Corn's rally ground to a halt on Wednesday amid warnings that market fears for this year's US harvest may be overblown, and that the grain may struggle to break $5 a bushel.

Rich Nelson, director of research at US broker Allendale, dismissed as "simply ludicrous" expectations that the US Department of Agriculture might cut to the mid-150s bushels per acre, from 162.50 bushels per acre, its forecast for this year's US corn yield.

While Chicago was hearing talk of harvest yields coming in "5, 10, over even 15 bushels" per acre lower than last year's, that did not translate into the kind of USDA cuts which would drive prices to "$8, $15, or even $20 per bushel".

Last year's crop had been marred by low test weights – the weight of corn per bushel, which is a volume measure – and high moisture. Both of these factors inflated the size of crops coming off combines, compared with that as accounted for by USDA metrics.

Different harvest 

"So yes, we are hearing of harvest yields being 10 to 15 bushels lower. But are you hearing of ridiculous moisture levels this year? No," Mr Nelson said.

"While we are collecting yield estimates we should also be collecting moisture and test weights. Test weights can vary 10% from normal in good and bad years."

He added that he expected a further drop of 1-2 bushels per acre in USDA yield estimates.

Best yet to come 

Separately, Alex Bos, farm commodities analyst at Macquarie in London, flagged $5 a bushel, which a Chicago spot corn contract has not stood above for nearly two years, as a "key psychological level" which investors may be cautious in breaching.

And while also expecting the USDA to cut its yield estimate a little further, Mr Bos highlighted that some of the best crops had yet to be harvested.

"There is good corn in the north and west of Iowa, which will balance out some of the weaker crop in southern Iowa," said Mr Bos, who took part in last month's ProFarmer tour of major American corn growing districts.

"Minnesota will have a near-record crop. And the Dakotas too look set up to have a decent crop."

Corn for December closed 0.25 cents lower at $4.95 ¼ a bushel in Chicago.