Herbicide weeds out mice in the womb. (Lawn Agent Cues Embryo Shortfall).

Having a lush, dandelion-free lawn may come at an unexpected price. According to a new report, minuscule amounts of over-the-counter weed killers impair reproduction in mice and therefore might also affect other animals, people included.

Previous studies have assessed the toxicity of high doses of single herbicide ingredients, but they haven't tested the chemical cocktails typical of commercial formulations, says toxicologist Warren Porter of the University of Wisconsin-Madison.

He and his colleagues measured the toxicity of an unnamed brand of weed-and-feed mix. It included the herbicides 2,4-D, mecoprop, and dicamba--one or more of which is in each of 1,500 commercial weed-killing formulations. Though herbicides target plants, says Porter, many attack basic biological functions common to both plants and animals.

The researchers fed solutions laced with the weed-killing mixture to four groups of laboratory mice throughout their pregnancies. Concentrations of 2,4-D, for example, ranged from 400 parts per million parts water in the strongest solution to 0.04 parts per million in the weakest. This weakest concentration is lower than some of those that turn up in the environment.

The scientists found that compared with mice receiving herbicide-free solutions, herbicide-fed mice had as few as 80 percent the number of pups. Surprisingly, mothers exposed to the lowest herbicide doses, in general, produced the smallest litters of mice. The findings are reported in an upcoming Environmental Health Perspectives.

Safety tests for chemicals generally start with high exposures and sequentially test different groups of animals with lower exposures until few or no toxicity effects are visible. "The dogma is, As the dose goes up, the effect gets worse," says Fred vom Saal at the University of Missouri in Columbia.

However, vom Saal notes, an increasing body of research shows different patterns. Some hormones, for example, cause the greatest effects at low and high doses, with little effect at intermediate exposures, This dose-response pattern, known as a U-shaped curve, has also been seen with hormonelike pollutants. Toxicologists are currently at a loss to explain it, says vom Saal.

Porter's team speculates that their mouse results resemble the low end of a U-shaped curve. They conjecture that small, but not large, concentrations of herbicides might interfere with hormones that control embryo implantation.

The researchers may be seeing a complicated mixture of responses by the body, says Ana Sore of Tufts University School of Medicine in Boston. "It's counterintuitive, but that doesn't mean the effect doesn't exist," she says.

"We don't have the answers to how this works," says vom Saal. However, he adds, toxicologists should redesign their standard experiments. To estimate the environmental exposure that poses no risk, they should collect precise data on very low as well as high doses.