Monday's 7-1 decision enables the U.S. Department of Agriculture to complete a study on whether the alfalfa will harm the environment before deciding on whether to approve the seeds for planting, a process that could go into next year.
The high court ruled that a federal judge in San Francisco went too far when he issued an order that overturned the Agriculture Department's decision to allow some farmers to plant Monsanto's Roundup Ready alfalfa seeds before the government had completed its full study of the environmental issues. The plants are resistant to the company's herbicide. An appeals court upheld the ban.
Soon after the Supreme Court decision was issued, both Monsanto and its critics claimed victory.
"This is a great day for farmers, as they can look to the government to set the regulatory rules as opposed to individual litigants," said David Snively, Monsanto's general counsel. "This should give growers great hope that new technologies in alfalfa are coming, and coming soon."
But environmental groups noted that the government must still approve the seeds' use for widespread planting and that its determination can be further challenged in court.
"To the extent Monsanto is claiming this as a victory, it's a very hollow one since no one can plant their crops," said Paul Achitoff, an attorney for the ecological law firm Earthjustice.
Andrew Kimbrell, executive director of the Center for Food Safety and a plaintiff in the case, said: "Nothing in practice has changed. This is still an illegal crop to plant. You cannot go out and plant this tomorrow."
The decision marked the first ruling by the nation's highest court over genetically modified organism, or so-called GMO, crops, and further fueled the long-running environmental dispute over such high-tech agricultural products.
It also comes at a time when Monsanto is fighting criticism over its genetically modified organisms and business practices on several fronts.
The company is embroiled in a similar fight in federal court in California regarding its genetically modified sugar beet products. The Justice Department is investigating Monsanto amid claims of antitrust violations and mounting complaints about unfair competitive practices.
At least three state attorneys general also have begun probes into whether the St. Louis firm has abused its market dominance to undermine rivals and raise prices.
U.S. farmers spent about $17 billion on seeds last year, up 56% from 2006, according to the USDA. Yet over the last decade the number of independent seed companies in the U.S. has shrunk to fewer than 100 from more than 300, according to the nonprofit Farmer to Farmer Campaign on Genetic Engineering, a network of 34 farm groups.
At the same time, environmental advocates and farm groups alike have voiced growing concern over the long-term effects of raising genetically modified crops that are immune to Monsanto's popular herbicide glyphosate, or Roundup.
Ninety-two percent of the U.S. soybean acres and 85% of the fields planted with corn are grown with seeds that use Monsanto technology, according to a 2009 report issued by Farmer to Farmer.
Yet a number of so-called superweeds — weeds that have developed an immunity to Roundup, including pigweed and horseweed — are growing on millions of acres of farmland in 22 states, including California.
That, in turn, has farmers using far more potent herbicides on their land and chemical companies starting to sell old chemical compounds that posed more environmental risks than Roundup.
USDA officials said Monday that the agency was ready to move forward on completing its environmental statement on Monsanto's variety of Roundup Ready alfalfa seed. "Nothing in the Supreme Court's decision affects that ongoing process," USDA spokesman Caleb Weaver said.
The USDA approved the modified alfalfa seeds' use in 2005, and more than 5,500 farmers planted the crop. But U.S. District Judge Charles Breyer in San Francisco found that the agency had violated federal law by not fully assessing the seeds' effect on the environment.
The seeds were eventually removed from the market. The U.S. 9th Circuit Court of Appeals upheld the ban.
Supreme Court Justice Samuel A. Alito Jr., writing for the majority, said Breyer overstepped his authority by issuing an injunction that was too sweeping.
Justice John Paul Stevens, in dissent, argued that the trial court was in a better position to conduct an inquiry into the facts of the case and that the justices should have deferred to its judgment.
Justice Stephen G. Breyer, who is the trial judge's brother, recused himself from the case.
Some Democrats in Congress condemned the decision. Rep. Dennis J. Kucinich (D-Ohio) said he would introduce a bill detailing a new regulatory framework for genetically engineered plants and animals.
Monsanto's Snively said the company was prepared to work with the government to accelerate making the seeds available to alfalfa growers as soon as the USDA grants its final approval.