Governor of California Jerry Brown signed a law Saturday banning the use of antibiotics to promote growth in livestock such as cows, chickens, and pigs. As of January 1, 2018, SB 27 will prohibit the use of antibiotics on livestock “unless ordered by a licensed veterinarian through a prescription or veterinary feed.” To date, this is the strictest US law limiting the use of antibiotics in livestock.

The overuse of antibiotics has led to the spread of dangerous drug-resistant superbugs, such as MRSA. According to the Centers for Disease Control and Prevention (CDC), antibiotic-resistant superbugs kill 23,000 Americans each year, and cause infection in 2 million. The rise of these superbugs is due largely to the overuse of antibiotics.

“Almost 80 percent of antibiotic usage in the United States is for livestock,” said Dr. David Agus in an interview with CBS This Morning. “That allows bacteria to become drug resistant and those bacteria can spread through the water, the air, through insects and from animal to human touch and actually make us sicker.”

This is a critical step in a much bigger movement of moving away from overuse of antibiotics in agriculture. Companies such as McDonald’s, Chipotle, and Panera have all begun touting under the tactic of selling chicken raised without antibiotics. Wal-Mart, Foster Farms, and Perdue are all taking similar steps to cut back on antibiotic use.

Antibiotics are used to promote the growth of livestock because they enable farmers to fatten animals on food they’re not evolved to eat. For example, in beef production, six months after a calf has been weaned, it’s brought from the pasture to a backgrounding pen where it’s taught to feed on corn. Said The Omnivore’s Dilemma author Michael Pollan in an interview with PBS, “You start giving them antibiotics, because as soon as you give them corn, you’ve disturbed their digestion, and they’re apt to get sick, so you then have to give them drugs.”

This high-calorie diet allows cows to be slaughtered at 12-13 months rather than the 18-24 months it takes to raise cows on grass and alfalfa.

In October of last year, Governor Brown vetoed a less-stringent version of this bill, which included provisions regarding growth promotion and veterinary oversight. The bill signed on Saturday went further by allowing regular use of antibiotics for prophylactic purposes. Brown deemed an earlier version of the bill “unnecessary since most major animal producers have already pledged to go beyond the FDA standard,” which the older version of the bill simply codified.

Under these new provisions, the law has more teeth and closes off what critics call loopholes in the FDA standard. Essentially, in the FDA standard, there were overlaps between dosages approved for growth promotion and disease prevention.

In an interview with Modern Farmer, West Coast Director for the Center for Food Safety (CFS) Rebecca Spector described the new law as “a huge step toward preserving medically important antibiotics in the U.S. and setting up restrictions on the use of these drugs.”