Evolution of herbicide-resistant weeds.

The first generation of species-selective phenoxy herbicides went into use at the same time as modern antibiotics, chlorinated hydrocarbon insecticides and new fungicides and rodenticides. No resistance has appeared to the still widely used phenoxy herbicides. However, resistance has developed, in many isolated areas around the world, to s-triazine herbicides and, in a few small areas, to bipyridillium herbicides. The parameters of population genetics that govern herbicide resistance and those that govern resistance of microorganisms and fungi to other xenobiotics are the same: generations, selection pressure and fitness. Even though generation times are longer with plants, a sufficient number has passed for resistance to be apparent. The only special factor that controls the development of herbicide resistance in weeds is the spaced germination of seed throughout many seasons. The reason that resistance has not developed to the phenoxy herbicides is probably because of a low effective selection pressure; germination of susceptible seeds occurs late in the season after the herbicide is biodegraded. The highly persistent triazines, and the monthly used, highly ephemeral bipyridillium, paraquat, have exerted much stronger selection pressures. Different modes of tolerance and resistance seem to have evolved in the same species. Crop and herbicide rotation can considerably delay the possibility of resistance development until it is effectively precluded.