Palmer amaranth (Amaranthus palmeri S. Wats.) is a pigweed species native the southwest portions of the United States. This non-native weed was first identified in Michigan in 2010, and later confirmed resistant to glyphosate and acetolactate synthase (ALS)-inhibiting herbicides. Field and greenhouse experiments were conducted in 2011, 2012, and 2013 to evaluate the biology and management of Palmer amaranth in Michigan. Palmer amaranth seed mortality in Michigan soils was at least 9% greater than the seed mortality of the native pigweed species Powell amaranth (Amaranthus powellii S. Wats.). Burial time also influenced Palmer amaranth seed mortality, with greater mortality the longer the seed was buried. Palmer amaranth seed mortality after 12 months ranged from 53% to 90%, depending on the year. The addition of soil amendments (rye, wheat, poultry compost, and dairy compost) had no influence on Palmer amaranth seed mortality. Palmer amaranth emergence in Michigan occurs from the second week of May through the first week in September. Once emerged, Palmer amaranth growth can occur rapidly. In growth chamber experiments, the time to reach 15 cm tall Palmer amaranth plants was only 22 d when the temperature was 25 C d/20 C night, but was much slower under lower temperatures. The increased prevalence of Palmer amaranth in Michigan will require an integrated approach to management of this weed. Palmer amaranth control with soil-applied flumioxazin was highest and most consistent over three years in field experiments. Control was 85, 64, and 99% in 2011, 2012, and 2013, respectively, 28 DAT. Sulfentrazone provided similar results in two out of the three years when applied at 0.28 kg ha-1. Rainfall also influenced the effectiveness of these herbicides. Season-long control of Palmer amaranth will also require the use of postemergence herbicides. To manage Palmer amaranth postemergence, fomesafen (0.26 kg ha-1) and glufosinate (0.60 or 0.74 kg ha-1) provided the best control when Palmer amaranth was 8 cm or less. Lower control was observed when these herbicides were applied to 15 cm tall Palmer amaranth. Applications of glufosinate at 0.74 kg ha-1 were needed when Palmer amaranth reached 15 cm tall. Glyphosate- and glufosinate-based herbicide programs were evaluated in glyphosate-resistant and glufosinate-resistant soybean, respectively. Results varied by year. During a dry year, no glyphosate-based program provided greater than 55% control. However, flumioxazin followed by two postemergence applications of glufosinate resulted in 80% control. In a year with adequate rainfall for incorporation of the soil-applied herbicides, >94% control occurred with all glyphosate-based programs, except for two applications of glyphosate. The addition of residual herbicides with the postemergence herbicide helped provide control of late-emerging Palmer amaranth and do not influence the activity of the postemergence herbicide. Results from these experiments have determined that reductions in the soil seedbank of at least 50% will occur in a single year in the absence of additional seed rain. To ensure seed rain is low, the use of herbicide programs will be necessary. Soil-applied applications of flumioxazin or sulfentrazone are encouraged, while postemergence applications of herbicides will need to be applied before Palmer amaranth reaches 8 cm in height. This may be difficult as Palmer amaranth growth occurs rapidly, especially under higher temperatures. A residual herbicide may also need to be tank-mixed with postemergence herbicides. Palmer amaranth management in soybean will need to integrate soil-applied, postemergence, and residual herbicides tank-mixed with postemergence herbicides to reduce selection pressure for new herbicide resistances while providing season-long control.
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