The U.S. corn harvest will be about 1.4 percent smaller than the government’s forecast, while average nationwide yields are heading for an eight-year low following an unusually wet spring and a hot summer, according to R.J. O’Brien & Associates analyst Randy Mittelstaedt.

Farmers are expected to harvest about 12.25 billion bushels of corn this year at an average yield of 145.9 bushels an acre, the Chicago-based Mittelstaedt said in an Oct. 26 report obtained by Vance Publishing. The projected harvest is down from the U.S. Department of Agriculture’s most-recent estimate, 12.43 billion bushels, in an Oct. 12 report. He also cut his soybean crop forecast.

Mittelstaedt’s estimated yield, which is based on late-harvest season conditions, is down from the 148.1 bushels-per-acre average the USDA projected earlier this month and would be the lowest U.S. yield since farmers averaged 142.2 bushels an acre in 2003. The USDA is scheduled to update its crop estimates Nov. 9.

Some analysts have scaled down their expectations after Midwest harvest results in recent weeks produced weaker yields compared with earlier in the season. Farmers this spring boosted corn acreage to the second-highest level since the end of World War II, but harvest prospects were trimmed by a July heat wave that coincided with the crop’s critical pollination phase.

“Some yield reports are moving lower as the harvest progresses,” Jack Scoville, an analyst with Price Futures Group in Chicago, said in an Oct. 26 report. “Yields to the east of Illinois do not seem as strong, and (Western Corn Belt) yields are starting to fade a little bit as the harvest there progresses.”

As of Oct. 23, about 65 percent of the U.S. corn crop was harvested, compared with the five-year average of 51 percent for that date, the USDA said in a report earlier this week.

Shrinking harvest expectations further reinforce the outlook for tight corn supplies that likely will keep corn prices historically high over the next year, raising feed costs for beef, dairy and pork producers. At the end of the 2011-12 marketing year next August, the nation’s corn stockpiles are expected to fall to a 16-year low, based on a USDA forecast.

In trading Oct. 26, corn futures for December delivery fell 13 ½ cents to $6.37 ¼ a bushel. December futures are still up 7.6 percent from a seven-month low of $5.92 ½ reached at the end of September. Corn futures hit a record $7.99 ¾ in June and are expected to trade above $6 at least through the summer of 2013, based on current prices.

Mittelstaedt’s projected corn production, at 12.25 billion bushels, is down 10 million bushels from his previous forecast, while his average yield is down 1.2 bushels an acre from a previous estimate.

The soybean harvest is expected to total 3.019 billion bushels at an average yield of 41 bushels an acre, Mittelstaedt said. Both figures are below the USDA, which earlier this month forecast a crop of 3.06 billion bushels and an average yield of 41.8 bushels an acre.

In trading Oct. 26, November soybean futures fell 15 cents to $12.10 ½ a bushel.