Rounding up resistance: weed sacrifices seeds to put up with a herbicide.

Use of a common agricultural herbicide is driving evolution of at least one weed species, a new study finds. In response to applications containing glyphosate, the tall morning glory (Ipomoea purpurea) grows increasingly impervious to that chemical, while sacrificing a measure of its fertility.

Morning glories are popular among gardeners but unwelcome on farms. There, they proliferate rapidly, shade crop plants, steal nutrients, and clog harvesting machinery.

To suppress the weed, farmers spray glyphosate-based herbicides such as RoundUp on about 80 percent of U.S. soybean fields, up from 5 percent in 1991.

To investigate the ecological consequences of glyphosate use, geneticists Regina Baucom and Rodney Mauricio of the University of Georgia in Athens examined offspring of 32 tall morning glories that had sprouted uninvited in a Georgia soybean field.

In a field nearby, they sprayed some of the offspring with RoundUp and kept others in a herbicidefree environment.

On the basis of the number of seeds each plant produced when exposed to the herbicide, the researchers conclude that the plants inherited varying degrees of tolerance for glyphosate. Such genetic variability invites natural selection that could give rise to increasingly herbicide-tolerant weeds, they say.

Among the unsprayed plants, those from lineages with relatively high glyphosate tolerance were up to 35 percent less prolific, on average, than were more-susceptible plants. That reproductive drawback suggests that glyphosate-tolerant plants prosper only where the herbicide is used heavily, Baucom reasons.

If glyphosate is used on large-enough areas of cropland, natural selection will favor the widespread evolution of tolerance, she says. Glyphosate use already exceeds that threshold, according to Baucom's rough estimate.

Baucom and Mauricio report their findings in an upcoming Proceedings of the National Academy of Sciences.

The study's implication, says John Stinchcombe of Brown University in Providence, R.I., is that "if spraying is common enough, increased tolerance and decreased effectiveness of the herbicide will result, despite substantial costs to the weeds of being tolerant."

The case for an evolutionary trade-off would be more airtight if the researchers had tried germinating seeds rather than just counting those that looked viable, says James H. Westwood of Virginia Polytechnic Institute and State University in Blacksburg. Nevertheless, he says, the new study "underscores a message that weed scientists have been saying for years: Growers must not rely exclusively on a single herbicide.

"If everyone relies on glyphosate as a sole tool, the day will come when it won't be an effective herbicide anymore," Westwood continues. Periodically using a different herbicide or cultivating some fields without any herbicide might slow or reverse the evolution of glyphosate-resistant weeds, he suggests.