LOS ANGELES — Calories. Nutrients. Serving size. How about “produced with genetic engineering?”

California voters will soon decide whether to require certain raw and processed foods to carry such a label.

In a closely watched test of consumers’ appetite for genetically modified foods, the special label is being pushed by organic farmers and advocates who are concerned about what people eat even though the federal government and many scientists contend such foods are safe.

More than just food packaging is at stake. The outcome could reverberate through American agriculture, which has long tinkered with the genes of plants to reduce disease, ward off insects and boost the food supply.

International food and chemical conglomerates, including Monsanto Co. and DuPont Co., have contributed about $35 million to defeat Proposition 37 on the November ballot. It also would ban labeling or advertising genetically altered food as “natural.” Its supporters have raised just about one-tenth of that amount.

If voters approve the initiative, California would become the first state to require disclosure of a broad range of foods containing genetically modified organisms, or GMOs. Food makers would have to add a label or reformulate their products to avoid it. Supermarkets would be charged with making sure their shelves are stocked with correctly labeled items.

Genetically altered plants grown from seeds engineered in the laboratory have been a mainstay for more than a decade. Much of the corn, soybean, sugar beets and cotton cultivated in the United States today have been tweaked to resist pesticides or insects. Most of the biotech crops are used for animal feed or as ingredients in processed foods including cookies, cereal, potato chips and salad dressing.

Proponents say explicit labeling gives consumers information about how a product is made and allows them to decide whether to choose foods with genetically modified ingredients.

“They’re fed up. They want to know what’s in their food,” said Stacy Malkan, spokeswoman for the California Right to Know campaign.

Agribusiness, farmers and retailers oppose the initiative, claiming it would lead to higher grocery bills and leave the state open to frivolous lawsuits. Kathy Fairbanks, spokeswoman for the No on 37 campaign, said labels would be interpreted as a warning and confuse shoppers.

“It’s not necessary. Worse, it leaves people with the impression that there’s something wrong with the food. That’s not the case,” she said.

The government approves genetically engineered plants and animals on a case-by-case basis, and the U.S. Department of Agriculture restricts the use of GMO crops that might harm other plants. The Food and Drug Administration can only require labeling if a genetically altered food is different — in taste, for example — from its non-engineered version or known to cause allergies.

The World Health Organization has said no ill health effects have resulted from GMO foods currently on the international market. The American Medical Association sees “no scientific justification for special labeling of bioengineered foods” but favors stricter testing before they hit stores.

Still, some consumers are wary and are increasingly demanding to know what’s on their dinner plates. With California a trendsetter on other issues, whatever happens in the nation’s most populous state could spill onto the national stage.

Already, at least 19 states this year have introduced GMO labeling bills, but none passed.

Alaska, with its dominant wild salmon industry, requires labels on genetically engineered fish, though none is currently on the market. Maine allows GMO-free products to be labeled as such.

The FDA is evaluating a petition to label genetically engineered foods nationwide; the group spearheading that effort is separate from California’s initiative.

The push comes as genetic engineering is expanding beyond traditional crops. Last year, agricultural regulators approved the planting of genetically modified alfalfa, angering organic farmers who feared cross-contamination. An application is pending on an Atlantic salmon that has been genetically manipulated to grow twice as fast as a regular salmon.

California’s ballot initiative would require most raw foods such as fruits and vegetables and processed foods by 2014 to bear the label “partially produced with genetic engineering” or “may be partially produced with genetic engineering.” Meat and dairy products would be exempt even if the animals are fed with biotech grains. Organic foods, restaurant meals and alcohol are also excluded.

Supermarkets and other retailers would be in charge of making sure products for sale are properly labeled. Spot checks would be carried out by California Department of Public Health inspectors. The nonpartisan California Legislative Analyst’s Office estimates that it could cost up to $1 million a year to regulate.

The initiative also allows individuals or groups to sue if they find food has been mislabeled. The California Grocers Association said supermarkets will do their best to comply if the measure passes, but noted it would be taxing on store owners. The group also fears being the target of lawsuits.

Association President Ronald Fong said it will be a burden for grocers to check the label of every box and keep track of their efforts in case they get sued.

“It’s going to be a complete paperwork nightmare,” he said.

The Center for Science in the Public Interest, which advocates for food safety, has not taken a side on the initiative. But Gregory Jaffe, the group’s biotechnology director, favors giving the government more regulatory power over biotech crops.

“The solution is not labels,” he said.