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ARS Home » Southeast Area » Tifton, Georgia » Crop Protection and Management Research » Research » Publications at this Location » Publication #319176

Title: Glysphosate-resistant Palmer amaranth (Amaranthus palmeri) morphology,growth, and seed production in Georgia

Author
item Webster, Theodore
item GREY, TIMOTHY - University Of Georgia

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2014
Publication Date: 1/25/2015
Citation: Webster, T.M., Grey, T.L. 2015. Glysphosate-resistant Palmer amaranth (Amaranthus palmeri) morphology, growth, and seed production in Georgia. Weed Science. 63:264-272.

Interpretive Summary: Palmer amaranth is native to the southwest US, but has moved into the Southeast US and become a significant weed problem, costing Georgia cotton growers in excess of $110 million annually. The first occurrence of glyphosate resistant Palmer amaranth was confirmed in Georgia in 2004 and in subsequent states over the next several years. Marginalization of potential herbicide tools has continued to occur due to cancellation of herbicide registration (e.g. cyanazine and proposed elimination of MSMA) and increased prevalence of herbicide resistant weeds. In response, research has expanded to include alternative means of affecting future Palmer amaranth populations by altering safe sites for Palmer amaranth establishment and reducing inputs to the seedbank population. The objectives of this study were to characterize Palmer amaranth morphology, growth, and seed production at five establishment dates and two levels of crop competition. Delayed establishment of Palmer amaranth simulates premature loss of herbicide efficacy and alters the competitive balance between cotton and Palmer amaranth. Median cotton yield loss occurred at 4.9 wks after cotton planting. While the time to median Palmer amaranth plant height was not affected by the presence of cotton, median plant width and biomass occurred within the first 3.8 wks of the growing season. This suggests that early control programs will have a large impact on Palmer amaranth potential growth. Median Palmer amaranth seed production also occurred early in the growing season, during the first two wks after cotton planting. Plants that established 12 wks after cotton planting were able to produce 2,740 seeds per plant. However, minimizing seed return to the soil seedbank must be coupled with reducing the ability of seeds to transition to established plants. In order to effectively implement soil seedbank reduction strategies, information on mechanisms that regulate Palmer amaranth seed production and seed persistence in the soil seedbank is needed.

Technical Abstract: Herbicide resistant Palmer amaranth has become the most economically detrimental weed of cotton in the Southeast US. With the continual marginalization of potential herbicide tools, research has expanded to include alternative means of affecting future Palmer amaranth populations by altering safe sites and reducing inputs to the seedbank population. The influence of delayed Palmer amaranth establishment on seed production potential has not been investigated in the Southeast US. Studies were conducted to evaluate the influence of time of Palmer amaranth establishment on morphology, growth, and seed production. The experiment was a factorial, with five levels of Palmer amaranth transplanting (0 to 12 wks after cotton planting) and two levels of crop type (cotton and non-crop). In the absence of crop competition, the first cohort Palmer amaranth produced 446,000 seeds per plant. This potential seed production was reduced 50% when Palmer amaranth plants were established nearly six weeks later. In contrast, the first Palmer amaranth cohort growing in competition with cotton produced 312,000 seeds, 30% less than what was produced in the absence of competition. The interference from cotton shifted median seed production to occur nearly four weeks earlier in the growing season, relative to plants without crop competition. Delayed establishment of Palmer amaranth simulates premature loss of herbicide efficacy and alters the competitive balance between cotton and Palmer amaranth. While the time to median Palmer amaranth plant height was not affected by the presence of cotton, median plant width and biomass occurred within the first 3.8 wks of the growing season. This suggests that early season weed control programs will have a large impact on Palmer amaranth potential growth and seed production, as well as crop yield. In order to effectively implement soil seedbank reduction strategies, information on mechanisms that regulate Palmer amaranth persistence in the soil seedbank is needed.