ANTIBIOTIC DOSE: Rampant use of antibiotics in livestock, combined with widespread use of manure as fertilizer, means some lettuce, corn, potatoes and other crops carry antibiotics. Image: ©iStockphoto.com/David T. Gomez
For half a century, meat producers have fed antibiotics to farm animals to increase their growth and stave off infections. Now scientists have discovered that those drugs are sprouting up in unexpected places: Vegetables such as corn, potatoes and lettuce absorb antibiotics when grown in soil fertilized with livestock manure, according to tests conducted at the University of Minnesota.
Today, close to 70 percent of all antibiotics and related drugs used in the United States are routinely fed to cattle, pigs and poultry, according to the Union of Concerned Scientists. Although this practice sustains a growing demand for meat, it also generates public health fears associated with the expanding presence of antibiotics in the food chain.
People have long been exposed to antibiotics in meat and milk. Now, the new research shows that they also may be
ingesting them from vegetables, perhaps even ones grown on organic farms.
The Minnesota researchers planted corn, green onion and cabbage in manure-treated soil in 2005 to evaluate the environmental impacts of feeding antibiotics to livestock. Six weeks later, the crops were analyzed and found to absorb chlortetracycline, a drug widely used to treat diseases in livestock. In another study two years later, corn, lettuce and potato were planted in soil treated with liquid hog manure. They, too, accumulated concentrations of an antibiotic, named Sulfamethazine, also commonly used in livestock.
As the amount of antibiotics in the soil increased, so too did the levels taken up by the corn, potatoes and other plants.
"Around 90 percent of these drugs that are administered to animals end up being excreted either as urine or manure," said Holly Dolliver, a member of the Minnesota research team and now a professor of crop and soil sciences at the University of Wisconsin-River Falls. "A vast majority of that manure is then used as an important input for 9.2 million hectares of (U.S.) agricultural land."
Manure, widely used as a substitute for chemical fertilizer, adds nutrients that help plants grow. It is often used in organic farming.
The scientists found that although their crops were only propagated in greenhouses for six weeks--far less than a normal growing season--antibiotics were absorbed readily into their leaves. If grown for a full season, drugs most likely would find their way into parts of plants that humans eat, said Dolliver.
Less than 0.1 percent of antibiotics applied to soil were absorbed into the corn, lettuce and other plants. Though a tiny amount, health implications for people consuming such small, cumulative doses are largely unknown.
"The antibiotic accumulation in plants is just another negative consequence of our animal agriculture industry and not surprising given the quantity fed to livestock," said Steve Roach, public health program director for the non-profit Food Animal Concerns Trust.
For highly processed plants such as corn, the drugs would most likely be removed, added Dolliver. But many food crops such as spinach and lettuce are not processed, only washed, allowing antibiotics to remain.
"Nobody particularly eats corn or soybean directly," said Satish Gupta, a University of Minnesota professor of soil science and study leader. "But there are crops I am much more worried about, like cabbage and lettuce, because these are leaves we eat directly and consume raw."
One finding that particularly worries food scientists is the accumulation of antibiotics within potato tubers. Tubers are an enlarged, underground stem that uptake and store nutrients from the soil. In crops like potatoes, carrots and radishes, it is the part humans eat.
"Since these tubers and root crops are in direct contact with the soil they may show a greater propensity for [antibiotic] uptake," said Gupta.
Health officials fear that eating vegetables and meat laced with drugs meant to treat infections can promote resistant strains of bacteria in food and the environment.