The relative calm in grain and oilseed markets could be about to end.

The US Department of Agriculture will at 07:30 Chicago time (13:30 UK time) on Thursday unveil its latest monthly Wasde crop report, part of a series which even in mild versions tends to move market.

This one looks set to be a spicy edition, which may not be able to help itself in moving prices.

Especially if the broad range of market expectations for the key numbers – US corn and soybean production – is a guide.

 'Way off beam'

"We have got this vast difference in estimates for corn and soybean production," Dan Cekander at broker Newedge said.

The gap between highest and lowest forecasts for soybean output, at 270m bushels, is equivalent to some 10% of the harvest.

"Somebody is going to be way off beam" when the actual numbers are released, Mr Cekander said.

That means that "even if we see numbers coming right on the consensus guess", investors positioned at the extremes of the range could twist the market by switching holdings to reflect the fresh USDA estimates.

Moving targets

The spread of forecasts - perhaps ironic so late in the harvest, which has answered some of the questions about the size of crops - is down the range of variables which were up for potential revision this time.

"It is not just yield, but you have got planted acres, harvested acres, demand, everything up for grabs," Don Roose, president of US Commodities, said.

And there is the potential for revisions to prove sizeable too, given the advancement of harvesting, which means both corn and soybean crops look set to be in the silo earlier than ever before.

USDA officials are "going to have all their test plots harvested", signalling that the production numbers on Thursday could be nearer to the final figures than normal, Jerry Gidel, chief feed grains analyst at Rice Dairy.

That implies that the USDA may be willing to implement larger changes than its traditional conservative approach might dictate.

'Consistently bigger yields'

Considering what farmers have been saying about their soybean crops, that could mean a big rise in the estimate for US production of the oilseed.

"The yield reports have been coming in consistently bigger than people expected," Mr Roose said.

Furthermore, many investors are expecting a rise in harvested acres of soybeans, given data from growers' returns to the Farm Service Agency, which handles farm support payments, indicating that spring sowings were higher than expected.

Such thoughts have fuelled a drop in soybean futures, which were on Wednesday looking at their lowest close in three months, down some $2.70 a bushel from their record high set in early September.

Power of two

And that has an impact on corn too.

"You get a much strong impact on prices when both corn and soybeans are in trouble," Mr Cekander said.

When supplies of only one are depleted, it can "start siphoning acres off the other", and remove some market tension.

That is part of the reason why corn has dropped more than $1 a bushel from its record high in August.

Another sap is that many investors see corn sowings having beaten earlier expectations too, meaning that even if turns out that farmers were forced to abandon more crop than had been thought, or yields turned out below expectations, the harvest could turn out than the USDA forecast last month.

Lesson from the past

But if it looks like bears hold all the cards, well, they don't.

History is on the side of bulls.

"Be advised that the trade has overestimated October corn production in four of last six years, including 230m-280m bushel over-shots in 2006 and 2010," Richard Feltes at RJ O'Brien said.

"More importantly, the trade overestimated Oct corn production in each of the last two droughty years."

For soybeans, analysts have overestimated  production numbers in October Wasde report, "in five of the last seven years including both of last two stress years in 2010 and 2011".

Only big crops get bigger?

Furthermore, a strong rise in the soybean yield figure - and upgrades of 4 bushels an acre close to 40 bushels an acre have been mentioned – would represent a break from the norm.

"That would a big adjustment. And it is the kind of adjustment you get in good years," Rice Dairy's Mr Gidel said, echoing the Chicago adage that "big crops get bigger" as the season proceeds.

"This is not a good year. And I am not sure Kansas and Missouri are going to add anything, or Illinois that much" to the production forecast.

'Fear of big numbers'

What bulls will also be hoping for is that, even if the USDA does raise its crop production forecasts, the upgrades fall short of the figures that bearish investors have position for.

"It could turn out estimates turn out supportive for prices if there is anything other than a large number," Newedge's Mr Cekander said.

"There is fear of big production numbers."

Fear among producers perhaps, although consumers will be hoping indeed that the late rains proved, once again, that soybeans deserve their nickname of the "miracle crop".