* USDA approves GMO alfalfa without restrictions
* Farmers can plant biotech alfalfa this spring
* Vilsack says no doubts about GMO crop safety
* USDA will research genetics to prevent GMO contamination
* Opponents pledge to sue to stop planting (Adds details, background)
By Carey Gillam and Christopher Doering
KANSAS CITY/WASHINGTON, Jan 27 (Reuters) - The United States said on Thursday farmers could proceed with planting genetically altered alfalfa without any of the restrictions that opponents say are crucial to protect organic and conventional farm fields from contamination.
The decision, closely watched by supporters and protesters around the world for its potential implications on biotech crop regulation, was seen as a boon to biotech crop developers and comes as research into additional biotech crops accelerates.
But opponents of biotech crops were disappointed.
The U.S. Agriculture Department had signaled last month that it might forge a first-ever compromise approval with a range of restrictions for planting, but there were no such conditions in the plan announced Thursday.
The USDA said the decision, made by its Animal and Plant Health Inspection Service, was made after analysis of various economic and environmental factors, and allows GMO farmers to get their crop in the ground this spring.
"After conducting a thorough and transparent examination of alfalfa ... APHIS has determined that Roundup Ready alfalfa is as safe as traditionally bred alfalfa," Agriculture Secretary Tom Vilsack said.
Alfalfa is the fourth-largest U.S. field crop grown, worth roughly $8 billion to $10 billion and grown on about 20 million acres as food for dairy cattle and other livestock.
The decision Thursday to allow planting of genetically altered version comes after years of court battles with opponents.
Developed by biotech leader Monsanto Co MON.N to tolerate treatments of Monsanto's Roundup herbicide, "Roundup Ready" alfalfa is preferred by many farmers because it makes killing weeds easier.
But opponents, including conventional and organic farmers, say the biotech alfalfa can easily contaminate their crops because alfalfa is pollinated largely by honey bees, making it difficult to isolate GMO fields from non-GMO strains.
Organic dairy farmers who feed their cows alfalfa say the biotech crop can kill their business.
Opponents also say increased use of herbicide is translating to increased weed resistance, and the rise of "super weeds."
A consortium of opponents led by the Center for Food Safety previously won a court decision against USDA for the government's failure to thoroughly account for the environmental and economic implications of the biotech alfalfa when it approved the crop for the first time in 2005.
A federal court ordered USDA to rescind its approval until the government thoroughly evaluated the impact of the crop.
Vilsack said Thursday the government has now done so, but opponents said they will sue again.
"It is very disappointing. It appears they just capitulated to the demands of the corporations," said Andrew Kimbrell, executive director of the Center for Food Safety, which has led litigation efforts against USDA. "They really are throwing conventional farmers under the bus."
Kimbrell said his group would immediately seek a court order vacating the government's approval.
VILSACK CITES CHOICE
While he acknowledged the concerns, Vilsack said U.S. farmers must have the "choice" to plant GMO alfalfa as well as conventional and organic varieties, and he said USDA would implement a series of measures to encourage trust between all the parties.
"I'm trying to bring people together," he said. "This set of actions, it seems to me, provides opportunities for preservation of choice in agriculture, creates a set of forms for building trusting relationships that could lead us to better policy in the future," said Vilsack.
Vilsack said USDA would promote research into how genetics might prevent contamination along with research designed to improve detection of any contamination that does occur.
He said USDA would also set up two advisory committees to try to help ensure availability of high-quality seed, as well as set up programs to try to protect the purity of alfalfa germplasm.
USDA will encourage voluntary, third-party audits and verification of "industry-led stewardship initiatives," Vilsack said.
"We welcome the Secretary's commitment to expand U.S. agriculture, to keep pace with the latest scientific developments, and to take into account the needs of all producers and all types of production," said Jim Greenwood, chief executive of the Biotechnology Industry Organization.
Opponents said the studies and advisory groups were a good step, but should be done before approval of the crop, not after.
"The USDA has also announced a series of measures to try to understand the way that GE alfalfa could contaminate other crops," said Food & Water Watch Executive Director Wenonah Hauter. "This is the first time the agency has acknowledged these issues, and, unfortunately, these steps should have been taken before the agency decided to release this crop into the food supply, not after." (Editing by Lisa Shumaker and Walter Bagley)