Corn resumed normal service, ie returning to downward movement, faster than many investors had expected.
There had been some idea that Chicago's December corn contract might at least stabilise after its bounce in the last session, on the US Department of Agriculture unexpected cut by 1.1 bushels per acre, to 154.4 bushels per acre, its forecast for the domestic corn yield this year.
"Chart watchers are looking at the big outside day up in December corn as supportive," Paul Georgy at Allendale said early on in the trading day, as the contract indeed posted further gains.
"However, many are expecting managed money to be sellers on rallies as the 50-day moving average is still above the 5.00-a-bushel area," he added.
And the bears proved on the money as December corn collapsed, setting a fresh near-three-year low.
There was one factor in row crops' favour on Tuesday, with the weather remaining drier than ideal over much of the western Corn Belt.
"Rains across the Midwest are expected to be limited over the next seven days while temperatures are seen running below average," Benson Quinn Commodities said.
While the short range North American Mesoscale model "is developing a thunderstorm cluster Thursday night into Friday morning over south eastern Nebraska and south western Iowa that drops 2.5inches of rain in south west Iowa, this is not supported by any of the other models", David Tolleris at WxRisk.com said.
"I do not think this rain is likely to verify."
'Skews the average'
However, that was seen as more of a support to soybeans, which are going through their sensitive pod-setting phase.
Corn faced selling inspired by some potential factual backing to ideas that the 154.4-bushels-per-acre corn yield forecast that the USDA came out with yesterday will represent a low water mark for 2013-14 estimates.
The USDA used, in making its estimate, a five-year average for corn ear weight, which spreads over a difficult period for farmers, including the drought year of 2012.
"That somewhat skews the five-year average," Darrell Holaday at Country Futures said.
"It does not mean that is the wrong this to do, but it is important that everyone looking at the data understands the method used to get the production numbers."
US Commodities said that, with the five-year average including "three short-crop years, ear weights should move higher" given that this year is looking far better for farmers.
"It is feared that ultimately this adjustment higher in ear weights could push the yield back to 160 bushels per acre, which could add 500,000 bushels back to the production, and push ending stocks back to 2.2bn-2.3bn bushels," the broker said.
And, after all, 2009, when the corn yield set a record 164.7 bushels per acre, was a late planting year with a cool summer, comparable to 2013 so far.
Whatever, investors found a food excuse to ignore Monday's USDA corn yield figure and renew selling, sending Chicago's December contract plunging 3.7% to $4.47 ¼ a bushel, its lowest finish since September 2010.
Although a little of the same think attached to soybeans as well as corn, the dryness during pod-setting and concerns over the vulnerability of the crop to frost remained a support.
The late soybean sowings, coupled with a cold summer and slow development, are seen as leaving the oilseed particularly open to losses to even a frost arriving at an average date in some states, more so than corn.
Of course, "it is still possible for US 2013-14 ending stocks to push back to 300m bushels and prices to drop back to $10-10.50 a bushel as harvest progresses and South America acres grow by 3-5% this fall", US Commodities said.
(Large South American soybean plantings certainly look an increasingly likely possibility.)
However, soybeans for November kept hold of positive territory, adding 0.2% to $12.27 ¾ a bushel.
That took the new crop soybean: corn ratio to an unusually high 2.75: 1, a reflection of the dismal fundamentals (from a bulls' perspective) on the grain.
However, investors proved unwilling to let another spread with corn, that with wheat, gain such elevated levels.
Wheat for December closed down 1.3% at $6.41 ½ a bushel in Chicago, keeping its premium below the sensitive $2-a-bushel mark. The September contract shed 1.1% to $6.28 ¼ a bushel.
Still, there was some reminder of resilient demand around, with Bangladesh buying 200,000 tonnes of wheat from Ukraine to enlarge its reserves.
In Paris, wheat for November added 0.3% to E183.75 a tonne.
However, in London, with UK import data coming in strong for June, spurring ideas of plentiful domestic supplies left over from last year, the November contract dropped 0.5% to £154.50 a tonne, within an ace of a 19-month low.
Among soft commodities, cotton extended its winning run helped by USDA downgrades to estimates for US and world stocks and separate data showing deterioration in the domestic crop.
The December contract added 1.8% to 91.72 cents a pound, its best finish in 17 months.
However, a weaker Brazilian real helped put the brakes on a rally in arabica coffee, which closed down 2.3% at 120.60 cents a pound for September delivery.
Raw sugar, however, resisted real weakness, adding 0.4% to 17.25 cents a pound with the help of technical factors.
"A long-term resistance upside trend channel was tested Friday and last nights close took us into breakout," Thomas Kujawa, co-head of the softs department at Sucden Financial, said.
"Furthermore, there seems to be a developing head and shoulders reversal indicator developing in the price action which a close above 17.50 cents a pound would suggest a test of 18 cents and higher."
On fundamentals, attention "seems to be centred on the potential for bad weather over Brazil, and the potential for damage to cane, and a possible increase in the recent shift away from sugar to alcohol", which was highlighted in results from Sao Martinho.