California has adopted what are said to be the strictest limits in the US on the use of antibiotics in animal husbandry.

The bill, signed into law by California Governor Jerry Brown, outlaws the routine use of antibiotics in livestock to prevent illness or to promote growth. 

Widespread concerns regarding the over-use of antibiotics, both in human medicine and animal rearing has led to the growth of so-called superbugs resistant to the drugs, and has prompted countries around the world to introduce limits on their use. 

The new Californian law, which comes into effect from January 2018, prohibits the use of antibiotics to boost growth in commercially raised livestock. Meat producers will only be allowed to administer the drugs with the agreement of a vet when animals are ill, or to prevent infections when there is an 'elevated risk.' However, they are forbidden from using the drugs "in a regular pattern."

Brown said the law "addresses an urgent health problem". He added: "The science is clear that the over-use of antibiotics in livestock has contributed to the spread of antibiotic resistance and the undermining of decades of life-saving advances in medicine.

"Recently, American poultry producers have shown leadership by voluntary committing to better husbandry practices and eliminating the subtherapeutic use of antibiotics. This is an example the rest of the livestock industry should follow."  

The new legislation has been met with little resistance from animal producers and farmers. "I think the bill is basically doing something that we in California have been doing all along, which is phasing out antibiotic use," Bill Mattos, president of the California Poultry Federation, which represents large poultry farmers, told Bloomberg. "It’s something that the industry is living with. We’re happy to get this bill the way it is, and I think we’re going to see more of this."

A number of manufacturers, retailers and foodservice operators doing business in the US have made announcements on the use of antibiotics this year amid rising consumer interest.

In March, McDonald's said it would within the next two years only be sourcing "chicken raised without antibiotics that are important to human medicine".

Tyson Foods said in April it would eliminate the use of human antibiotics in its US broiler chicken flock by the end of September 2017.

In May, Wal-Mart outlined its position on the use of antibiotics in animal husbandry. It argued antibiotics should be used "responsibly" – avoiding the use of "medically important" antibiotics and not using them for "growth promotion".

In June, Foster Farms added branded organic and antibiotic-free ranges to its portfolio in a bid to meet rising consumer interest in the categories.

A month later, US poultry processor Perdue Farms said it would cut the use of animal-only antibiotics as it plans to eliminate the use of all antibiotics from its chicken production.

"I think we’re seeing the marketplace change, and this legislation will continue to push it in that direction," Jason Pfeifle, public health advocate at the California Public Interest Research Group, a consumer group that supports the new law, told Bloomberg.