The rush to increase planting of biofuel feedstock is rapidly killing off unique grasslands and pastures in the central United States, researchers report.

Christopher Wright and Michael Wimberly of South Dakota State University -- analyzing satellite images of five states in the U.S. corn belt -- determined 1.3 million acres of grassland had been replaced by corn and soybean fields between 2006 and 2011, NewScientist.com reported.

Demand for both crops has risen with incentives to use them as biofuel resources instead of food, the researchers said.

The loss of native grasslands and pastures was greatest in South Dakota and Iowa, they said, with as much as 5 per cent of pasture being converted to cropland each year. This is having an impact on wildlife, the researchers report, especially on ground-nesting birds that use grasslands and their surrounding wetlands as breeding grounds.

One area of wetlands in South Dakota known as the Prairie Pothole Region is especially at risk from encroaching crop fields, they said.

"Half of North American ducks breed here," Wright said.

Other environmentalists agreed the loss of grasslands was worrisome.

"Exchanging real environmental impacts for the dubious benefits of biofuels is counterproductive," Bill Henwood of the Temperate Grasslands Conservation Initiative in Vancouver, British Columbia, said. "Last year's record drought in the corn belt all but wiped out the crops anyway."