Chicago investors have been reluctant to stick their necks out too far in early deals ever since, on Monday, seeing an early rally, based on crop tour results, founder.

Caution reigned early on Thursday too, with the knee-jerk reaction to take profits after the rally of the last session, but with some reluctance to take the trend too far.

As an extra reason for investors to tread warily, external markets were a little negative, with shares closing down 1.0% in Tokyo, 1.2% in Seoul and 1.1% in Sydney, amid nerves ahead of the central bank get-together in Jackson Hole, starting on Friday.

The prospect of an Italian bond auction, of E6.5bn, later is also a focus for attention, in taking the temperature of eurozone debt jitters.

Russia meeting

Wheat markets face their own crucial meeting on Friday too, when farm officials in Russia discuss grain supplies, a session seen as potentially heralding measures to curb grain exports, a move which would likely tip neighbouring Ukraine into copycat measures to avoid its silos getting ransacked by importers instead.

"Expect Ukraine to implement some type of tariff structure, if the Russian implement a shift in export policy," Benson Quinn Commodities said.

"It appears Ukraine has exported 1.2m tonnes of wheat during July and August, roughly twice the amount sold during the same period last year."

Australia concerns

And the growing ideas of demand remain alive too, with Taiwan tendering for 55,000 tonnes of US wheat from the US, extending the run of buying interest this week from the likes of Saudi Arabia, Jordan and Tunisia.

"There are good tenders around the world," Mike Mawdsley at Market 1 said.

Furthermore, there is continued fretting about Australia's wheat prospects, following dryness.

"On the wheat front, be advised that the trade will be increasingly sensitive to September rainfall prospects across Australia where private sector 2013 production forecasts are already 2m-3m tonnes below US Department of Agriculture's August forecast," Richard Feltes at RJ O'Brien said.

"The wheat market is already nervous over eroding 2013 wheat production prospects across the European Union and former Soviet Union amid renewed reports of disappointing wheat yields in Germany, Siberia, Kazakhstan and the Urals area."

Rains where they needed

However, there was enough on the negative side of the ledger to keep investors from getting carried away.

Prospects for India's monsoon have improved, with rainfall in the next two weeks set to top averages, according to the state-run Indian Institute of Tropical Meteorology.

Then there is the Hurricane Isaac rain which, while a threat to many standing crops such as corn and cotton and to an extent soybeans, is improving soil moisture ahead of US winter wheat seedings.

Data later

And the market had some nerves too over the US export sales data later, which have taken on increasing importance as a sign of the extent to which rationing has kicked in at higher prices.

For wheat, sales are seen at least matching the previous week's 475,000 tonnes, old crop and new, and potentially hitting 600,000 tonnes.

For corn, 250,000-450,000 tonnes are expected, in line with the 325,500 tonnes last time.

Soybean export sales are pegged at 600,000-800,000 tonnes, compared with 718,700 tonnes the previous week.

Price moves

Furthermore, Friday is the last trading day of the month, a time when investors often tidy up positions and withdraw money from the market, with month beginning associated with cash injections.

Wheat, the leader in the last session, fell, but hardly steeply, down all of 0.3% at $9.03 a bushel for December, but signally remaining above its 20-day moving average, at a little $8.96 a bushel, regained in the last session.

Corn, for which Hurricane Isaac is an extra setback, threatening ear falls and lodging besides slowing harvesting, dropped 0.1% to $8.13 a bushel for December delivery, again staying over its 20-day moving average, at $8.10 a bushel.

November soybeans eased 0.4% from the last session's contract closing high to stand at $17.45 ¾ a bushel.

Blowing on bolls

Volatility was hardly the watchword among soft commodities either, although New York cotton for December added 0.5% to 77.00 cents a pound, as investors injected some premium in case of damage to the US crop from Hurricane Isaac.

Isaac, dropping rains of up to 20 inches, is working its way through many of the main cotton-growing states, at a time when boll-opening renders crops vulnerable to damage.

Raw sugar for October dropped 0.3% to 19.70 cents a pound, extending losses of the last session, as improved hopes for India's monsoon raised hopes for its cane harvest too.