LAUSR

Abstract Alopecurus myosuroides Huds. is the most abundant herbicide-resistant weed species in Western Europe. Herbicide resistance management programs may rely, amongst other things, on information about fitness of resistant sub-population i.e. the number of offspring a genotype produces and contributes to next generation. Growth and phenological development of susceptible (S) and fenoxaprop-P resistant (R) A. myosuroides sub-populations from a non-target site resistant (NTSR) Danish population was studied for two years, while the primary seed dormancy of the populations was evaluated for three years. The genetic background of the S and R sub-populations was controlled using a plant cloning technique. No statistically significant differences were observed in the development of vegetative growth stages between the R and S sub-populations. In contrast, the R sub-population required more thermal time (at least c. 40 °C d) compared with the S sub-population to reach the reproductive phenological stages including booting and flowering. There were no vegetative (biomass) and reproductive (seed production) fitness costs in the R sub-population, however, the harvest index of the R sub-population was significantly lower than of the S sub-population. Moreover, the R sub-population had a lower primary dormancy than the S sub-population. The lower primary dormancy of the R sub-population can be considered as agronomical fitness costs of the NTSR to fenoxaprop-P, if pre-sowing seedbed preparation are implemented. The results can be implemented in models simulating phenological development and in models predicting the proportion of resistant individuals in a population.