Sellers regained the upper hand in Chicago, driving corn to a fresh 2011 low, as data from broker Intl FCStone only added to the downward pressure spilling over from external markets.

FCStone, overnight, raised its estimate for the US corn yield to 148.7 bushels per acre, from 146.3 bushels per acre last month, crystallising ideas that the harvest may not be as bad as analysts had thought.

The production total was lifted by 200m bushels to 12.553bn bushels. (The US Department of Agriculture has the harvest at 12.497bn bushels on a yield of 148.1 bushels per acre.)

For soybeans, the broker was more generous too than the USDA, which will revisit its forecasts next week in its benchmark monthly crop reports.

The yield estimate was lifted to 42.8 bushels per acre from 41.05 bushels per acre, and output to 3.157bn bushels from 3.030bn bushels. (The USDA has 41.8 bushels per acre for yield, producing a 3.085bn-bushel harvest.)

'Picking up the pace'

That tempered any, potential, bullish thoughts from USDA data showing just 21% of the US corn crop, and 19% of soybeans, had been harvested as of Sunday, compared with averages of 23% and 25% respectively.

The corn figure was well below market expectations too, set at 25-30%, and a reflection of a difficult sowing period.

Indeed, weather remains on the side of growers, who are expected to play catch up.

"Harvest progress is picking up the pace and should more than double next week as the majority of the producers in the main core of the Corn Belt finish up their soybean harvest this week," Jon Michalscheck at Benson Quinn Commodities said.

"Nearly ideal weather the past two-to-three weeks in a large portion of the belt has the corn drying down, and that should give the producer the green light to not waste any time in getting it out of the field."

Quick combines mean harvest pressure on prices, from a spike in supplies, besides reducing the risk of frost if crops are left out in the field.

2008 or 2010?

And this before taking into account external markets which continued to crumble, presenting a difficult backdrop for any attempt to rally in grains.

Barclays Capital summed up the macro situation so: "Both business sentiment and financial markets seem to stand  at a pivotal point, with investors wondering whether the  near-term outlook is for a rebound similar to July 2010 or for a further deterioration in sentiment, taking us into the dangerous spiral of 2008."

Shares continued to fall on Tuesday, with Tokyo's Nikkei index shedding 1.1%, while Brent crude eased 0.2% to $101.50 a barrel.

Ominously, the Vix index, the so-called gauge of fear, closed the last session at 45.45 points, its highest since early August.

One-year low

There were some plus points for crops, such as a purchase by South Korean feedmakers of 120,000 tonnes of corn for arrival in January, indicating demand at lower levels.

"However, global economic worries tied to Greece's unresolved debt issues and seasonal selling could continue to add pressure to corn prices," Lynette Tan at Phillip Futures said.

Chicago corn futures stood 1.1% lower at $5.86 ¼ a bushel for December delivery, as of 07:20 GMT (08:20 UK time), earlier setting a 2011 low for a spot contract of $11.68 ¼ a bushel.

November soybeans were 0.4% down at $11.72 ¾ a bushel, and in Kuala Lumpur oilseed peer palm oil fell 1.3% to 2,807 ringgit a tonne, its lowest for nearly a year, before recovering some ground to 2,840 ringgit a tone, down 0.2%.

Besides the broader market weakness, the vegetable oil is being undermined by seasonal ideas of export weakness, now a period of Asian festivals is passing.

'Limit the aggressiveness'

Back in Chicago, wheat fell 0.4% to $6.17 ¼ a bushel for December, with the prospect of rains for drought-stricken areas of the US South easing fears over a pace of winter wheat sowing which, thanks to the dryness, remained far behind the pace as of Sunday, USDA data showed.

Plantings were 26% completed, compared with an average of 42%. In Ohio, sowings, usually 18% finished by now, have barely started, while in Texas, they are 25% done, half the normal pace.

However, standing in the grain's favour is the amount of selling in futures that has already been done, implying that pressure for further short positions may be limited.

Fund's "large net short fund position does merit some attention and may limit the aggressiveness of the sellers as long as the market remains oversold", Brian Henry at Benson Quinn said.