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?