Antibiotics afield.

Livestock farmers usually feed their animals small doses of antibiotics to promote growth. The livestock typically shed a large share of the drugs--up to 90 percent--in their feces, leading some researchers to question whether using this manure as fertilizer might lead to the uptake of antibiotics by crops or to the development of disease-resistant microbes. New studies validate both concerns.

Kuldip Kumar, Satish C. Gupta, and their colleagues at the University of Minnesota in St. Paul applied swine manure to greenhouse softs used to grow corn, green onions, and cabbage. Some of the soils were fertilized with manure from animals raised without growth-promoting drugs, the rest with manure from pigs treated with the growth-promoting antibiotic chlortetracycline.

In the drug-suffused soil, all three crops absorbed substantial amounts of chlortetracycline into their aboveground tissues, the researchers report in the November-December Journal of Environmental Quality.

Clay, a component of most soils, can firmly bind antibiotics. In tests with crop soils seeded with several types of bacteria, including Salmonella, the Minnesota scientists found that the antibiotic tended to persist in a crop's root zone and continued to kill microbes there. Indeed, the researchers note, this may explain the elevated concentrations of antibiotic-resistant bacteria that other scientists have observed in softs fertilized with manure.