So-called "superweeds" resistant to the herbicide glyphosate also may be more resistant to disease pressures.

Those are the findings of research conducted by weed science professor Bill Johnson, associate weed science professor Steve Hallett and graduate student Jessica Schafer, all of Purdue University in West Lafayette, Ind., according to a news release.

The group subjected giant ragweed, horseweed and common lambsquarter to glyphosate treatments.

Part were conducted in a laboratory, using sterile soil.

The other half was conducted in the field under field conditions.

In each soil type, they tested both strains known to be susceptible to the herbicide and strains known to be resistant.

What they found were both types of giant ragweed were more heavily damaged by glyphosate in the field.

The susceptible strain of lambsquarter also was more heavily damaged in the field, and horseweed fared the same in the field, regardless of the strain.

What the group theorizes is microbes may plan a role in glyphosate activity, probably by invading herbicide-weakened plants.

The results also suggest that glyphosate-resistant plants also may be more resistant to diseases.

"We may be selecting not only for glyphosate resistance, but inadvertently selecting for weeds that have disease resistance as well," Hallett said in the release.