IMPACT OF POSTEMERGENCE HERBICIDES ON WEED COMMUNITY DIVERSITY WITHINCONSERVATION-TILLAGE SYSTEMS

The influence of herbicides on weed community dynamics is an issue of agroecological concern. A field study was initiated in 1986 with zero- , minimum-, and conventional-tillage systems. In 1989 and 1990, the fo urth and fifth years of study, weed community diversity and relative c omposition were assessed prior to and following the application of pos t-emergence non-residual herbicides. Weed communities were assessed at the level of tillage system to balance the effect of different crop s equences and crop-herbicide combinations within each comparison. In ge neral, herbicides did not reduce community diversity when compared by Shannon's H' and dominance-diversity curves. An increase in species ri chness and evenness, as assessed by Shannon's E and Margalef's D-MG, o ccurred in some cases. Differences in relative community composition a mong tillage systems were apparent before the application of herbicide s, but disappeared following herbicide application. The use of herbici des in this study reduced weed densities, maintained weed diversity, a nd inhibited community changes that were due to changing tillage syste ms.