Herbicide resistance jeopardizes the usefulness of valuable chemical tools and, therefore, weed management in many crop systems. Models must be developed to evaluate management tactics that prevent, delay, or reduce resistance. The complexity of biological processes involved in herbicide resistance also requires models to focus research and to integrate experiments. A population model was developed that improves upon previous attempts to predict herbicide resistance dynamics. The model incorporates plant population demographics with the Hardy-Weinberg concept for gene segregation. The model simulates the evolution, spread, and subsequent dynamics of resistance in the presence and absence of a herbicide. Analysis of model simulations identified two sets of biological processes as key factors in the evolution and dynamics of herbicide-resistant weed populations. These are processes that influence ecological fitness and gene flow. Several options are suggested as examples for the management of resistant weed populations.

Studies of weed and crop competition can be used to predict yield losses from weed presence and to determine optimum levels or periods of weed control. However, competition is a complex phenomenon that is governed by various biological, environmental, and proximity factors. The factors of proximity include plant density, species proportion, and spatial arrangement among individuals. Several experimental methods have been developed that attach different levels of importance to proximity factors. These methods are described, and the advantages and disadvantages of each are discussed. Density, proportion, and arrangement of plants influence the outcome of competition experiments and should be incorporated into studies of crop-weed interference, since differing estimates for the effects of weeds on crop productivity can be obtained, depending upon the experimental method used.

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SUMMARY We conducted intensively replicated field experiments in the Willamette Valley. Oregon, USA, in order to study the role of light in triggering seed germination during soil tillage. We found that the normal practice of cultivating agricultural land during daytime can increase germination of buried seed populations between 70 and 400 above the levels recorded following nighttime cultivations. Experimental reduction of the irradiance under the tillage implements during daytime cultivation decreases the number of dicotyledonous seedlings emerged. while strong artificial illumination (>300μmol m−2 s−1; 400–800nm) of the soil surface under the implements during nighttime tillage significantly increases seedling densities. These results suggest that the enhancement of seed germination caused by daytime tillage, compared with nighttime tillage, is due to light that penetrates into the soil during the actual disturbance. The detection by the seeds of this very short exposure to sunlight requires a high photosensitivity. which provides an adaptive‘purpose’for the evolution of the very‐low‐fluence response mechanism in phytochrome‐controlled seed germination. Seedling emergence induced by nighttime control tillage was considerable in some experiments. suggesting that light perceived by seeds after cultivation or other microenvironmental factors affected by tillage may be important in triggering germination in disturbed soil.

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