Hopes for a so-called "Turnaround Tuesday" foundered as funds one again fled the nest in Chicago, encouraged by further strength in the dollar.
Often, strong trends on a Monday in Chicago reverse the next day, according to traders' lore.
But hopes of a rally this Tuesday, to match the strong losses in the last session, evaporated as funds continued to sell down positions.
As US Commodities noted at the opening, the market had failed to react much in Asian trading hours to data showing US corn and soybean harvests still way behind – delays which should encouraged buying.
"The market failed to respond to bullish news overnight," the Iowa broker said. "This is a telling sign."
And the sell off accelerated as trading continued.
Even with an hour to go, funds were reported by Vic Lespinasse, GrainAnalyst.com's Chicago marketwatcher, as sellers of about 3,000 wheat, 5,000 corn and 4,000 beans lots.
If funds needed any encouragement, the dollar gave some, as it headed to two-week highs against the euro. The shift, which cut the competitiveness of US exports such as crops, reflected disappointing US consumer confidence data, which encouraged investors back to the safety of the dollar.
Weather forecasts also continued to show brighter spells next week, which should help the harvest play catch up.
"The outlook for next week looks a bit drier than it did previously," Midwest Market Solutions said.
Mr Lespinasse said approaching the close of Chicago trade: "I have only seen one updated forecast so far and it is dry for next week, which would allow rapid harvesting of corn and beans."
Wheat, the more fund-driven grain at present, led the decline, ending down 4.5% at $5.03 ¼ a bushel for December, wiping out in two days two weeks' gains.
December corn shed 1.9% to $3.70 ¾ a bushel, with soybeans down a relatively modest 1.3% to $9.73 ½ a bushel.
Soybeans faced the extra headwinds of a 2.2% slide, to 2,170 ringgit a tonne, in Kuala Lumpur palm oil, a major rival in the vegetable oil market, and of an estimate by Safras e Mercado, the Brazilian analysis group, that bean planting in the South American farm powerhouse is 20% complete, compared with an average of 11%.
Safras e Mercado has pegged Brazil's 2009-10 soybean harvest at 64.8m tonnes, compared with a Washington forecast of 62m tonnes.
Some of the poor sentiment made its way across the Atlantic, where Paris wheat for November closed down E1.25 at E127.75 a tonne, with the weaker euro at least providing some support.
London wheat ended down £1.50 at £102.00 a tonne.
And the strong dollar was also blamed for another weak close in many soft commodities, including sugar and coffee, which did far worse in New York than London.
Orange juice, once again, dodged the bullet, closing up 2.0 cents at $1.174 a pound on continued concerns about an expected slide in Florida citrus production.