Some critics of biotechnology say that a victory for Mr. Bowman could weaken what they see as a stranglehold that Monsanto and some other big biotech companies have over farmers, which they say has led to rising seed prices and the lack of high-yielding varieties that are not genetically engineered.

Patents have “given seed companies enormous power, and it’s come at the detriment of farmers,” said Bill Freese, science policy analyst for the Center for Food Safety, which was an author of a brief on the side of Mr. Bowman. “Seed-saving would act as a much needed restraint on skyrocketing biotech seed prices.”

Farmers who plant seeds with Monsanto’s technology must sign an agreement not to save the seeds, which means they must buy new seeds every year.

Monsanto has a reputation for vigorously protecting its intellectual property.

The Center for Food Safety, which has tracked the cases, said Monsanto had filed more than 140 patent infringement lawsuits involving 410 farmers and 56 small farm businesses, and had so far received $23.67 million in recorded judgments. The center says there are many other cases in which farmers settled out of court or before a suit was filed.

Monsanto says it must stop infringers to be fair to the large majority of farmers who do pay to use its technology.

But Monsanto typically exercises no control over soybeans or corn once farmers sell their harvested crops to grain elevators, which in turn sell them for animal feed, food processing or industrial use.

Mr. Bowman said that for his main soybean crop, he honored Monsanto’s agreement, buying new seeds each year containing the Roundup Ready gene, which makes the plants immune to the herbicide Roundup. That technology is hugely popular, used in more than 90 percent of the nation’s soybeans, because it allows farmers to spray fields to kill weeds without hurting the crop.