SAN FRANCISCO - A federal judge Thursday barred the planting of genetically engineered alfalfa nationwide until the government can adequately study the crop's potential impact on organic and conventional varieties.

U.S. District Court Judge Charles Breyer made permanent a temporary ban he ordered in March on alfalfa with genetic material from bacteria that makes the crop resistant to the popular weed killer Roundup.

Breyer said the U.S. Department of Agriculture must conduct a detailed scientific study of Roundup Ready alfalfa's effect on the environment and other alfalfa varieties before deciding whether to approve it.

USDA spokeswoman Rachel Iadicicco said the agency would conduct the study, which could take up to two years to complete.

In North Dakota, members of the Dakota Resource Council, one of plaintiffs represented by the Center for Food Safety in the lawsuit, praised the ruling.

"Judge Breyer's ruling forces USDA to do its job - that is, to conduct the research necessary to determine the effects of Roundup Ready alfalfa on the environment," Dean Hulse of Fargo, the past chairman of resource council, said in a statement. The council statement also quoted organic alfalfa seed producer Blaine Schmaltz, of Rugby, who said the ruling helps farmers.

"The judge's order to make public the location of Roundup Ready alfalfa fields is a critical part of the decision," said Schmalz. "It allows GM-free and organic producers like me make sound planting decisions."

The Center for Food Safety in Washington, D.C., had sued on behalf of farmers who argued that the genetically engineered seed could contaminate organic and conventional alfalfa varieties. Alfalfa farmers grow the crop primarily for livestock feed.

Monsanto Co., the St. Louis-based biotech company that developed the crop and makes the weed killer, and Forage Genetics International, which is licensed to sell it, argued that the alfalfa would actually benefit the environment because fewer weed killers would be used.