District Court Judge Charles Breyer of the Northern District of California said his initial injunction against planting the biotech giant's herbicide-resistant Roundup Ready alfalfa should stay in place until government studies on its environmental effects are concluded.

The preliminary injunction was ordered in March, in a monumental case that overturned the US Department of Agriculture's (USDA) approval of a biotech alfalfa variety.

The judge found USDA failed to abide by federal environmental laws when it approved the crop, developed by Monsanto and Forage Genetics, without conducting a full Environment Impact Statement. He ordered an immediate halt to sales of the seed, on the grounds that the genetically engineered gene could contaminate organic and conventional alfalfa.

The latest decision was met with support from environmental groups, but Monsanto said in a statement it was "disappointed".

"We support a farmer's right to choose biotechnology, organic or conventional crops with the proper stewardship practices that make coexistence feasible, " it said.

Alfalfa is grown on over 21 million acres, and is worth $8bn per year (not including the value of final products, such as dairy), making it the country's third most valuable and fourth most widely grown crop. Alfalfa is primarily used in feed for dairy cows and beef cattle, and is also used in pork, lamb, sheep, and honey production. Consumers also eat alfalfa as sprouts in salads and other foods.