Monsanto Co. has asked a San Francisco federal court to allow it to continue selling its genetically modified Roundup Ready Alfalfa while the USDA conducts a court-ordered environmental impact study. Monsanto, its seed distributors and growers stand to lose up to $250 million if the alfalfa, which was designed to survive the company’s Roundup herbicide, is taken off the market for the two years it takes to complete the study, the company said in court papers filed late Friday. Earlier this month, U.S. District Judge Charles Breyer halted the sale of the modified alfalfa at the request of farmers, environmentalists and consumer advocates who say that it could harm the U.S. economy and the environment.