Fitness of triazine susceptible and resistant Solanum nigrum L. in maize.

The introduction of selective herbicides has led to a rapid adoption of chemical weed control by farmers. The success of these herbicides is based on high reliability and low costs but the sole reliance on herbicides for weed control in cropping systems has resulted in a fast increase in the incidence of herbicide resistant weeds worldwide. In the case of triazine resistance, the resistance trait is linked with a fitness penalty. Aim of this study is to assess perspectives to use this fitness penalty in the development of management strategies to control triazine resistant weed populations.The determination of fitness requires knowledge on different life history processes. Therefore, experiments on the germination, emergence, growth, seed production, and seed longevity of triazine susceptible and resistant biotypes of S. nigrum in maize were performed. In germination experiments, a lower dormancy and a lower minimum germination temperature requirement of the resistant biotype was found compared to that of the susceptible biotype. The germinated seeds of the resistant biotype emerged better from different soil depths than seeds of the susceptible biotype. A growth analysis with both biotypes at low light levels under controlled conditions showed a lower final biomass production of the resistant biotype after 29 days while the relative growth rates of both biotypes in the young growth phase were similar. A lower biomass and seed production of the resistant biotype compared to the susceptible biotype was also observed in field experiments where both biotypes were grown in competition with a maize crop. Seed longevity appeared to be much greater for susceptible seeds than for resistant seeds during the summer, while in the winter, differences in seed longevity were minimal.The integration of the different life history processes of both biotypes into a population dynamics model showed a strongly reduced fitness of the triazine resistant biotype compared to the susceptible biotype of S. nigrum in an agro-ecosystem with continuous maize cropping. The performance of the resistant biotype was in most of the life history processes inferior to that of the susceptible biotype which in combination resulted in a strong fitness reduction. The relative fitness of the resistant biotype compared to that of the susceptible biotype was 0.65. The most important determinant of the observed fitness difference between susceptible and resistant plants was the difference in seed characteristics.The simulation model showed the importance of measures to reduce selection pressure for the development of a triazine resistance management strategy. Management strategies exploiting the observed fitness differences between triazine susceptible and resistant biotypes of S. nigrum in the continuous cropping of maize should be aimed at exhaustion of the seed reserve of the resistant population in the seed bank