ORLANDO, Fla. — Shoppers who think all organic milk comes from cows lazily munching away on verdant pastures in bucolic settings may need to think again, some consumer groups warn.

As the organic carton label rises in popularity, a cadre of policy groups has amplified its criticism of two large organic milk suppliers.

One outspoken critic, the Cornucopia Institute, a Wisconsin-based farm policy group, has filed a string of federal complaints against Horizon Organic and Aurora Organic Dairy, claiming the large suppliers run industrial-style feedlots that fall short of organic standards.

Last week, Cornucopia registered a complaint against Horizon with the U.S. Department of Agriculture, alleging that the company does not offer sufficient grazing time on pastures at two of its large dairy farms.

That filing comes on the heels of a separate complaint to the USDA filed by Cornucopia against Aurora Organic, alleging in July that Aurora isn't giving its more than 3,300 dairy cattle on a Texas farm enough grazing time or space and is using questionable feed.

USDA spokeswoman Joan Shaffer said the agency is reviewing the July filing and studying the August filing. The agency is investigating a third complaint filed last November by Cornucopia against a large Aurora farm in Colorado, Shaffer said. A separate complaint against Aurora in 2005 resulted in no action.

Both Horizon and Aurora say the criticisms of their farms are without merit and point out that their dairies are fully certified organic and meet USDA standards. Federal regulations call for organic dairy cows to be free of hormones and antibiotics, given organic feed and allowed access to pasture.

"We do meet the organic regulations, and as it is today, we already go above the standards," said Horizon spokeswoman Molly Keveney, adding that about 80 percent of the company's milk comes from family farms, the vast majority of which have 500 cows or fewer.

Aurora spokeswoman Amy Barr called the complaints against her company's farms "absolutely bogus." She added: "We're up to the letter of the law and beyond in every case."

USDA doesn't specify how long cows must graze on pasture and is considering making the regulations more precise based on recommendations from the National Organic Standards Board, an advisory board. Horizon said it would support more stringent grazing standards.

Concerns about conditions at large farms run by Horizon and Aurora have prompted the Organic Consumers Association, a Minnesota-based advocacy group, to call for a boycott of the two organic milk producers.

"Consumers are being defrauded, and honest organic dairy farmers are being put at a disadvantage," said Ronnie Cummins, director of the Organic Consumers Association.

As large retailers like Wal-Mart expand their organic offerings, critics are calling for increased scrutiny of large organic suppliers who may be tempted to skirt the law in order to meet surging demand.

"What we're trying to counter right now is a corporate hostile takeover of organics," Cornucopia co-founder Mark Kastel said.

Organic milk, which sells for a premium over conventional milk, is one of the fastest-growing organic products. In 2005, sales of organic milk and cream totaled $1.1 billion, a 25 percent increase over the previous year, according to the Organic Trade Association.

Horizon, which is based in Colorado but owned by Texas-based dairy giant Dean Foods, accounts for nearly half of the organic milk sold in the United States.

Colorado-based Aurora provides its milk to leading retailers, which sell it under their private labels. Those include High Meadows brand, Wal-Mart's Great Value brand and Kirkland Signature at Costco.

So far, grocery stores that carry Horizon and Aurora milk are keeping it on their shelves. Wal-Mart said the two dairies follow the organic certification process and meet USDA standards.

"We believe that both organic and conventional agriculture provide safe, healthy and sustainable products for customers, and we believe it's up to our customers to choose which type of product they want to buy," Wal-Mart spokeswoman Karen Burk said in a statement.