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In my area, the most common weeds that strongly resists (N-(phosphonomethyl)glycine) (glyphosate) are the horseweed, or mare's tail, Conyza canadensis, and Canada thistle, Cirsium arvense There are several other weeds with similar resistance. I use the brand Roundup on jobs where a complete kill is necessary. However, I sometimes have to go through again, with glufosinate, to control these weeds. I'd prefer not to, as the glufosinate lingers much longer in the soil.

Glyphosate inhibits an enzyme used in the synthesis of the aromatic amino acids tryptophan, tyrosine, and phenylalanine. It is taken in by the stomata in the leaves, and is moved throughout the plant to all the points of growth, acting fastest on those plants which are undergoing fastest growth.

I can't seem to find an article on how the weeds mentioned can tolerate this treatment. How do these weeds resist the glyphosate?

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I found a paper investigating the mechanism of glyphosate resistance in Conyza canadensis. They used 31P NMR to investigate the fate of the glyphosate in vivo. What they found is that the resistant plants are able to transport glyphosate into the vacuole:

The following view of horseweed resistance to glyphosate emerges from the data presented herein. Glyphosate enters the cytoplasm of both R and S plant variants at the same rate. Within hours, however, glyphosate begins to occupy the vacuole in the R but not the S biotype. The identical pH values of R and S vacuoles speak against the possibility of a pH-driven process. This, coupled with the preferential movement of glyphosate from the cytosol to the vacuole in R tissue but not in S, suggests the presence of a transporter for glyphosate either specific to R or at a substantially greater concentration in R than in S tissue.

Another paper titled "Two non-target mechanisms are involved in glyphosate-resistant horseweed (Conyza canadensis L. Cronq.) biotypes" found a different mechanism for glyphosate resistance in addition to the sequestration into the vacuole.

Glyphosate was metabolized faster in the R (C004) biotype than in the S biotype. The herbicide disappeared completely from the R (C004) biotype by conversion into glyoxylate, sarcosine and aminomethylphosphonic acid within 96 HAT.

The resistant plants seem to be able to metabolize glyphosate more effectively.

It is of course not guarenteed that these are the only mechanisms for glyphosate resistance, there are likely more variations.

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This answers concession to the diversity of evolution and the humble state of human endeavors is awesome. –  caseyr547 Jul 28 at 23:25

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