In the tussle between U.S. farmers and big data purveyors, farmers are winning some control over details about crop and growing conditions on their land, but most data sellers are retaining ultimate say over how they can use information that could be worth billions of dollars.

Although companies like Deere & Co. and Monsanto Co.’s Climate Corp. are giving some ground by putting legal teeth behind promises made during sales pitches, they are refusing to back away from claims they have an absolute right to all data collected as combines, tractors and other equipment work fields across the country.

Some farmers suspect they are not getting their share.

“It’s a ploy; it’s marketing. They’re still after what they really want, and that’s to get the data for free,” said Billy Tiller, a farmer and founding member of the Grower Information Services Cooperative, a farmer cooperative focused on data.

The controversy centers on the emerging new market for digital information systems sold by Climate Corp., Deere and other vendors. Mounted onboard farm machines as they work the fields, the new tools assist with precision planting, improved fuel efficiency, cost-efficient fertilization and other improvements to modern agronomy. They also collect and feed back the farm data, which data vendors can repackage and sell.

Big data firms from the heartland could turn into big business for the handful of companies that dominate the market: equipment makers Deere and CNH Industrial and seed companies Monsanto and DuPont Pioneer. Monsanto has said Climate Corp. could become part of a $20 billion market, and DuPont Pioneer projects $500 million in annual sales over the next decade.

Deere’s careful recasting of contracts this spring highlights the dispute. Contracts for the agriculture giant’s telematics service no longer grant Deere open-ended rights to use all the data it collects for unspecified “business purposes.” Instead, they grant Deere the right to use data only “as needed to provide telematic services.”

But in a separate document Deere provides to customers—the company’s data services and subscriptions policy—Deere declares it is free to use all the data it collects, however it sees fit, so long as it strips away personally identifiable information.

So far, farmers have been willing to share data to gain specific details on farm conditions, and companies have said they do not intend to pay farmers for data. But with big data companies planning to sell customer-made seeds or peddle precise crop production estimates, farmers now want a share of the value stream.

The American Farm Bureau Federation in January adopted an official policy calling for more transparency and farmer control. Farm Bureau officials met with several data companies this spring to drive home the point.

“Transparency is the golden rule of this data privacy conversation,” said AFBF economist Matt Erickson.

Company responses vary. Climate Corp. rewrote parts of its privacy policy this month, retaining the right to use farm data, promising not to use it to speculate on commodities prices and stating that it will obtain explicit consent for uses other than making improvements to Climate Corp.’s own products.

“We believe our policies are both responsive to farmer concerns and lead the industry in the clarity with which we outline how we will and won’t use farmer data,” CEO David Friedberg said in a statement.

CNH Industrial this month updated its user agreements to explicitly state that farmers own their agronomic data.

“I think the data policies from a number of the companies will evolve rapidly over the next few years,” said Dave Larson, CNH vice president for agricultural equipment portfolio strategy. “The market’s going to push it.”