Under pressure from supermarket chains and their customers, California cows are going drug-free, the Los Angeles Times reports.

The giant Central Valley dairy co-op that produces 4 of every 10 glasses of milk drunk by Californians is phasing out the use of a synthetic bovine growth hormone that increases how much milk cows produce.

Despite evidence that the rBST hormone doesn’t harm humans, California Dairies Inc. said its biggest customers such as Vons and Safeway didn’t want it in the cows. The co-op also supplies brands such as Foster Farms, Knudsen Farms and Producers Dairy.

The demand for milk from these hormone-free cows is part of a nationwide consumer swing toward products that are either labeled organic or are perceived to be more natural. “That’s a good decision as far as I’m concerned,” said David Callahan, a self-employed artist from Eagle Rock who was shopping in Glendale. “The milk I buy is always free of hormones and preferably is organic. It’s a big issue with me.”

The dairy co-op has to comply if it wants to keep its customers, Chief Executive Richard Cotta said. “Demand for this milk has exceeded our ability to supply it.” The phaseout of the hormone made by Monsanto Co., the St. Louis-based chemical giant, will be in effect Aug. 1.

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