Washington has dismissed corn farmers' hopes of near-doubling yields over the next 20 years, saying that such a sharp increase would require an "extraordinary deviation" from historical trends.

America's 300,000 corn farmers may well be on track to hit the average yield of 175 bushels per acre targeted by the National Corn Growers' Association, given an accelerating rate of crop improvement.

A long-run annual rise in corn yields, which have "trended upward dramatically" ever since hybrid varieties were introduced in the 1930s, could potentially accelerate to more than 2 bushels per acre and "be consistent with historical experience", the US Department of Agriculture said.

However, the annual yield increase of 6 bushels per acre needed to reach a NCGA target of 300 bushels per acre by 2030 would be "completely unprecedented" the department said.

Scale factor 

While farmers had already achieved yields as high as 365 bushels per acre in small plots entered into crop contents, these required a high level of grower attention.

"Extensive irrigation of corn over large areas may not be practical given the availability and price of water," the USDA said.

"Furthermore, the levels of inputs and time spent in managing contest plots may not be profitable in commercial farming."

Official data shows that it took 40 years to double yields to the current record of 160.4 bushels per acre hit five years ago.

Extra plantings 

The NCGA has maintained that yields are "on the trend line" to hit the 2030 target, claiming in a report in September that yield growth had further accelerated since 1990, helped by genetically modified varieties.

One factor in further improvement will be an increase in sowing density which, in trial plots, was being tested at 65,000 plants per acre, twice the current rate, the NCGA said.

The USDA's comments came in a report on the implications on of US ethanol targets, forecasting that greater use of the petrol alternative would raise sowings to 94.7m acres by 2015, about 3.2m acres beyond plantings likly without ethanol incentives.

Most of the increase will occur in America's Northern Plains and Corn Belt, the report said, forecasting hits to rice and sorghum sowings, and to land held in environmental programmes.