Alfalfa is a perennial livestock fodder crop and one of the mostly widely grown crops in the United States. The commercialization of the biotech variety angered environmental groups as well as organic farmers, consumer groups and others who fear the biotech alfalfa will contaminate organic and conventional varieties, create "superweeds" that don't respond to herbicide, and damage export business.

Breyer had issued a preliminary injunction in March, ruling that US regulators acted illegally in allowing the commercialization of the biotech alfalfa without a thorough examination of its impact.

The court action marked the first time a federal court overturned USDA approval of a biotech seed and halted planting, according to The Center for Food Safety, a consumer advocacy group.

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

Neither party would provide any immediate comment.

The USDA, a defendant in the case, also did not immediately comment on the ruling.

Monsanto and Forage Genetics had asked the court to lift the ban, arguing there was a low risk of contamination. But the judge rejected that argument.

"The harm to these farmers and consumers who do not want to purchase genetically engineered alfalfa or animals fed with such alfalfa outweighs the economic harm to Monsanto, Forage Genetics and those farmers who desire to switch to Roundup Ready alfalfa," the judge wrote.

The ruling Thursday does not stop the harvesting of Roundup Ready alfalfa that already has been planted and is contracted to be sold for seed back to Forage Genetics. About 76 farmers have such contracts, the court ruling states.

To minimize the risk of "genetic flow" between the genetically engineered alfalfa in the ground and conventional and organic alfalfa crops, Breyer ordered that several conditions be implemented, including the segregation of the biotech alfalfa immediately after harvest.

He also ordered the public disclosure of field locations where the crop was planted.