Competitive Relationships Between Atrazine Resistant and Susceptible Populations of Amaranthus Retroflexus and A. Powellii From Southern Ontario

SUMMARY The relative competitive abilities of atrazine resistant and susceptible populations of Amaranthus retroflexus and Amaranthus powellii were compared in a greenhouse experiment using a de Wit replacement series. Both inter and intraspecific differences in competitive ability were apparent when the populations were grown in two‐way combinations of varying proportions but constant overall density. Within both species, the susceptible population had a greater competitive ability with respect to biomass and seed production than the resistant population. The susceptible population of Amaranthus powellii was more competitive than either the susceptible or resistant populations of Amaranthus retroflexus. The resistant population of Amaranthus powellii was more competitive than the resistant population of Amaranthus retroflexus but appeared to have a competitive ability equal to that of the susceptible population of Amaranthus retroflexus.