"We are building in record crops in South America, and we are washing out some spec length. Nobody can look at a chart objectively and say they look positive," said Jason Ward, an analyst with Northstar Commodity.

At the Chicago Board of Trade as of 12:15 p.m. CST (1815 GMT), March wheat was down 9-1/2 cents at USD4.98-1/4 per bushel. March corn was down 5 cents at USD 3.66-1/2 a bushel and March soybeans were down 11-1/4 cents at USD9.57 a bushel.

Wheat posted the biggest percentage losses of the three, with the lead contract dipping below USD5 for the first time since mid-October. World wheat production is projected at a record high for 2014/15, and the US Agriculture Department this month raised its forecast of global ending stocks.

Also bearish, storms were forecast to bring moisture to the US Plains wheat belt this weekend, while snowfall expected in the Midwest should insulate dormant crops from a cold spell next week, the Commodity Weather Group said.

Worries about global growth hung over the market.

"The Brazilian real and Russian rouble are soft while the Canadian dollar and Mexican peso have fallen to new lows. This reinforces the theme that financial markets are struggling with deflation and slow economic growth in many nations," Helen Pound, a vice president with Wedbush Securities, wrote in a note to clients.

For the month, wheat was on track to fall 15.5 percent, its biggest drop since September 2011. Corn was on pace to fall 7.6 percent and soybeans were down 6.1 percent, the largest respective declines for each product since September.

Crop weather has improved in South America, with scattered showers in Brazil this week easing concerns about drought curtailing soybean yields.

Grains analyst Safras & Mercado lowered its forecast for Brazil's 2014/15 soy crop to 95.0 million tonnes, down from 95.9 million in December but still a record high. The USDA this month raised its Brazilian soy forecast to 95.5 million, from 94 million last month.