Differential Resistance Mechanisms to Glyphosate Result in Fitness Cost for and .

Multiple mechanisms of resistance to glyphosate are exhibited by populations of spp. worldwide. Association of resistance with growth and reproductive fitness is an important predictor for long-term success of glyphosate-resistant (R) versus glyphosate-susceptible (S) biotypes. Numerous studies were conducted on R- and S-biotypes of Italian ryegrass () and perennial ryegrass () to characterize the underlying mechanism(s) of glyphosate resistance and associate this with growth and reproductive fitness. . expressed both altered uptake and translocation as well as a genetic change at 106-Pro to -Ser, This pattern for two resistance mechanisms is unique. also exhibited altered uptake and translocation as well as duplication of EPSPS gene copies. Reduced plant biomass and height for R-versus S-biotypes of both species was evident over two growing seasons. This resulted in S- versus R- producing up to 47 and 38% more seeds in 2014 and 2015, respectively. S- produced up to 20 and 30% more seeds in 2014 and 2015, respectively. Both non-target site and target-site mechanisms of glyphosate resistance can render spp. at a competitive disadvantage. This has long-term implications for the success of glyphosate-resistant plants in the absence of selection pressure.