Weed Thresholds: The Space Component and Considerations for Herbicide Resistance

As an extension of weed threshold models in which crop losses are based on weed density, an altemative model for grass weeds in cereal crops is proposed that incorporates the theoretical importance of selection for herbicide resistance, initial weed population frequency, and weed seed dispersal. Simulations suggest optimum weed population levels (thresholds) for maintaining genotypes that are susceptible to control practices and which minimize crop yield reductions. Weed population frequency, in combination with dispersal and competitive traits may determine optimum weed management strategies. Model simulations indicate that understanding how agricultural practices select for "weedy" traits (e.g. herbicide resistance, competitive ability, dispersal potential) may be important in determining weed density thresholds. Nomenclature: green foxtail, Setaria viridis (L.) Beauv #3 SETVI; spring wheat, Triticum aestivum L. Additional index words: Population models, seed dispersal.

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