"It will be interesting to see if the buying late in the session that we have seen recently occurs today," Darrell Holaday at broker Country Futures mused, as Chicago futures headed weak into the last hour of trading.

The buying didn't arrive, leaving grains closing at three-week lows, and the benchmark December wheat contract in particular doing something it has not done since June 15.

That is, to fall beneath its 50-day moving average, at a little under \$8.78 a bushel, and indeed stay there, closing down 2.1% at \$8.67 ¾ a bushel, its ninth negative finish in the last 10 sessions.

In Europe, Paris wheat for November lost 1.0% to end at E260.00 a tonne, while London November wheat shed 0.4% to £203.65 a tonne.

Rain gain

Wheat's particular issue is that while Hurricane Isaac proved a mixed blessing for many US crops - flattening Louisiana cotton for instance, but calming enough by the time it reached the Midwest to give Illinois soybeans just helpful moisture – it proved an unequivocal boon for winter wheat.

As Dan Cekander, director of grain marketing analysis at Newedge told Agrimoney.com, the rains did "a good deal to rebuild soil moisture" needed to get winter grain sowings off to a good start.

"Illinois had more than three inches of rain – there is not a puddle anywhere. It just soaked in," he said, forecasting soft red winter sowing for the 2013 "will be higher" than those for this year.

'Still very dry'

OK, winter wheat conditions are still hardly ideal, with Gail Martell noting the shortfall in Kansas, the top wheat producing state.

"Kansas field moisture has improved with increasing showers the past couple of weeks though conditions are still very dry," she said, noting that 74% of topsoil  in the state was still "short" or "very short" of moisture as of Sunday, according to US Department of Agriculture data.

"Rainfall needs to increase to fully replenish field moisture following a hot, dry summer," she added.

"When field conditions are very dry, soil erosion is common. Strong gusty winds in winter may claim a significant portion of wheat particularly in Kansas' western crop districts where the climate is semi-arid."

'Rain needed soon'

But Texas A&M University nonetheless forecast a 2m-acre rise, to 58m acres, in overall US wheat sowings for the 2013 harvest, good news for users, but another reason for bulls to stay on the sidelines.

Not that their cause is lost, given the background concerns over the crop in Australia, which "is dry as we near key reproductive periods", US Commodities noted.

UK grain merchant Frontier said: "Australian rainfall is being watched with keen interest as August rainfall was a half of that normally seen and warm temperatures are starting to reduce yield ideas.

"To get anywhere near the USDA estimate of 26m tonnes, rain is needed soon. Can this key wheat exporter really escape the weather issues that have plagued every other world producer?"

Take the money…

Nor did wheat receive much support from the row crops, which were sapped by a feeling that growers are increasingly willing to settle for current prices, enhancing the pressure on values seen anyway when combines roll and supplies increase.

US Commodities added: "The big market inversions are sending a signal to the producer to sell off the combine."

Newedge's Dan Cekander said "Farmers can get more than \$8 a bushel for corn off the combine. They are going to sell corn, and also soybeans, at \$17 a bushel cash off the field.

"They have never been able to get these prices before."

'Extremely overbought'

Indeed, as Country Futures' Darrell Holaday noted, "the cash basis is weakening and that is a concern".

Corn for December ended down 1.8% at \$7.90 ¾ a bushel, and this time not even soybeans, which have continued to set fresh records as grains have eased, could hold out.

The November soybean contract lost 1.1% to \$17.47 ½ a bushel, feeling a technical headwind too.

"The technical picture in the soybean market became extremely overbought early yesterday and the sell-off at this point is a correction to that technical situation," Mr Holaday said.

Softening softs

Weakness was felt among many soft commodities too, such as New York cotton, which felt pressure from relatively low levels of damage to Hurricane Isaac, as revealed in USDA data overnight, besides a caution from the International Cotton Advisory Committee that demand destruction is "plaguing" the sector.

The December contract shed 0.4% to 75.35 cents a pound.

New York raw sugar for October dropped 1.7% to a three-month low for a spot contract 19.01 cents a pound, sapped by continued ideas of ample supplies, as signalled by an International Sugar Organization report on Friday.

"The sugar bulls still haven't managed to get a convincing story together that threatens the prevailing Brazil/ global surplus story," Thomas Kujawa at Sucden Financial said.

"There was news of bad weather in Thailand on the wires and a potential reduction in the previously estimated crop, but otherwise it's still going nicely for the bears."

Sweeter cocoa

However, cocoa bucked the trend, soaring 4.0% to \$2,654 a tonne in New York, boosted by further buying by speculators.

Besides concerns for the 2012-13 crop in West Africa, the main growing area, where weather has not been ideal and there have been some disease reports, Macquarie on Tuesday cautioned over the risk of a self-feeding price spiral upwards, thanks to the mechanics of the Ivory Coast marketing strategy.