Corn and sugar proved the winners of a difficult day for farm commodity investors, as the writhings of the dollar alternated ascendancy between bears and bulls.

It took broader issues - including Thai unrest - to keep commodities higher.

It was advantage long investors early on, as tensions over eurozone debts and economic growth eased, sending the dollar lower and raising the attractions of assets denominated in the currency, including a host of commodities.

Crude oil topped $72.50 a barrel at one point, a cheery sign for many commodities, but in particular crops such as corn used in making biofuels.

Short-selling curbs 

However, you can't keep a bad thing down.

As in all the best horror scenarios, eurozone jitters sprang from their deathbed to revive the dollar later on.

A ban by German authorities on some short selling activities accelerated the demise, being viewed as showing a lack of confidence.

Indeed, the euro had fallen 1% against the dollar by late trading in Chicago, with the greenback gaining more than 0.5% against a basket of currencies.

Kansas rain 

The decline sorted out the men commodities, in a bullish sense, from the boys.

Some Chicago crops received some support from worsening weather forecasts.

"The weather forecast is bullish near term because the current system that is poised to move through the Plains and Midwest has become much wetter," Darrel Holaday at Country Futures, said.

While poor for sowing progress, such news was mixed for wheat, with some 1-2 inches of rain set to fall in the hard red winter wheat states, such as Kansas, which have been short of a drink.

July wheat eased 2 cents to $4.88 a bushel.

Chinese corn update 

However, corn had the extra impetus of strong demand in China, whose return to importing the grain from the US in the face of soaring domestic prices has been the talk of Chicago.

A weekly Chinese corn auction, from state stockpiles, sold out, while July corn jumped 1.4% on China's Dalian exchange.

"Chinese corn values continue to rise on tightening supplies," US Commodities said.

Estimates cut 

Furthermore, US crop progress data released late on Monday fell short of market forecasts, both on sowings but, with plantings nearly complete, more importantly on crop condition, which came in below average levels despite what had seemed near-ideal weather.

"The first crop ratings of the year on corn were under what the trade had estimated," US Commodities.

"After the crop ratings some analysts have reduced optimum corn yields by 1-3 bushels per acre, and soybeans by 0.5-1 bushels per acre."

The picture was enough to help July corn add 1.1% to $3.59 � a bushel.

Soybeans, however, failed to cling on to strong early gains, ending 1.5 cents lower at $9.39 � a bushel, a fresh one-month low for a near-term contract.

Paris vs London 

The somewhat confused picture spread across the Atlantic to Europe, where the weakening of the euro came a little too late to prevent Paris wheat ending E0.25 lower at E135.00 a tonne, for August delivery.

This was even after Algeria confirmed 500,000 tonnes of wheat for June-July shipment, stating it was "exploiting the weak euro", according to Glencore's UK grain arm.

London wheat for July, meanwhile, adding �1.60 to �105.00 a tonne," with slow first-hand selling helping to keep the market firm", Glencore said.

Thai buy

However, sugar investors were in no doubt about their bullish intentions, driving New York's July contract up 6.6% to 14.80 cents a pound, the best finish for two weeks.

The knock-on effects of Thai unrest helped persuade investors to stay on even after the dollar faded.

Thailand is the world's second-ranked sugar exporter, and some 3,000 tonnes of white sugar shipments have been delayed because trucks got held up in road blocks manned by anti-government protestors, according to reports.

London white sugar for August ended up 2.8% at $481.10 a tonne.