Emergence Patterns of Giant Ragweed (Ambrosia trifida) in Various Crops and Crop Rotations.

In the Midwest, biotypes of giant ragweed (Ambrosia trifida) resistant to multiple herbicide sites of action have been identified. Weeds with multiple herbicide resistances reduce the efficacy of existing herbicides and necessitate the development of alternative weed control strategies. From 2012-2014 in southern Minnesota, we determined the effect of six crop rotations containing corn (C), soybean (S), alfalfa (A), and wheat (W):  (CCC, SCC, CSC, SWC, SAC, AAC) on giant ragweed emergence patterns. To study seed-bank depletion over the course of the study, all emerged giant ragweed was eliminated to prevent seed input into the seed bank. Giant ragweed emergence was highly variable among years due to the overall depletion in the seed bank over time and the influence of variable soil temperature and moisture, with 11.4, 58.4, and 3.9 seedlings m^-2 yr^-1 emerging in 2012, 2013, and 2014, respectively. Across all treatments in 2013, 94.8% of the total giant ragweed emerged by June 3^rd, while 82.6% emerged by the same date in 2014. Wheat and alfalfa, which were established early in the growing season in 2013, had lower giant ragweed emergence than later-planted corn and soybean. Cumulative giant ragweed populations in 2013 were 93.3, 61.5, 42.7, 39.8, and 19.7 seedlings m^-2 in corn, soybean, wheat, seedling alfalfa, and established alfalfa, respectfully. The low level of emergence in wheat, seedling alfalfa, and established alfalfa is likely due to early season soil conditions being less conducive to giant ragweed emergence. Established alfalfa also provides conditions more favorable for seed degradation by seed predators and soil microorganisms. These results show that herbicide-resistant giant ragweed infested fields can be managed by utilizing various crop rotations in addition to strategic timings of mechanical and chemical weed control options, which can be part of an integrated weed management plan directly targeting early emerging weeds.