Agriculture Department spokesman Mike Louisell said the seeds have been turned over to the department and tests for genetic modification will be completed by Friday at its Yakima, Wash., seed lab.
"At this point, it may have been mislabeled," Louisell said. "That's when the farmer contacted our agency."
Genetically modified alfalfa is legal to grow and sell in the U.S., contrasting May's discovery of genetically modified wheat in an Oregon field. Modified wheat is illegal in the U.S. outside of licensed test fields.
Consumers have shown increasing interest in avoiding genetically modified foods, so it has been important to separate them from products that are unmodified. There has been little evidence to show that foods grown from engineered seeds are less safe than their conventional counterparts, but several state legislatures are considering bills that would require them to be labeled so consumers know what they are eating.
The alfalfa farmer was seeking to sell unmodified alfalfa. "He was under the impression that it was not a genetically engineered crop," Louisell said.
Brokers test alfalfa for a number of reasons, he said, including the protein content for a crop that serves to feed livestock. That's when the broker discovered the alfalfa was genetically modified.
The farmer contacted the Agriculture Department in late August, and tests began after Labor Day. The department has not identified the farmer.
Pesticide-resistant alfalfa was developed by Monsanto Co. and has been licensed to several companies.
Monsanto spokesman Thomas Helscher said Thursday that major importers of U.S. alfalfa, including the United Arab Emirates, Japan and South Korea, have no restrictions on genetically modified crops, and negotiations with China over imports of modified alfalfa are ongoing.
Forage Genetics, a major alfalfa seed company that sells genetically modified alfalfa seed, said in a statement that genetic modification is permissible in conventional alfalfa seed.
"Varietal purity standards allow low-level presence of impurities, including (genetically modified) traits, in conventional alfalfa seed," spokeswoman Rebecca Lentz said in a statement. "The potential presence of impurities is clearly stated on the label. If growers are growing alfalfa for sensitive markets and want to purchase seed with non-detectable presence of GM traits, such seed is available in the marketplace."
A group in Washington calling for more rigorous food labeling said the broker's test shows the need for more scrutiny.
"Our overseas markets are concerned and sensitive to our products," said Elizabeth Larter, spokeswoman for the Yes on 522 campaign, which calls for labeling at the seed level. "Hopefully labeling all the way down to the seed level will give more confidence to our overseas market."
Reach reporter Nigel Duara at http://www.twitter.com/nigelduara . Manuel Valdes in Seattle and Rachel La Corte in Olympia, Wash., contributed to this report.