WASHINGTON -- The Supreme Court has sustained Monsanto Co.'s claim that an Indiana farmer violated the company's patents on soybean seeds that are resistant to its weed-killer.

The justices, in a unanimous vote Monday, rejected the farmer's argument that cheap soybeans he bought from a grain elevator are not covered by the Monsanto patents, even though most of them also were genetically modified to resist the company's Roundup herbicide.

Justice Elena Kagan says a farmer who buys patented seeds must have the patent holder's permission. More than 90 percent of American soybean farms use Monsanto's "Roundup Ready" seeds, which first came on the market in 1996.

Monsanto has a policy to protect its investment in seed development that prohibits farmers from saving or reusing the seeds once the crop is grown. Farmers must buy new seeds every year.

The case had been closely watched by researchers and businesses holding patents on DNA molecules, nanotechnologies and other self-replicating technologies. But Kagan said the court's holding only "addresses the situation before us."

Farmer Vernon Hugh Bowman bought the expensive, patented seeds for his main crop of soybeans, but decided to look for something cheaper for a risky, late-season soybean planting.

He went to a grain elevator that held soybeans it typically sells for feed, milling and other uses, but not as seed.

Bowman reasoned that most of those soybeans also would be resistant to weed killers, as they initially came from herbicide-resistant seeds too. He was right, and he repeated the practice over eight years. In 2007, Monsanto sued and won an $84,456 judgment.

Bowman said he should not be liable, in part, because soybeans naturally sprout when planted.

Kagan said the court did not buy that argument. "We think the blame-the-bean defense tough to credit," she said.

The case is Bowman v. Monsanto Co., 11-796.