Corn prices plunged 4% in Chicago, where wheat futures tumbled below $6 a bushel to a four-month low.
Oat futures tumbled the daily limit at one point to their lowest since September last year.
"It has been a meltdown in the grains today," Darrell Holaday at broker Country Futures said.
At North America Risk Management Services, Jerry Gidel said: "It is another of those ‘the sky is falling’ days."
The declines reflected weakness in many risk assets, with Wall Street shares trading 1.3% lower in afternoon deals, after a weak Spanish bond auction whetted concerns over the eurozone debt crisis.
Chicago closing prices
Oats: $3.00 a bushel, -5.7%
Corn: $6.14 ½ a bushel, -4.4%
Wheat: $5.92 ½ a bushel, -3.9%
Soybeans: $11.68 ¼ a bushel, -1.6%
Prices for front-month contracts
Spain, which needed a yield of 5.43% to get away 10-year bonds at its last auction, in October, this time required 6.97% – just short of the 7% mark which has come to rings alarm bells on financial markets – and even then sold fewer bonds, by E440m, than the E4bn maximum target.
Prices of many raw materials suffered too, with the CRB commodities index trading 2.5% lower.
However, grains fared particularly badly after a US weekly export report showed sales of wheat sales, at 334,560 tonnes, including a few forward sales for the 2012-13 crop year, short of market forecasts.
Corn export sales, at 208,950 tonnes, were well below even the most pessimistic trade estimate, of at worst a 350,000-tonne figure.
The data touched a raw nerve among investors over the weak performance of US exports so far in 2011-12, when South American and Black Sea rivals have backed by strong crops and low prices, been picking up significant trade that would, historically, have been seen as American.
"The problem is the corn market as [US] exports sales remain extremely slow," Mr Holaday said.
Japan, the top corn importer, is believed to have bought 800,000 tonnes of the grain from Ukraine, the biggest non-US order for a decade.
While soybean exports have picked up, and at 751,200 tonnes in Thursday’s report beat expectations, they are also seen as running behind the curve, sapped by competition from Brazil, whose bumper early-2011 harvest has allowed it to compete on export markets further into the calendar year than it has done previously.
Taiwan booked a cargo of soybeans from Brazil overnight for January delivery," Benson Quinn Commodities noted.
"The window for the US to step up the export pace is closing fast."