Land unfit for growing food could make a big contribution to the United States' biofuel needs with wild-growing, non-grain crops, a study said Wednesday.

So-called marginal land in 10 states in the American Midwest could produce as much as 5.5 billion gallons of ethanol per year, a team of researchers wrote in the journal Nature.

This would represent a quarter of the country's stated 2022 target for cellulosic biofuels -- derived from wood, grasses or the inedible parts of plants.

"Marginal lands... can serve as prime real estate for meeting the nation's alternative energy production goals," said a statement from Michigan State University, which contributed to the study.

As countries seek to move away from expensive and CO2-polluting fossil-fuels, concern is growing that the production of biofuel from crops like maize or soybeans may jeopardise food production without saving much on Earth-warming greenhouse gas emissions.

The new study used computer modelling to show that certain wild, herbaceous plants, including the goldenrod, red clover, Indian hemp, wild carrot and several types of grasses, can be planted for biofuel with a similar greenhouse-gas effect to traditional crops and much lower than that of fossil fuel.

"This also is the first study to show that grasses and other non-woody plants that grow naturally on unmanaged lands are sufficiently productive to make ethanol production worthwhile," said the statement.

Importantly, this method has no impact on food production.

The study "suggests that marginal lands could be a viable alternative to fertile cropland for biofuel production -- which would be extremely useful given the limited land resources," Klaus Butterbach-Bahl and Ralf Kiese from the Institute of Meteorology and Climate Research in Germany wrote in a comment published by Nature.

But the German duo pointed out the study was not clear on whether marginal land could be exploited without harming biodiversity and the environment.

"Moreover, land that is fallow today might be needed in the future for agricultural production, to offset the demands of the world's growing population," they wrote.