Food safety: From Mexican farm, to Costco, to your plate

American concerns about food safety in imports have created a whole new ethic among Mexican farmers eager to sell in the US.

The tour at the sprawling Aguilares Ranch is about to begin. Hair pulled back. Check. Watches and bracelets off. Check. The employers are so meticulous about the sanitary standards of these lettuce, broccoli, celery, and garlic fields that a visitor wearing a wedding ring is asked to remove it mid-circuit.

And you, global consumer, should be relieved. The vegetables grown and packaged at this exporting farm in the vegetable heartland of Mexico's Guanajuato State may have been, or could one day be, consumed by you. They go east to Europe, west to Japan, and north to Canada and the United States. The iceberg lettuce basking in a field on a recent sunny day could end up in an Olive Garden salad bar or a Big Mac; the flowering cauliflower might roll down the checkout belt at Costco or Wal-Mart.

The problem is that not all Mexican growers are required to follow the same rules, says Miguel Usabiaga, driving a pickup across his 1,800-acre ranch past workers adding gypsum enrichment to the soil with John Deere tractors and cleaning out ditches. And one bad tomato can trigger panic that affects the nation's entire growing industry. "We are very worried about this," Mr. Usabiaga says. "If there is one problem with an item in Mexico, all of Mexico gets hurt."

In Pictures: The foreign and domestic food chain

Political commentators have long noted that there are two Mexicos when it comes to wealth distribution. So, too, are there two Mexican agricultural worlds, with small, often uneducated farmers producing for the local market, and bigger agribusinesses growing for big chain stores in Mexico and export to the US and beyond.

Most exporters follow standards dictated by their clients abroad, and they say the guidelines for everything from pesticides to irrigation water purity are as good if not stricter than those facing farmers in the US. But neither the US nor Mexican governments oversees any of this.

At Aguilares Ranch, fields are fenced with chicken wire that goes to the ground, so that no animals can get through. Each day the fences are checked for holes. Domestic produce farms in Mexico are often left wide open, where animals can roam freely. You will see no donkeys, horses, or dogs frolicking in the garlic fields here, or small children for that matter.

All employees, like Monica Rodriguez – who, on a recent day with 70 other fieldworkers, bundles cauliflower leaves to prevent sun damage – are trained for at least a half day on basic sanitary measures, including no painted nails, no glass bottles, and no sharing water vessels. "It is strict here," says Ms. Rodriguez. She and her fellow fieldworkers earn an average of $16 a day, which is well above Mexican minimum wage.

Portable toilets are set up around worker sites, with basins and strict rules outlining how to wash hands before heading back to the fields. There must be one toilet for every 25 workers, no more than five minutes' walking distance apart. At farms with no such requirements, workers often urinate on the edge of fields. Here, all of the water to hydrate crops comes from a deep well source that is also filtrated. It is tested at both the source and the point of irrigation.

The cost is high. All of this requires $150 to $200 per acre, estimates Usabiaga, whose family produces the Mr. Lucky brand that has been in his family since the 1960s. They are one of the biggest growers in Mexico.

But smaller growers who also export follow similar guidelines. Alejandro Aboytes, the president of Las 5 Estaciones, near Celaya, says his farm also has fences and separate work areas for employees, and uses well water for irrigation, all standards he says are set and audited by the certifier PrimusLabs.com, based in Santa Maria, Calif. He says the costs are high, but it is all that sets apart exporters targeting huge markets from peasants supplying local markets. "You do not just have to be the biggest guy to do this."

To that end, Mr. Aboytes, who also works with various grower organizations, is trying to lobby the state to demand certification of all growers, exporting or not. "They are a danger to our business," says Aboytes, who primarily grows baby carrots that end up in Wal-Marts in the US. "If there is any problem in Mexico, we are all going to suffer for that."

Mexico offers a certification option, but it doesn't require it. The US does spot checks – on farms and at the border – but requires no standards certification. The main exporters face standards dictated by third-party auditors demanded by American clients. But some growers, they say, do not have direct relationships with the end client in the US. They sell to brokers, and some are more lax than others. That, Usabiaga says, is where the danger to all looms.

In Pictures: The foreign and domestic food chain