Soybean prices plunged the maximum permissible in Chicago, while corn futures fell 4% to a two-month low with wheat matching it for pace, undermined by factors from weather to a Sino-US trade dispute.
"Ag markets are awash in negative news," Richard Feltes at RJ O'Brien said, as soybeans at one point fell the maximum $0.70 a bushel permitted in Chicago, with corn and wheat suffering bigger losses in percentage terms.
For wheat, sellers seized on forecasts for rain for Western Australia, Australia's main wheat-producing state, where dryness has stoked fears for a near-halving in production in the forthcoming harvest.
Furthermore, parts of the southern Plains received much-needed rain ahead of winter wheat sowings.
"A strong wave of showers pushed through the Southern Plains over the weekend producing welcome rainfall in Texas and Oklahoma," Gail Martell at Martell Crop Projections said.
'Open for harvest'
Corn suffered as fair weather over the weekend, in boosting thoughts of immediate supplies, added to concerns springing from a weak technical close to the last session.
"The next seven days is expected to be open for harvest," broker US Commodities said, noting that farmers "have been and are expected to continue to sell grain off the combine".
Crop prices as of 12:15 Chicago time (18:15 UK time)
London wheat (November): £204.50 a tonne, (-1.2%)
Paris wheat (November): E260.25 a tonne, (-2.4%)
Paris rapeseed (November): E507.25 a tonne, (-2.4%)
Chicago wheat (December): $8.90 a bushel, (-3.7%)
Chicago corn (December): $7.52 ½ a bushel, (-3.8%)
Chicago soybeans (November): $16.69 a bushel, (-4.0%). Limit down
The "carry in corn is not sufficient to hold for opportunities" of higher prices, with Chicago contracts up until the start of the July 2013 harvest priced close to the spot December lot.
Meanwhile concerns over aflatoxin, a toxic fungal residue being found in relatively high concentrations in the US crop, "the poor quality of corn and the insurance being set in October are all adding up to selling off the combine in corn".
But soybeans suffered a particular cocktail of downbeat news, besides thought of good harvesting weather meaning ready supplies of the oilseed for now.
Yields for the early US harvest of the oilseed are beating forecasts cut after a surprisingly large downgrade last week by the US Department of Agriculture to its estimate for the soybean yield.
"Yields are better than expected as the harvest gains momentum. This is especially true in the western Corn Belt on soybeans," US Commodities said.
Furthermore, forecasts hardened for rains in central Brazil where moisture is badly needed to get the sowing season off to a good start, after dryness which in some areas has lasted for some 120 days.
'Possible Beijing retaliation'
However, soybean prices also suffered from talk that China is to step up its programme of sales of the oilseed from state stockpiles, with rumours of 3m-4m tonnes to be released at auctions.
The country has until now being offering regular chunks of 400,000 tonnes of soybeans for sale.
And there were fears for a backlash after China filed a complaint over a new US law on tariffs, hours after Washington announced a wide-ranging complaint against Beijing support for car exports.
"Ag markets are feeling headwinds from possible Beijing retaliation," Mr Feltes said.
China is the top importer of crops including cotton, rubber and soybeans, and a major buyer of the likes of palm oil too.
Chicago soybeans for November fell the exchange maximum to $16.69 a bushel at one point, for November delivery, a fall of 4.0%.
Corn for December dropped to $7.47 ½ a bushel at one stage, the weakest for a spot contract since July, and a fall of 4.4%.
Wheat for December dropped to $8.86 a bushel, a decline of 4.1% on the day.