WASHINGTON—A campaign is calling on Starbucks Corp., one of the world's largest coffee chains, to exclusively serve organic milk fed from cows that are not fed genetically-modified (GMO) crops.

The organization behind the movement, GMO Inside, said its social media campaign prompted General Mills' plans to drop genetically-modified ingredients from basic Cheerios.

Earlier this week, GMO Inside launched a campaign website to pressure Starbucks to rely exclusively on organic, non-GMO milk. Starbucks, which boasts more than 20,000 stores in 62 countries, didn't respond to requests for comment on the campaign. GMO Inside has asked consumers to sign a petition that is directed to Starbucks CEO Howard Schultz.

"GMOs have yet to be proven safe for humans, animal, or the planet by independent long-term studies," declared GMO Inside, which also noted GMOs tolerated to withstand weedkillers have led to the emergence of "super weeds," requiring the application of more rather than fewer herbicides to crops.

A representative for the dairy industry disagreed with the implication that GMOs are unsafe to eat.

"All the scientific evidence that has been put forth indicates there are no human health concerns or environmental concerns that are explicit or exclusive to the growing of genetically-modified crops or vegetables," said Chris Galen, spokesman for the National Milk Producers Federation, which represents dairy producers.

Greg Jaffe, director of the Biotechnology Project with the Center for Science in the Public Interest (CSPI), cited international consensus that GMO crops are safe to eat. A number of organizations have reached such a conclusion, including the U.S. Food and Drug Administration (FDA), European Food Safety Authority (EFSA) and National Academy of Sciences among others.

Organic Milk Supply

Galen pointed out a practical problem with the Starbucks campaign: that the coffee giant would be unable to serve only non-GMO organic milk even it wanted to.

He said organic milk production represents roughly 3% of all milk production in the United States. According to the Organic Trade Association, the organic dairy category comprises 6% of the total dairy market.

"It's a niche product at the production level. It's a niche product at the consumption level," Galen said of organic milk. "The bottom line: I don't think they [Starbucks] would be able to fulfill that commitment anytime in the near future … because the organic milk supply is not there."

Galen also said organic production is more expensive, with less yields on crops and less-productive cows.

"It's a less-productive system of making food or in this case milk than in what we see in other forms of dairy production," he said.

In a 2013 survey, the Organic Trade Association said the category is growing slowly, indicating that rising feed costs are a potential barrier to conventional farmers growing organic crops. But increasing consumer interest in non-GMO foods could prompt more retailers to embrace organic ingredients in spite of potentially higher costs.

According to GMO Inside, Pret a Manger, a quick-service food and coffee chain with locations in the United States, Hong Kong, United Kingdom and France, has made a commitment to serve only organic milk in its stores. GMO Inside said Pre a Manger only serves organic dairy and soy. The company did not immediately return a phone call seeking comment.

The Starbucks campaign was unveiled on the heels of General Mills' announcement earlier this year that its original Cheerios will no longer be made using GMOs. The company has no plans to remove GMO ingredients from other U.S. cereal products, including other Cheerios varieties.

"For our other cereals, the widespread use of GM seed in crops such as corn, soy or beet sugar would make reliability moving to non-GM ingredients difficult, if not impossible," General Mills said.

Over the years, U.S. farmers have become increasingly reliant on certain GMO crops such as corn. Gaffe said American farmers grow eight distinct GMO crops: alfalfa, canola, corn, cotton, papaya, soybean, squash and sugar beets. According to the Center for Food Safety, up to 85% of corn is genetically modified, while the percentages are even greater for soybeans (91%) and cotton (88%).