The impact of Palmer amaranth in Indiana

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Palmer amaranth has been confirmed in seven counties in Indiana since the fall of 2012, with five of these counties occurring in Northwestern Indiana. The confirmation of Palmer amaranth in Northwestern Indiana plus recent confirmations in Michigan, Ohio, and Illinois has raised concern that Palmer amaranth has the potential to become a major agronomic weed in Indiana within the next couple of years. The economic hardships that this weed has caused Southern United States cotton and soybean producers should be a warning to Indiana corn and soybean producers. The remainder of this publication will cover the current status and location of Palmer amaranth in Indiana and the biology of Palmer amaranth.

Current Status of Palmer Amaranth in Indiana

Palmer amaranth was first confirmed in Indiana in 2011 with populations occurring in the river bottoms of Posey and Vanderburgh counties. Plants grown from seed collected from one of the river bottom fields were able to survive 20 pounds of acid equivalent per acre of glyphosate (equivalent of 7 gallons per acre of generic glyphosate) in greenhouse screenings conducted at Purdue University. In the fall of 2012, 51 fields across 5 counties in Northwestern Indiana were confirmed to have Palmer amaranth plants that were not controlled by management tactics used during that growing season. The majority of fields (and the heaviest infestations) were confirmed in Jasper County. Many of the fields observed had received multiple applications of glyphosate and attempted rescue applications of PPO inhibiting herbicides.

The initial transport of Palmer amaranth to Northern Indiana is proposed to have been in manure spread on fields from dairy or beef operations that feed cotton seed hulls from the Southern U.S. that were contaminated with Palmer seed. The exact timing of the initial event is unknown, but is estimated to have happened at least two to three years ago due to the severity of infestation in multiple fields. Further spread of Palmer seed has and will occur through farm equipment, specifically combines. Wildlife can also spread the seed into new, previously uninfested fields. It is likely that more Palmer amaranth populations exist within Northwestern Indiana, as well as other regions, but these areas have yet to be properly identified.

Biology and Impact Of Palmer Amaranth

Palmer amaranth is an aggressive, weedy invader that is native to the desert regions of the Southwestern United States and Northern Mexico. It has slowly infiltrated into the Southeastern United States and has become the most significant weed pest of cotton and soybean producers with the majority of populations having resistance to glyphosate and ALS herbicides. The adaptability and invasiveness of Palmer amaranth is further observed by the success of the populations in Michigan and Northern Indiana where average temperatures are below the preferred temperatures of native Palmer populations. The following biological distinctions highlight the success of Palmer as a weed.

• Genetic Diversity: Dioecious reproduction in which individual plants are either male or females, which forces outcrossing and genetic diversity. This gives Palmer amaranth the ability to adapt and quickly spread herbicide-resistance genes when selection pressure is applied, such as repetitive use of single mode of action herbicides.

• Seed Production: Palmer amaranth is a prolific seed producer with plants producing at least 100,000 seeds in competition with a crop, and upwards of a half million seeds in non-competitive scenarios.

• Seed Size: Palmer amaranth seeds are rather small and thrive in no-till or minimum tillage fields where seeds are allowed to stay in the ideal emergence zone in the top inch of soil. The small seeds are easily transported by humans either through grain, seed, or feed contamination; and they are also transported on equipment such as combines.

• Aggressive and Competitive: Palmer exhibits aggressive growth and competitiveness with plants growing 2-3” per day in ideal conditions and creating yield losses of 11-91% in corn and 17-79% in soybean when allowed to compete throughout the growing season.

• Herbicide Resistance: Palmer populations have conferred resistance to multiple herbicide modes of action including: ALS inhibitors, triazines, HPPD-inhibitors, dinitroanilines, and glyphosate, with a large majority of southern populations being glyphosate- and ALS-inhibitor-resistant.

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