US farm officials acknowledged they were stumped by the surprisingly large domestic corn stocks unearthed two weeks ago, which sent the market plunging, even as they faced fresh criticism over the data.

The comments came ahead of an annual event at which investors were set to grill US Department of Agriculture statisticians over what appear to be data anomalies. 

The USDA revealed several explanations as to why the country's corn stocks as of September 1 came in at 1.13bn bushels – more than 200m bushels higher than previously thought, and well above market expectations.

Reasons for the unexpectedly large inventories - which the USDA said implied the lowest US feed and residual use of corn in the July-to-September period in at least 35 years – included a higher quality of last year's crop, meaning consumers needed to use less.

Alternatively, the anomaly may have been down to buyers switching early to newly-harvest 2011 crop, meaning more of last year's output was left unused, or to a drop of about 50m bushels in exports.

"The slow pace of September corn exports suggests that less corn than usual was in transit, leaving more corn in more visible storage positions," USDA analysts Thomas Capehart and Edward Allen said.

'Speculative explanations'

However, they admitted that "none of these potential explanations are definitive and even together may not fully explain all of the reduction year-to-year in feed and residual disappearance during the last half of the 2010-11 marketing year".

All the explanations proposed were "speculative and not necessarily supportable by available data".

The comments come amid continued investor disgruntlement over the data, the latest in a series to have puzzled the market, and which sent corn futures down the exchange maximum in Chicago on the day.

On Monday, Chicago trader Matthew Pierce – who dismissed as "a joke" a USDA cut last week to its estimate for domestic feed wheat use despite resilient animal numbers – said the department's credibility was at stake.

"The sad fact is people are losing faith in the USDA due to questionable data," he said.

He also questioned the impact on data reliability of proposals by the US Senate to cut by 5% funding for the domestic National Agricultural Statistics Service division, and 25% reduction for the Census Bureau, which collects trade data on good including agricultural commodities.

'Could have repercussions'

Separately, on Monday, US Commodities termed the USDA's estimate on wheat feeding "suspect", while an analyst at a major London investment bank told Agrimoney.com that he remained astonished by a the USDA's decision last week to upgrade to 182m tonnes its forecast for China's corn harvest, in line with Beijing estimates, despite large doubts about the reliability of Chinese data.

These concerns re-emerged on Monday, when the chairman of China's potash industry association was revealed to have predicted Chinese production of the nutrient of 6m tonnes this year – 50% above the forecast of sector giant PotashCorp.

In the US, a broker told Agrimoney.com of a belief that the quality of US data had fallen with the retirement of a succession of seasoned statisticians.

"You may now have people without that actual experience and keener on computer models which are just not as robust," the broker said, pointing out the USDA's cut last month to its estimate for world cotton stocks, only to reverse that change, and add more, last week.

"It's a shame, because people are losing confidence in the data, and that could have repercussions."

The USDA is designed in part to encourage orderly crop markets, where investors rely more on official data rather than speculation which can bring false pricing, and therefore send the wrong signals to growers about whether to invest in increased plantings or not.

USDA officials on Monday met trade representatives at the annual Data End Users Meeting in Chicago. (Link to official site.)