The rain delayed, and frost-nipped, US autumn harvest may prompt Washington to cut up to 360m bushels (9.14m tonnes) from its corn production forecast.

The US Department of Agriculture's revised global crop supply and demand estimates will peg America's corn harvest at 12.66bn bushels (326m tonnes), analysts at JP Morgan believe.

The Wall Street bank is the gloomiest among major brokers and analysts polled ahead of Tuesday's report, which will reveal Washington's first estimates since US corn and soybean crops were hit by frost, which may have halted maturity early, and persistent rain, which has raised further questions over quality.

"October was a terrible weather month as the corn and bean growing season came to an end. Too much rain and an early month freeze and frost," Tim Hannagan at PFGBest, the Iowa-based broker, said.

Data revealed a week ago showed the condition of the crops declining to its worst this season.

Soybean raise? 

And some observers forecast there will be an increase, to judge by historical data.

"In the seven years since 1974 when the corn harvest was less than 50% completed as of the last week of October, yields were revised down from the October estimate in only three instances," Goldman Sachs said.

"The average revision was in fact an increase of 1.8 bushels an acre.

"Moreover, just as wet weather during the harvest months tends to reduce yields, warm weather in November – which is currently forecasted – has a tendency to contribute positively to yields."

Indeed, analysts forecast Washington raising its hopes for US soybean production by 12m bushels (327,000 tonnes) to 3.26bn bushels (88.7m tonnes).

"The fractionally higher average may be attributed to thinking beans were far enough along in their maturity to avoid weather's worst effect,"  PFGBest's Tim Hannagan said.

 Objective data 

The report will be particularly keenly watching given that it will be based largely on actual harvest data, rather than the more subjective observational techniques required when crops are in their early stages.

"This report will be more meaningful."

Other areas set to attract interest include US wheat exports, which some analysts believe are vulnerable to a downgrade, and feed use of corn, which some speculate may also be revised down thanks to the growing popularity of rival distiller's grains, a biorefinery byproduct.