Waning hopes for America's corn crop could be poised to send prices back above $4.20 a bushel, a leading agricultural academic has said, as US officials highlighted a decline in the crop's condition.

Darrel Good, farm economist at the University of Illinois, has warned that investors may need to reconsider forecasts of yields reaching a bumper 170 bushels per acre.

The 163.5 bushels-per-acre yield figure that US Department of Agriculture officials have pencilled in is already above trend levels, on University of Illinois estimates.

Yet wet "excessive precipitation" last month, and expectations that this month's weather will "not be as favourably cool" for pollination as in 2008 and 2009, have dampened yield prospects.

Prices to soar? 

"Yield expectations may need to be trimmed a bit from those lofty early-season projections," Mr Good said, adding that this was likely to boost prices, provided demand for the grain held up.

"A bit of a shortfall in the average yield might be enough to push prices back to the late winter highs".

Chicago's near-term corn contract topped $4.26 a bushel in December last year.

The current spot lot stood at $3.61 � a bushel in early deals on Wednesday, having already gained 11% over since the US Department of Agriculture a week ago unveiled estimates for America's corn stocks and sowings which fell short of market forecasts.

'Poor-looking crop' 

Mr Good's comments came as the USDA reported that the proportion of the US corn crop in "good" or "excellent" condition fell by two percentage points to 71%.

While still representing a high figure, and in line with that last year's, the third successive week of declining ratings surprised many investors.

Some analysts had been "looking for an improvement� due to the season's most extended period of dry weather and warm temperatures", Terry Roggensack at Hightower Report said.

Mike Mawdsley at Market 1 said he was "hearing more and more talk about some poor-looking crop in parts of the Corn Belt.

"Would make one wonder if acreage and yield will slip as we go forward."

US Commodities said: "It is too wet in the north and too dry in the south."

The broker added: "Typically it is hard to achieve trend yields in a La Nina year," referring to a weather pattern linked to Pacific water temperatures.