SAN FRANCISCO (Reuters) - A U.S. judge questioned whether he should lift a ban on the sale and planting of Monsanto Co.'s genetically modified alfalfa without a government study of the crop's potential impact.

U.S. District Court Judge Charles Breyer on Friday told lawyers defending the use of Monsanto's alfalfa that it was up to the government -- not him -- to determine whether use of the seed posed a potential threat to the environment.

He also said that lifting his preliminary injunction before such a study was complete could lead to greater harm to the environment. He challenged defense lawyers to show him case law establishing a precedent for him to do so.

Breyer, who has already ruled that the government acted illegally in approving the biotech alfalfa, issued the preliminary injunction in March and set April 27 as a date to consider whether to make it permanent. He did not indicate when he might make a final decision on the ban.

"It is not the court's function to do an environmental impact study," Breyer said during the hearing. "That hasn't been done, and I don't know if the court ought to do it. The government ought to do it, and that is what I held.."

In March, Breyer issued a preliminary injunction banning the sale and planting of the alfalfa, which has been genetically altered to tolerate treatments of Monsanto's Roundup herbicide.

CONTAMINATION FEARS

Many farmers, environmentalists and consumer activists fear the biotech alfalfa will contaminate organic and conventional varieties, create "superweeds" that don't respond to herbicide and damage export business. Alfalfa is a perennial livestock fodder crop and among the most widely grown crops in the United States.

The judge's order in March said the USDA had not done a thorough job in evaluating the potential impact of the crop, and he vacated the USDA's 2005 approval of Monsanto's alfalfa.

His decision marked the first time a federal court overturned USDA approval of a biotech seed and halted planting, according to The Center for Food Safety, among the groups seeking the injunction.

Lawyers defending use of the crop, however, urged the judge to lift his injunction, saying at the hearing on Friday that the important factor was that any likelihood of injury was low and that farmers relying on the seed would be harmed.

"There are some significant environmental and beneficial effects in Roundup Ready Alfalfa," Janice Schneider, a lawyer representing Monsanto, told the judge.

Monsanto has presented testimony from scientists who say there is an "extremely low" risk that Roundup Ready Alfalfa would pollinate conventional crops if "appropriate stewardship measures" were taken.

Monsanto has also argued that a continued ban on Roundup Ready seed would force farmers "to plant lower-yield alfalfa breeds that pose more complicated and costly weed control problems and require the use of more toxic or environmentally problematic herbicides."

The Roundup Ready alfalfa genetic trait was developed by Monsanto and licensed to Forage Genetics International, which produces and markets the seeds.