* USDA option on GM alfalfa goes beyond law, says letter
* Farm groups say USDA strays from science as GM guide
* Vilsack says fewer suits if growers agree to co-exist
* Issue to get airing at House panel on Thursday
By Charles Abbott
WASHINGTON, Jan 19 (Reuters) - The Agriculture Department is straying beyond the law by suggesting it can set planting restrictions on genetically modified crops that are deemed safe, said three senior U.S. lawmakers on Wednesday.
USDA has raised the possibility of planting restrictions on a GM alfalfa strain developed by Monsanto MON.N Co. And Agriculture Secretary Tom Vilsack says growers of traditional and GM crops should find a way to co-exist without repeated lawsuits.
"This is the first time these measures would be included in a regulatory decision where the crop did not pose a plant pest or health risk," wrote the Republicans to Vilsack.
They said if a GM crop is ruled safe after review by USDA, the Environmental Protection Agency and the Food and Drug Administration, "USDA has no authority to impose further restrictions." There already are rules to settle cropping disputes, they said.
Signing the letter were Republican Frank Lucas, chairman of the House Agriculture Committee and senators Pat Roberts and Saxby Chambliss, the Republican leaders on the Senate Agriculture Committee.
Vilsack was scheduled to speak at a House Agriculture Committee forum on Thursday on regulation of GM crops.
A dispute over Monsanto's "Roundup Ready" alfalfa strain went to the Supreme Court and a U.S. appeals court is hearing a case on biotech sugar seeds. In both cases, USDA was accused of improperly approving the crops for deregulation.
In a speech last week, Vilsack said an even-handed compromise among growers would be better than repeated litigation.
"Every farmer ought to be able to do what he or she wants to do on their land, so we are going to continue to have that conversation," he said.
Seven major farm groups, as well as the Biotechnology Industry Organization, wrote to the White House in early January to object to USDA's path. The farm groups said it was wrong "to use motives other than science to impose conditions on this crop."
On Dec 16, USDA released a final environmental impact statement on the GM alfalfa and said it was considering two alternatives for deregulation -- total approval or a partial deregulation that could include isolation distances from other crops, set geographic limits on where the crop is grown, spell out harvest periods and regulate equipment use. (Reporting by Charles Abbott;editing by Sofina Mirza-Reid)