We asked if there was a difference in growth or reproduction between two populations of glyphosate-resistant (R1 and R2) and two populations of glyphosate-susceptible (S1 and S2) Conyza canadensis.  The herbicide glyphosate is commonly used in land management.  Conyza canadensis has evolved more glyphosate-resistant (GR) biotypes than any other weed in the US, and may reduce crop yields by up to 90%.  Though resistance is not fully understood, there may be multiple physiological mechanisms responsible.  Mechanisms can be placed in two categories: target site resistance involves a change in amino acid 106 in the EPSPS, and non-target site resistance is caused by reduced translocation of the herbicide to the active site.  Both mechanisms could be present in biotypes of C. canadensis and may be responsible for changes in plant fitness.  Differences in plant fitness may only be apparent with biotic interactions, such as competition or herbivory.  We planted four populations of C. canadensis in a random block design in two habitats, a soybean field (agrestal) and an old-field habitat (ruderal).  We analyzed survivorship, growth rate, maximum height, rosette diameter, number of leaves, number of flowering plants, and percent herbivory as surrogate measures for fitness. 

Results/Conclusions

We found that there was no indication of a fitness cost in the GR populations, and some GR biotypes may even show greater fitness than GS biotypes.  Survivorship analysis shows that there was no difference between populations within each habitat; but more individuals survived in the ruderal habitat, possibly because fewer plants reached reproductive stage.  Height was greater for all populations in the agrestal habitat.  Rosette diameter was smaller in the ruderal habitat, with no difference between populations; while in the agrestal habitat, R2 had the greatest diameter, and S1 had the smallest diameter.  R2 had a greater number of leaves in the agrestal habitat, while S1 had more leaves in the ruderal habitat.  In the agrestal habitat, reproduction was greatest in R2, least in S1 and S2, and intermediate in R1.  There was no difference in percent herbivory by habitat type or population, suggesting that herbivore host preference is not altered by GR mechanisms in these biotypes.  Lack of a fitness cost or greater fitness for GR plants has important implications in management decisions, and may translate to higher population growth rates.