SUSTAINABILITY, CONSERVATION TILLAGE AND WEEDS IN CANADA

The sustainability of conservation tillage is dependent on the extent of changes in weed community composition, the usage of herbicides, and the development of integrated weed management (IWM) strategies, inclu ding biological weed control. The objective of this paper is to review research on conservation tillage and weed management in light of thes e factors. Recent Canadian research has found that changes in weed com munities due to the adoption of conservation tillage are not necessari ly those expected and were not consistant by species, location, or yea r. Changes reflected the use of different selection pressures, such as different crop rotations and herbicides, within the studies to a grea ter extent than weed life cycle groupings. Therefore, research that de termines the reasons for change or the lack of change in weed communit ies is required to provide the scientific basis for the development of IWM strategies. Documented herbicide usage in conservation tillage va ries from less than to more than conventional-tillage systems. Potenti al to reduce herbicide usage in conservation-tillage systems exists: F urthermore, the herbicides used in western Canada are different from t hose causing ground water contamination in the United States, are less volatile, and are used at lower rates. The presence of surface crop r esidues in conservation tillage may provide a unique environment for c lassical and inundative biological control agents. Some insects, fungi , and bacteria have the potential to survive to a greater extent in un disturbed plant residues. Residue management and conservation tillage systems are evolving in Canada. Research must keep pace by providing w eed management strategies that enchance the sustainability of these sy stems.