressuring prices.
All three commodities were on track to post gains for the week, but Friday's bearish tone spurred investors to move money to the sidelines.
"Some traders (are) headed to the exits rather than get bruised by more volatile trading into the weekend," Farm Futures senior analyst Bryce Knorr said in a note to clients.
Soybeans notched the biggest declines as the South American export pace was expected to pick up after slowing this week.
At least 11 ships are facing delays in loading soybeans at Brazil's northern ports after rains washed out roads and disrupted the progress of trucks carrying beans from the center-west region, Brazilian officials said on Thursday.
"The export flow may pick up again next week, provided weather conditions have allowed enough road repairs to take place," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia.
At 10:05 a.m. CST (1605 GMT), Chicago Board of Trade May soybean futures were down 5-3/4 cents at $10.31-1/2 a bushel. For the week, soybeans have risen 0.6 percent.
CBOT May corn was down 1/4 cent at $3.79-1/14 a bushel. Corn was on track for a weekly gain of 4.1 percent, its biggest weekly rally since mid-October.
Soybean and corn futures rose earlier this week, supported in part by reports of potential changes to U.S. biofuel policy to boost production. Corn is the primary U.S. feedstock for ethanol and soyoil is used in biodiesel.
"The biofuel rumors in the U.S. have thrown uncertainty and confusion back into the market. They come as U.S. farmers are deciding which spring crops to grow," said David Sheppard, managing director at UK merchant Gleadell.
CBOT May wheat was 2 cents lower at $4.50-3/4 a bushel. Wheat futures have risen 0.3 percent so far this week.
Russia, among the world's largest wheat exporters, is considering exporting part of its 4 million ton state grain stockpile to free up storage space before the new crop arrives, industry sources said.