California already has some of the most rigorous laws in the country regarding farm animal welfare, but the state’s rules surrounding pigs, chickens and cows could get even stricter.

On Tuesday, the Humane Society of the United States introduced a ballot initiative called the Prevention of Cruelty to Farm Animals Act, which calls for a requirement that all pork and veal sold in California be produced without restrictive crates, and that all eggs produced and sold in the state be cage-free. It would make California the only state other than Massachusetts, which passed similar legislation last year, to have such regulations on farm animal welfare.

“California is a bellwether state that sends a very strong signal to the rest of the nation,” said Paul Shapiro, vice president of policy engagement at the Humane Society of the U.S.

The Humane Society and other welfare groups that introduced the legislation said that the goal is to strengthen the state’s existing regulations: Approved by voters in 2008 and implemented in 2015, Proposition 2 requires that hens, pigs and calves in California farms must be able to stand up, lie down and fully extend their limbs. A follow-up law enacted by the state Legislature, AB1437, requires that those same standards be applied for any eggs sold in the state.

“In the decade that has transpired since (Prop. 2 passed), not only have other states like Massachusetts implemented stronger laws, but the marketplace has largely moved beyond the Prop. 2 standard,” said Shapiro. He also said the proposed law, which has more enforceable language, would improve the welfare of egg-laying chickens since after Prop. 2 was passed egg producers can still use cages, but they just must allow more space for each bird.

“Most egg-laying chickens (in California) are still confined in cages to this day,” said Shapiro.

United Egg Producers, the country’s largest egg industry group, did not take a position on the ballot initiative.

“Our farmer-members support all types of hen housing,” said Chad Gregory, United Egg Producers CEO and president, in a statement. “Changes in hen housing are complex and costly, and they require close collaboration with customers. Our focus remains on proper management of hen health and well-being, and meeting or exceeding all food safety requirements.”

Richard Blatchford, an assistant cooperative-extension specialist at UC Davis’ department of animal science, said the proposed law appears to have more enforceable language than Prop. 2, but he noted that cage-free systems for hens are not without their flaws, even in Europe where they’ve been used for a while.

“Aggression (among hens) tends to be really high. You tend to have higher mortality overall,” he said.

However, the biggest potential impact of the initiative could be on pork. California does not have a large pork industry, and most of the pork sold here comes from out-of-state producers who would have to comply with the regulations when selling their product here.

As of 2012, an estimated 85 percent of breeding sows in the United States were kept for the majority of their lives in gestation crates, a type of housing that animal welfare advocates say is cruel because it restricts the ability of the pigs to move and sometimes even lie down. But according to Jim Monroe, spokesman for the National Pork Producers Council, many members of the pork industry favor gestation crates because they say they prevent aggression among sows, injury and mortality.

“Any change in production practices should be based on signals from the marketplace,” said Monroe. “Pork producers, not activists, know far better what’s good for their animals.”

The marketplace will dictate some changes to the national pork industry, however, now that large food-service companies like McDonald’s, Subway, Denny’s and many others have pledged to stop serving pork raised in confinement systems in the next five to 10 years.

Monroe said there would be “significant cost” when changing from one housing system to another. One estimate predicted that a total national ban on gestation crates and the resulting transition would cost the pork industry between $1.9 billion and more than $3.2 billion.

“Not surprisingly, those costs can be passed along from the producer to the consumer,” said Monroe, who pointed to a 2016 Cornell University study that showed California’s ban on battery cages added 49 cents to the cost of a dozen eggs.

Other studies show a potentially smaller impact of a ban on gestation crates. The Humane Society cites a study from the Department of Agricultural Economics at Oklahoma State University that showed that the cost of building new housing would be the same for farmers building a gestation crate system or a group housing system, which allows sows more room to move around.

The next step is for organizers to get more than 365,000 signatures within 180 days in order for the initiative to be placed on the statewide ballot in November 2018.

The Humane Society’s Shapiro said he is confident that the initiative can pass in California, where Prop. 2 passed by 64 percent of the votes and where voters have approved many other animal protection laws.

Blatchford, a member of the Center for Animal Welfare at UC Davis, said the proposed law could have a large impact on out-of-state egg producers since many California producers are already cage-free.

“I had a feeling we were heading that way in advance of the rest of the country anyway,” Blatchford said.

Tara Duggan is a San Francisco Chronicle staff writer. Email: tduggan@sfchronicle.com Twitter: @taraduggan