US foreign staff have, again, challenged official estimates of a bumper Chinese wheat harvest last year, citing disease damage, and cautioned that ideas of the corn crop may be too high too.

The US Department of Agriculture's Beijing bureau restated an estimate made in November that China's wheat harvest fell by more than 9m tonnes last year, to 108.0m tonnes, "due to head blight", or fusarium, outbreaks in major growing provinces such as Anhui, Henan and Hubei.

The estimate is more than 12m tonnes below the USDA's official estimate, which is in turn in line with the figure from China's own National Bureau of Statistics of 120.58m tonnes.

And the bureau cited as evidence of the squeeze on wheat supplies a rise in prices of some 9% rise to 2,360 remninbi a tonne in Chinese wheat prices between August and January, quoting data from analysis group JCI.

"This is a strong indication that wheat production and total available supplies are lower than Chinese official production estimates," the bureau said in a report.

Chicago wheat prices fell by more than 10% over the same period.

'High levels of vomitoxin'

Some commentators have noted, in defending ideas of a higher wheat crop, the relatively low rate of Chinese imports, which the USDA bureau acknowledged could fall nearly 15%, to 2.0m tonnes, in 2012-13.

However, the report also flagged the impact of sales from state wheat reserves in cushioning the impact of last year's poor harvest, especially on quality shortfalls given that these sales are largely of crop from previous years rather than 2012 crop of which "some still may be infect with head blight".

Indeed, the "possibility of high levels" of vomitoxin - a toxic residue from fusarium infections - in last year's crop has prompted Chinese officials to order state grain companies "to strictly follow domestic safety standards while purchasing wheat".

The impact of last year's poor crop may not be felt until further ahead, if the 2013 harvest also disappoints, forcing Chinese authorities to turn to turn to stored 2012 crop, with its vomitoxin risk.

"If 2013-14 production is less than expected or suffers from a similar disease outbreak, depending on how much of the 2012-13 wheat crop may be infected with head blight and comprise current reserve levels, there is a possibility that China may need to further increase imports in order to meet domestic demand."

'Pests, typhoon and drought'

The data dispute is the latest in a series of wranglings over the accuracy of Chinese harvest statistics, which critics claim tend to offer inflated estimates thanks to a subsidy programme which rewards regional authorities by output, so encouraging over-reporting.

Typically, the spotlight has fallen on discrepancies in corn – in which China's balance sheet is particularly important to markets given the country's likely move from being self-sufficient to a perennial importer, and in quantity.

USDA estimates on Monday forecast Chinese corn imports growing from 2.0m tonnes this season to 19.5m tonnes in a decade's time, overtaking the likes of Japan and Mexico to become the world;s top buyer.

The bureau estimated last year's Chinese corn crop at 200m tonnes, up 4.2% year on year, but 8m tonnes below the official USDA number.

"According to agricultural sources, yields in some areas were affected by factors such as army worm outbreaks, a typhoon and drought," the report said.

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