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.