SAN FRANCISCO -- A federal appeals court in San Francisco Tuesday upheld an injunction requiring a full environmental study before a genetically engineered alfalfa seed can be sold and planted.

The 9th U.S. Circuit Court of Appeals ruled that a federal trial judge in San Francisco held adequate hearings last year before ordering the study of a seed known as Roundup Ready alfalfa, developed by St. Louis-based Monsanto Co.

The seed is engineered to tolerate Roundup, a chemical pesticide also made by Monsanto, and is intended to enable farmers to grow alfalfa while using Roundup to kill weeds.

A coalition of farmers, consumers and environmental groups claimed in a lawsuit in 2006 that the seed should be studied further because the engineered gene could contaminate organic and conventionally grown alfalfa through pollination by bees and wind.

The environmental impact study is to be conducted by the U.S. Department of Agriculture, which has authority to approve the seed.

The injunction by U.S. District Judge Charles Breyer allowed harvesting of a limited amount of Roundup Ready already planted in 2007, but bars any further planting and harvesting until the study is completed.

Kevin Golden, a lawyer with the Center for Food Safety in San Francisco, said, "This ruling is a major victory for farmers and consumers."

Golden said the decision is the first time an appeals court in the nation has ruled on an environmental lawsuit concerning a genetically engineered crop.

Golden said the study may be completed next year, after which the Agriculture Department will make a new regulatory decision on whether to approve the seed.

The Center for Food Safety is one of nine plaintiffs in the case.

Spokespersons for Monsanto and the Agriculture Department had no immediate comment.