Across the Midwest, weeds resistant to multiple herbicides continue to become more widespread. Not only do weeds with resistance to multiple herbicides reduce the utility of existing herbicides, but they also necessitate the use of alternative weed control strategies. From 2012-2015 in southern Minnesota, we determined the effect of six 3-year crop rotations containing corn (C), soybean (S), alfalfa (A), and wheat (W): (CCC, SCC, CSC, SWC, SAC, AAC) on herbicide-resistant giant ragweed seed bank depletion and emergence patterns. Crop rotation had no effect on the amount of seed bank depletion when a zero weed threshold was maintained, with 96% of the giant ragweed seed bank being depleted within 2 years (Table 1). However, this quantity of seed bank depletion was primarily through seedling emergence in annual crop rotation treatments. Multiple years of alfalfa exhibited less seedling emergence while maintaining a high level of seed bank depletion, possibly indicating an increase in seed predation or fatal germination of seedlings (Table 1). In comparison to rotations containing just corn or soybean, total emergence of giant ragweed was reduced by an average of 38% when wheat or alfalfa were included in the rotation (Table 1). Giant ragweed emerged early across all treatments, with 90% emergence occurring by 4 June on average. These results indicate that corn and soybean rotations are more conducive to giant ragweed emergence than rotations containing wheat and alfalfa, and that adopting a zero weed threshold is a viable approach to depleting the weed seed bank. This presentation will discuss current research focusing on how crop rotation and timing of field operations can be used as part of an integrated weed management plan to improve herbicide-resistant giant ragweed control. Specifically, alfalfa will be highlighted as being an important tool to deplete the weed seed-bank while maintaining profitability.
Soil Science Extension
University of Wisconsin Madison