Analysts may have overegged their hopes for US corn yields, a leading farm academic has warned ahead of a barrage of crop revisions viewed as among the most crucial of the year.

While many brokers believe the US Department of Agriculture, in estimates due later, raise its forecast for America's average corn yield to some 166 bushels per acre - which would represent a fresh record - they are underestimating the impact of weather setbacks, Darrel Good said.

In fact, the failure of this year to repeat last summer's ideal cool and wet weather meant the average yield was likely to end up at 158.1 bushels per acre.

"Unlike last year, when the crop yielded better than it looked because of very favourable summer weather, our analysis suggests that the 2010 yield will be lower than implied by crop condition ratings alone," Mr Good, an agricultural economist at the University of Illinois, said.

"The expected shortfall is due to excessive June precipitation and above average summer temperature."

'Lower production number'

In fact, USDA ratings for the condition of US corn, while still indicating a crop in fine health, have been falling, a factor Alaron analyst Tim Hannagan said was a reason for caution over estimates of a higher corn crop.

Analysts on average believe the USDA will, in its monthly Wasde report on global crop supply and demand, lift its estimate of American corn production in 2010-11 by 34m bushels to 13.28bn bushels.

North American Risk Management analyst Jerry Gidel also supported thoughts of lower numbers, saying that estimates for the trend yield, on which initial USDA estimates were built, were skewed by 2009 figure which was lower than has been reported.

The production shortfall was shown up in official stocks data last month which came in significantly short of market forecasts, he told Agrimoney.com.

'Aggressive' revisions?

Mr Good, whose forecasts based on computer modelling on factors such as planting dates and weather, said that his estimate for the US soybean yield was, at 43.7 bushels, in line with brokers' expectations.

However, analysts also flagged the potential for demand estimates for both crops to provoke surprises, given better export hopes and, for corn, increased use by bioethanol plants.

On soybeans "the government has been guilty of underestimating new crop usage", Mr Hannagan said, noting the scope for "aggressive" upward revisions.

Benson Quinn Commodities analyst Jon Michaelscheck noted analyst expectations that US feed grain export numbers for 2010-11 could be lifted by more than 150m bushels, as end buyers attempt to replace feed wheat supplies lost from  the Black Sea exporters.