One of the most common weed killers used on suburban lawns in the United States depresses reproduction in lab mice. The effect is most severe at lower doses, according to new research, including concentrations similar to what might be found in the environment.
When toxicologists test chemicals for health effects with lab animals, they generally begin at lethal doses and decrease the dose until they see no effect. But many animal and human studies show responses that do not increase steadily with dose as expected, and some of them show effects to be strongest at doses lower than what is generally tested. With chemicals that affect the endocrine system, this is likely because hormones are geared to respond to minute differences in chemistry, endocrinologists say.
For that reason, toxicologists María Fernanda Cavieres of the Universidad de Valparaiso, Chile, and James Jaeger and Warren Porter of the University of Wisconsin, Madison, made sure to include very low doses when they examined an off-the-shelf weed-and-feed mix used widely by homeowners and lawn-care companies. The mix consisted of three chemicals common to many commercially available herbicides: 2,4-D, mecoprop, and dicamba. Pregnant female lab mice received the brew in their drinking water at four concentrations, from 400 parts per million (ppm) down to less than 0.04 ppm--equivalent to one drop of pesticide in 500 bathtubs of water, Porter says.
The average litter size was reduced by 10% to 20% at all doses compared with controls, but it was most reduced at low doses. The researchers saw the same U-shaped dose-response pattern in the number of embryos implanted in the uterus, leading them to speculate that the herbicide mix interferes with hormones controlling implantation. The study will appear in the November issue of Environmental Health Perspectives. Porter says the results call for closer attention to effects on humans of pesticides at low levels found widely in the environment.
It is hard to know what accounts for the mixture's U-shaped dose-response curve without knowing the individual dose-response curves of each of the three chemicals, says toxicologist Edward Calabrese of the University of Massachusetts, Amherst. Although the study's methodology is somewhat unusual, he adds, the findings contribute to a growing appreciation that dose-response curves don't always follow a straight line.