Crop biotechnology giant Monsanto Co. has the most at stake in the first of an unprecedented series of public meetings that the antitrust wing of the Justice Department is holding across the Farm Belt.

In January, the Justice Department launched a formal antitrust investigation of the St. Louis company’s handling of the most widely planted genetically modified crop in the U.S., a herbicide-immune soybean.

Now, Justice’s tight-lipped antitrust division is taking the unusual step of inviting competitors, farmers, politicians and activists to air any gripes about Monsanto-and to suggest ways to limit the company’s reach before a high-profile audience.

The Obama administration disclosed Wednesday that Attorney General Eric Holder will speak Friday at the first of five such meetings, billed as joint “workshops” with the Department of Agriculture on competition issues.

Friday’s meeting in Iowa will focus on genetically-modified seeds, the 14-year-old market largely created and led by Monsanto, which has at least one of its patented genes in about 90% of soybeans grown in the U.S. and in about 80% of U.S. corn.

Monsanto declined to make Hugh Grant, its chairman and chief executive, available for comment, but issued a statement that “an objective review of the agricultural sector will reveal that competition is alive and flourishing.” A Monsanto vice president is scheduled to speak Friday.

Farmers and the seed companies that license genes from Monsanto have long complained about the prices it has been able to command. The price of a bag of soybean seed, for example, has roughly quadrupled since Monsanto began licensing genes.

Pioneer Hi-Bred, the seed unit of Wilmington, Del., chemicals concern DuPont Co., has alleged that Monsanto is trying to use gene licenses to limit competition. Monsanto has also tried in recent months to dispel fears among some farmers and seed breeders that Monsanto will make it hard for them to use generic versions of genetically modified crops after the company’s patents expire.

Monsanto’s Roundup Ready soybean is genetically modified to survive dousing by a weedkiller made by Monsanto called Roundup. Introduced in 1996, the seed made it so easy for farmers to chemically weed their fields that many stopped using other herbicides or mechanically tilling their fields. With that seed losing its ability to draw royalties after 2014, Monsanto is trying to get farmers to switch to a second generation of Roundup Ready seed that it has patented.

Mr. Holder will be joined Friday by Christine Varney, his antitrust chief, U.S. Agriculture Secretary Tom Vilsack and several states’ attorneys general, some of whom have been investigating Monsanto’s business practices for years.

“Seed technology is pretty heavily consolidated,” said Mr. Vilsack, a former governor of Iowa, a state where Monsanto and Des Moines-based seed giant Pioneer Hi-Bred are locked in a bitter fight for farmer loyalty that includes dueling lawsuits in a federal courthouse. “I’m not taking sides,” Mr. Vilsack said Wednesday. “What I’m really concerned about is farmers getting a fair shake.”

The USDA estimates that U.S. farmers spent $17.2 billion on seed in 2009, up 56% from $11 billion in 2006.

President Obama promised early in his administration to “reinvigorate” antitrust enforcement, which involved the Justice Department disavowing Bush era guidelines. His antitrust chief has largely pushed on her own for a closer look at agriculture, where everything from hogs and cattle to corn, soybeans, milk and seeds are processed by a handful of big concerns.