Nov. 27 (Bloomberg) -- Australia, the world’s fourth- largest wheat exporter, may harvest less of the grain than previously forecast after extreme weather in some states trimmed yields, National Australia Bank Ltd. said today.

Production may be about 21 million metric tons, 2 percent less than last year and down from the 23 million tons forecast in October, the bank said in an e-mailed report. The Australian Bureau of Agricultural and Resource Economics, the nation’s commodity forecaster, tipped a crop of 22.7 million tons in September.

The lower forecast echoes a cut by Commonwealth Bank of Australia, which last week trimmed its wheat crop estimate to 21.6 million tons. Yield expectations are being trimmed after frosts in some regions were followed by unusually hot weather in other states during the final pre-harvest period.

“Frosts in late September appear to have had a significant influence on grain development in parts of southern and central New South Wales,” Frank Drum, the bank’s agribusiness economist, wrote in the report. “Extremely hot temperatures in Victoria and South Australia have also resulted in a downward revision to yield estimates in those areas.”

Wheat futures for March delivery fell 2.1 percent to $5.60 a bushel on the Chicago Board of Trade in after-hours electronic trading at 11:55 a.m. Sydney time. Prices are down 8.4 percent this year.

To contact the reporter on this story: Madelene Pearson in Melbourne on mpearson1@bloomberg.net

Last Updated: November 26, 2009 20:37 EST