A recent call about the poor control of marestail (horseweed, Conyza canadensis) to glyphosate (Roundup®) wasn't surprising, but that paraquat didnt do the trick was.  It turns out that there is multiple resistance to the materials.  If horseweed is resistant to glyphosate it is possibly going to be resistant to paraguat  which also means that hairy fleabane which has glyphosate resistance could also show resistance to paraquat.  A recent study reports on the increased Conyza resistance to paraquat (Distribution of Conyza sp. in Orchards of California and Response to Glyphosate and Paraquat, Moretti et al, 

As a farm advisor many growers will ask, “What is that in my grove- horseweed or fleabane?” These two weeds have been running rampant in Southern California, as well as other parts of the state and can be seen in every cropping system imaginable. Their population numbers can be alarming and can start to become troublesome to growers as they can easily become A challenge to control if left untreated. These two members of the of the Conyza family are particularly fond and thrive in permanent tree crop environments due to lack of tillage and lack of a crop rotation schedule due to the trees permanent...