Chicago traders were left wrong-footed, selling grains heavily only to be forced into a swift u-turn, after a mildly bearish streak to a long-awaited set of US crop data was trumped by more weather concerns.

US soybean inventories of the start of the month were, at 667m bushels, up 48m bushels, and above market expectations, the US Department of Agriculture said.

Data for soybean sowings also signalled more generous supplies than investors had pencilled in, with plantings pegged at 76.08m acres, some 2.2m acres more than initially estimated.

Estimates for US corn plantings too were given a healthy upgrade of 541,000 acres, to 95.86m acres – a combination of data which initially sent prices of both crops into negative territory.

Down, and up 

Indeed, the immediate market reaction was to send corn for December, which had stood some 1% higher before the reports, plunging to $6.22 a bushel, 1.6% in the red.

However, the prices rebounded strongly within five minutes to send December corn back up to $6.55 a bushel – a 5% swing – with soybeans posting a smaller gain, as attention swung to forecasts for hot and dry US weather, and to a more upbeat interpretation of the data.

"The market moved 6-8 cents lower on corn in the first three seconds after the report and then shot up 15 cents higher on threatening weather," US Commodities said.

At PFB Best, Tim Hannagan said:  "The weather traders came in and bought the break driving corn and beans 20-plus cents higher before settling back at midsession."

The price moves appear to bear out concerns that Chicago's extended opening hours, which since this month has left them trading live when major reports are released, would lead to large volatility, and losses for traders caught on the wrong side of market swings.

The stocks reports have a particular history of proving difficult to anticipate, provoking average prce movements of 9%, according to Macquarie.

'Depends where you were looking' 

"You can understand how people initially sold into this. The soybean stocks number was bearish. You could even say the corn stocks were bearish," Jerry Gidel at broker Rice Dairy said.

The more price positive data in the report included a forecast that, even though corn sowings had proved higher, the area making it through to harvest would come in at 88.85m acres, lower than initially expected.

Also, farmers turned less to spring wheat that had been thought, leaving total wheat seedings, at 56.02m acres, some 660,000 acres below market forecasts.