ABSTRACT
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Ecological Consequences of Modern Weed Control Systems . . . . . . . . . . . . 63
Weeds in the Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Weed Adaptation to Management Practices . . . . . . . . . . . . . . . . . . . . 64 In Search of New Approaches to Weed Management. . . . . . . . . . . . . 64 The Role of Mathematical Models in Predicting Weed
Population Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Spatial and Temporal Dynamics of Weed Populations. . . . . . . . . . . . . . . . . . 66
The Dynamics of Weed Invasion and Spread. . . . . . . . . . . . . . . . . . . . 66 Predicting Weed Invasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Seed Dispersal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
The Dynamics of Weed Population Density. . . . . . . . . . . . . . . . . . . . . 69 Optimum Weed Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Extrinsic Factors Affecting Weed Populations . . . . . . . . . . . . . . . . . . . 73 Weed Control Decision Thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Timing of Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Optimal Weed Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Integrated Weed Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Required Advances in Modeling Weed-Crop Interactions . . . . 78 Biological Control of Weeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Weed Adaptation to Management Practices . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Adaptation to a Single Control Measure. . . . . . . . . . . . . . . . . . . . . . . . 81 Adaptation to Integrated Weed Management Systems . . . . . . . . . . . 83
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
INTRODUCTION
In recent years, two very different approaches to controlling weeds have developed. On the one hand, there has been the introduction of herbicidetolerant crops in North America with their specific reliance upon herbicides. Clearly, however, the widespread application of such techniques will alter the dynamic equilibrium which normally exists in vegetation. Thus, a key research issue must be the long-term ecological consequences of the regular use of nonselective herbicides on the community structure of seminatural vegetation (Willis, 1990). In direct contrast, in response to both public and industry concerns, there has been the development of sustainable systems of crop production, in which the emphasis has been on minimizing herbicide use. Instead, a mixture of biological, chemical, and mechanical methods are combined to control weeds, pests, and diseases to provide stable long-term protection to the crop (Lockhart et al., 1990; Swanton and Weise, 1991; Gressel, 1992; Wyse, 1994; Holt, 1994; Viaux and Rieu, 1995). Fundamental to this latter approach is a sound understanding of weed demography and of the efficacy and impact of different control methods. Although the two approaches represent very different strategies to weed control, both require an understanding of the population biology of weeds, including evolutionary aspects (Jordan and Jannink, 1997), and the dynamics of weed populations. Accordingly, this chapter summarizes current understanding on these matters, including the effects of crop rotation, tillage systems, and herbicide use on weed communities.
