Relative competitiveness of glyphosate-resistant and glyphosate-susceptible populations of hairy fleabane, Conyza bonariensis

Hairy fleabane is a weed found increasingly in agronomic crops and rural areas. Although glyphosate resistance is now widespread in this species, the ecological fitness of glyphosate-resistant (GR) and glyphosate-susceptible (GS) populations has not been compared. When seeds of both biotypes were subjected to different alternating temperature, germination occurring and seedling vigor did not differ between the biotypes with maximum germination at 10/20 and 15/25 °C. Under noncompetitive conditions, growth and seed production of the GR and GS biotypes were similar. The competitive ability of the two biotypes, as determined by a replacement series experiment, was also similar. Initially, the relative crowding coefficient (RCC) between the biotypes was close to one. However, the RCC values for each characteristic increased with time indicating that the GR had a small competitive advantage as plants matured. Overall, there was no apparent fitness penalty associated to glyphosate resistance. Therefore, the GR populations are likely to persist unless effective management strategies are adopted.