"Palmer amaranth (Amaranthus Palmeri S. Wats.) was first identified in Michigan in 2010, it is a non-native pigweed species that has been detrimental to row crop production throughout the southern and Great Plains regions of the United States. In 2013, the failure of atrazine to control a suspected glyphosate- and ALS-resistant Palmer amaranth population identified in Barry County, MI prompted further investigation into the possibility that this Palmer amaranth population was resistant to three different herbicide sites of action. Field, greenhouse, and laboratory experiments were conducted in 2013, 2014, and 2015 to quantify the levels of resistance, identify the mechanisms, study the biology, and identify possible management strategies for Palmer amaranth in Michigan corn production systems. The resistance factor (RF) values for the suspected multiple-resistant Barry County (MR) Palmer amaranth population were 12, 43, and 9X for POST applications of glyphosate, thifensulfuron, and atrazine, respectively, compared with a known susceptible population. The MR population was also highly resistant to PRE applications of atrazine, RF = 112X. These results confirmed that this population was resistant to three different herbicide sites of action. Laboratory experiments identified target-site based resistance for glyphosate and the ALS-inhibiting herbicides via gene amplification and amino acid substitution, respectively. Resistance to atrazine was not target-site mediated, with no observed nucleotide substitutions within the psbA gene, leading us to believe that atrazine resistance in this population may be metabolism-based. In the field, sole reliance on a PRE or single site of action POST herbicide application did not provide season-long control of Palmer amaranth. Several one-pass EPOS weed management programs effectively managed multiple-resistant Palmer amaranth. However, these programs must contain at least two effective herbicide sites of action with foliar activity tank-mixed with a residual herbicide for season-long control. The most consistent and effective management strategies for control of this multipleresistant Palmer amaranth population in corn were two-pass herbicide programs, PRE followed by POST. These strategies included at least one effective herbicide site of action PRE and two effective foliar sites of action POST plus a soil residual herbicide for season-long Palmer amaranth control. The HPPD-inhibiting herbicides will be a major component of a Palmer amaranth management program in corn. The effectiveness of the POST HPPD-inhibiting herbicides for Palmer amaranth control were tolpyralate > tembotrione = topramezone > mesotrione. The addition of atrazine to mesotrione and tembotrione was synergistic for Palmer amaranth control. This suggests that even in an atrazine-resistant Palmer amaranth population the addition of atrazine to some of the HPPD-inhibiting herbicides would be beneficial for Palmer amaranth control, especially as Palmer amaranth size increases. In a three year crop rotation experiment, Palmer amaranth emergence started at 2803031 GDD10 (late May/early June) and did not cease until September in central Michigan. The total number as well as the duration of Palmer amaranth emergence was greater in corn than in soybean. The initial growth rate of Palmer amaranth was greatest for early emerging cohorts in corn, however those that emerged 2 wks later in the season were more competitive in soybean. Seed production declined with each successive cohort and was greatest for early emerging Palmer amaranth in soybean with >64,000 seeds plant-1 . Palmer amaranth that emerged in August after wheat harvest produced seed that added to the soil seedbank. When corn is included in the rotation the availability of more effective herbicide sites of action paired with reduced Palmer amaranth growth and lower seed production can lead to greater success in managing resistant Palmer amaranth populations."--Pages ii-iv.
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