Different weeds have different genetic mechanisms for overcoming glyphosate herbicide, explains weed scientist Jeff Stachler.

Resistant horseweed (marestail), for example, has a gene that sequesters the herbicide in one part of the plant, allowing the reproductive parts of the plant to keep growing.

Resistant pigweeds have multiple copies of the gene that produces EPSPS, an essential enzyme that plants use to make amino acids. Glyphosate normally disrupts this enzyme.

These varying mechanisms show the complexity of glyphosate resistance, Stachler says.