Seed Bank Depletion and Emergence Patterns of Giant Ragweed (Ambrosia trifida) in Midwestern Cropping Systems.

In the Midwest, biotypes of giant ragweed (Ambrosia trifida) resistant to multiple herbicide biochemical sites of action have been identified. Weeds with resistance to multiple herbicides reduce the utility of existing herbicides and necessitates the development of alternative weed control strategies. From 2012-2015 in southern Minnesota, we determined the effect of six three year crop rotations containing corn (Zea mays) (C), soybean (Glycine max) (S), alfalfa (Medicago sativa) (A), and wheat (Triticum aestivum) (W):  (CCC, SCC, CSC, SWC, SAC, AAC) on 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 91% of the giant ragweed seed bank being depleted in two years. However, this quantity of seed bank depletion was primarily through seedling emergence in annual crop rotation treatments, while similar seed bank depletion totals were observed alongside low levels of seedling recruitment after two years of alfalfa, possibly indicating an increase in seed predation or fatal germination. Giant ragweed emerged early across all treatments, with 90% emergence occurring by June 4th.  In comparison to corn or soybean, total emergence was reduced when wheat or alfalfa were planted, indicating that seedling recruitment is affected by crop rotation. These results indicate that various crop rotations are more conducive to giant ragweed emergence than others, and that long term giant ragweed management can be accomplished by implementing a zero weed threshold to deplete the weed seed bank.