Risks and Reliability of Using Herbicides at Below-Labeled Rates1

A growing awareness of environmental issues in Canada has had a major influence on government policies. An initiative was launched by the government of Ontario to promote research toward the development of an integrated weed management (IWM) system. Research in IWM must take all aspects of the cropping system into consideration and evolve in a progressive manner. This approach must encompass the role of conservation tillage, knowledge of the critical period of weed interference, alternative methods of weed control, enhancement of crop competitiveness, modeling of crop-weed interference, influence of crop rotation and seed bank dynamics, and education and extension of the findings. The complexity involved in addressing these issues requires a multi-disciplinary approach.

There is substantial environmental variance at small spatial scales (1 m or less) in both natural and disturbed environments. We have investigated the spatial structure of physical variables at larger scales (up to 106 m). We analysed surveys of edaphic properties of Wisconsin forest soils, of the water chemistry of lakes in Ontario and Labrador, and of temperature and precipitation in northeastern North America. We found no clear indication that the variance among sites approaches some maximal value as the distance between them increases. We suggest instead that the variance of the physical environment tends to increase continually with distance. The slope of the log-log regression of variance on distance provides a means of comparing the heterogeneity of different environments with respect to a given factor, or of comparing different factors within a given environment. This slope provides a useful measure of environmental structure that can be related to the biodiversity or plasticity of native organisms.

The awareness and adoption of conservation tillage is one of the most important changes taking place in agriculture today. There are, however, concerns regarding weed species shifts under conservation tillage. Under conservation tillage, shifts toward grass, perennial, wind-disseminated weeds and volunteer crop plants have been observed. Shifts in weed species composition may either represent long-term ecological succession or temporary fluctuations in species composition; few long-term studies have examined the ecology of these shifts in detail. Further studies are needed to identify mechanisms driving these shifts to determine whether they are fluctuational or successional and to develop more sophisticated management strategies. In this paper, we present a research approach for studying ecological processes such as competition within a hierarchical framework of all possible causes, processes, and defining factors related to weed succession under conservation tillage. Succession management strategies can be developed to act at the causal level in the successional hierarchy. Three primary causes are site availability, colonization, and species performance. Site availability may be controlled through “designed disturbance”, while differential species availability may be regulated through “controlled colonization” and species performance may be regulated through “controlled species performance”. In general, the goals of succession management would involve reducing populations of the weed species most likely to proliferate under conservation tillage. Comprehensive ecological research, within the hierarchical framework outlined here, would identify potential problems and enable management strategies to account for the numerous factors that may be influencing fluctuations and succession of weeds under conservation tillage.