Kurt Schweitzer, president of Keystone Fruit Marketing, is proud that his company’s Vidalia onion farm in Walla Walla, Wash., which supplies Costco, has been certified. He said the workers had received extensive training and were paid at least $15 an hour. “This really shows these workers that their jobs are important,” he said.

“I love the whole concept that Costco or Walmart are going arm-in-arm down the aisle, saying we have a responsibility and want to work with you, rather than just saying, give us a once-a-year audit and we know things are fine, and we’ll put our blinders on,” Mr. Schweitzer said.

In Vermont, not every farmer is as enthusiastic. Many dairy farmers say they feel beleaguered — most of all from seesawing milk prices, but also from environmentalists, animal rights activists and now labor groups.

Tom Gates, a manager at St. Albans Creamery, a cooperative of about 430 farms in Vermont that supplies Ben & Jerry’s, said the farmers shared the ice cream maker’s vision of treating workers well. He notes that at most dairy farms, the owners work side by side with the migrant workers. “Some groups think farmers don’t value their labor — nothing could be further from the truth,” he said.

Migrant Justice, Ben & Jerry’s and the farmers still have many issues to thrash out. On farms with only two or three migrant workers, for example, it might not be so easy to give workers one day off each week because cows need to be milked three times a day.

But Victor Diaz, a dairy laborer from Mexico who works 12 hours a day, seven days a week, expressed optimism that things would soon get better.

“Ben & Jerry’s is feeling the pressure from many people,” he said. “If it wasn’t feeling all that pressure, everything would just remain the same.”