The region's biggest dairies are rushing to rid their bottled milk of artificial growth hormones in a bid to draw back customers who have switched to organic milk.

Dean Foods, which operates the Garelick plant in Franklin, and H.P. Hood, which operates a plant in Agawam, are demanding that regional farmer cooperatives supply them with milk from cows that haven't been injected with synthetic hormones that boost milk production.

Over the next few weeks, jugs of Hood and Garelick milk with labels pledging "no artificial growth hormones" should start filling supermarket shelves — a strategy the dairies hope will satisfy the chief concern of consumers going organic and do so at less than half the retail price of organic milk.

"The phenomenal success of organic milk, with growth rates of 20 percent or more, is driving our demand for milk from cows not treated with artificial growth hormones," said John Kaneb, the chief executive of Chelsea-based Hood.

Under federal standards, organic milk is from cows not treated with synthetic hormones or antibiotics; these cows are fed only organically grown food and have access to pastures.

By halting the use of synthetic hormones, which are marketed under the brand name Posilac by Monsanto Corp. of St. Louis, Hood and Dean are bringing their milk one step closer to the organic standard. Their milk is also screened for antibiotics.

Smaller dairies have previously marketed conventional milk produced without synthetic hormones, but the changes taking place now in southern New England represent the first large-scale conversion in the country. If more dairies jump on board, it could be a tipping point in the long-running debate about the safety of using synthetic hormones to spur milk production.

"Even though conventional milk is completely safe and POSILAC is completely safe, some people don't feel comfortable with it," said Marguerite Copel, a spokeswoman for Dallas-based Dean Foods. POSILAC is a trademarked bovine protein product marketed by Monsanto.

Copel said Dean intends to see how consumers react to its new Garelick milk before expanding its hormone-free operations. Dean operates 100 dairy plants nationwide; eight will now be operating with milk from artificial-hormone-free cows.

Whole Foods Markets sells organic milk and a store-brand milk produced from cows not treated with POSILAC. At the chain's Boston store near Symphony Hall, most consumers yesterday were opting for the less expensive store-brand milk.

But Nasser Hussain, a teacher from Boston, said he buys organic milk largely because he opposes industrial farming. "Organic to me means they let the cows out of the pen," he said.

Monsanto won Food and Drug Administration approval for POSILAC in 1993. The product, also known by the acronym rBST, is a synthetic version of the milk-inducing growth hormone that cows produce naturally. The company says a third of the nation's dairy herd is injected with POSILAC.

Even though the FDA and other researchers say POSILAC is perfectly safe for humans, many consumers remain troubled by its use. They fear that synthetic hormones cause cancer or premature development in children, and many have shifted to organic milk to avoid exposure. Other countries have banned POSILAC — though primarily because of harmful side effects in cows.

Sales of organic milk represent only about 2.4 percent of total milk sales, but they are growing quickly, particularly in the Northeast. Nationally, sales of organic milk have nearly doubled over the last three years to $1.1 billion, according to the Organic Trade Association in Greenfield, despite the fact that organic milk costs $6 to $7.50 a gallon, compared with $2.50 to $3.50 for conventional milk. By contrast, sales of conventional milk have been flat.

Wal-Mart recently showed just how big the organic milk business is becoming by unveiling a private-label product that will cost less than most branded organics. Even Dean and Hood produce organic milk. Dean, in fact, owns Horizon Organic of Boulder, Colo., the nation's largest organic milk producer and owner of Organic Cow milk.

Monsanto sued Oakhurst Dairy in Maine in 2003 after it put labels on its milk saying its farmers pledged not to use artificial growth hormones. The case was settled when Oakhurst added a statement to its labels saying the FDA has found "no significant difference in milk from cows treated with artificial growth hormones."

Monsanto isn't saying what it plans to do in response to Dean and Hood. A spokesman for the company said the two dairy processors are depriving farmers of an FDA-approved technology.

There is no test that dairies can use to distinguish between artificial and naturally occurring growth hormones, which is why processors ask farmers to sign affidavits stating that they don't use artificial growth hormones.

The big farmer cooperatives in the Northeast are not telling their members to stop using POSILAC, but they are warning that their milk may have to be used in lesser-value dairy products.

Some dairy processors plan to pay a slight premium for milk from untreated cows, but farmers who are using POSILAC fear any increase in price will be more than offset by a dropoff in production.

Richard Woodger, a dairy farmer in Granville, said his gross income is already off $20,000 this year because of depressed milk prices. If he takes his cows off POSILAC, he said, his production is likely to drop by 25 percent.