Weed population dynamics in diversified cropping systems of the Northern Great Plains

Abstract

Small grain/fallow-based cropping systems of Montana and the Northern Great Plains are largely viewed as unsustainable from both an economic and environmental perspective. As a result, interest in alternative cropping systems has increased. A component of these alternative systems is increased crop diversity, but a major obstacle to adopting more diverse crop rotations is concern about weed management during the transition to the new system. I investigated changes in weed population dynamics during this transition period to some alternative systems in MT. Temporal dynamics of wild oat and redroot pigweed populations as affected by crop rotations and management intensity (conventional vs. reduced input levels) were examined. I found that crop diversity and accompanying crop management practices can be effective weed management tools during the transition to reduced input cropping systems. Crop rotation alone, however, in both the conventional or reduced input systems had little impact on weed populations when it was decoupled from corresponding chemical weed management practices. In addition, I examined the temporal and spatial dynamics of wild oat, Persian darnel, and redroot pigweed as part of the same field study. I quantified metrics that described weed population growth or decline in different crop rotations across conventional, reduced, and organic input cropping systems. A methodology for graphically depicting the combined temporal and spatial dynamics of a weed population was developed. This data set was then used to investigate the importance of variables hypothesized to be important drivers of observed wild oat population dynamics. Using multiple linear regression techniques, the best predictors of wild oat seedling abundance were determined to be wild oat seedling density and proximity to wild oat seedling density the previous growing season. The overall predictive power of our models however was low, but may indicate that wild oat populations persist in cropping systems by continuously forming new colonies in response to management more than any other single factor. Results of these studies highlight the gaps in our knowledge base related to identifying the mix of fundamental ecological processes and management that drive observed weed population dynamics. These results also demonstrate the data requirements needed to develop an understanding of weed population dynamics in agroecosystems.