by Ken Smith and Jason Norsworthy

SITUATION: The first glyphosate-resistant Palmer amaranth in Arkansas was identified in 2005 in Mississippi County. Poor spray coverage was initially suspected, but it became evident that symptoms were because of resistance, not coverage. There appear to be various degrees of resistance in that first field, i.e. individuals in that field are suspected to be segregating. In Lincoln County, plants survived 194 oz/A of glyphosate as Roundup WeatherMax®. Seedlings from these plants did not appear to be damaged by glyphosate at 22 oz/A. This population does not appear to segregate and appears more resistant than the Mississippi County population, suggesting the possibility of two mechanisms of resistance. Populations in the Mississippi flood plain (where the first population was found) are of special concern because of the potential for spread along the river.

DISTRIBUTION: Glyphosate-resistant Palmer amaranth is present at 28 sites in at least 15 counties in eastern Arkansas.


  • Level of awareness: Producers are very aware of the problem of resistant weeds in Arkansas.
  • Management issues: Producers want recommendations for management of specific resistant weeds, much as they got for horseweed. They indicate they will make changes in production practices, including dropping Roundup Ready® technology, if that is what is necessary to maintain their farms. However, it was pointed out that most producers will not treat resistance as a problem until they actually have the problem.
  • Education program: Arkansas has established a strong resistance education program. A resistance committee was established in Arkansas, and brochures about herbicide resistance and pigweed resistance have been published and distributed.


  • Resistance confirmation and levels: Of 21 accessions from fields in northeast Arkansas, a low percentage of plants in most accessions survived the labeled use rate of glyphosate, which should have provided complete control. LD50 values ranged from 41 to 339 g/ha glyphosate, and mean survival rate over all the accessions, including 5 accessions that had no survivors, was 2.2%. LD50 of the two most tolerant accessions, AR18 and AR19, was 312 and 339 g/ha glyphosate, and mean survival frequencies were 6.3% and 11.8%, respectively. Following an additional cycle of selection with glyphosate, 44.3% of the progeny from AR19 survived glyphosate at 870 g/ha, and the LD50 value increased to 646 g/ha glyphosate.
  • Absorption/translocation: Absorption of glyphosate did not differ among resistant and susceptible biotypes.
  • Control: Arkansas research suggests that residual herbicides are needed for control of Palmer amaranth. Control from fomesafen (Flexstar®, Reflex®) applied 7 days preplant was at least 95% at 34 days after emergence. A residual program in cotton could include fomesafen or flumioxazin (Valor®) applied preplant followed by (fb) metolachlor (Dual®) applied early over-the-top fb prometryn (Caparol®) post-directed and flumioxazin (Valor®) at layby. In Liberty Link® (glufosinate-resistant) cotton, a potential program is fluometuron (Cotoran®, others) preemergence fb glufosinate (Ignite®) plus metolachlor (Dual®) early over-the-top fb glufosinate late over-the-top and flumioxazin (Valor®) at layby. In soybean, start clean with tillage or burndown with paraquat (Gramoxone Inteon®), flumioxazin, or a mixture; apply metolachlor, flumioxazin, or pendimethalin/trifluralin (Prowl®/Treflan® others – if no resistance to dinitroanilines) preemergence; and apply metolachlor + glyphosate (Sequence®), metolachlor + fomesafen (Prefix®), or fomesafen postemergence. A rotation to rice may be effective, but it is necessary to control Palmer amaranth on the levees. A fall-seeded rye cover crop may also be effective for early suppression of Palmer amaranth in cotton.
  • Future direction: Programs to study genetic similarity among resistant populations (population structure); dispersal mechanisms; inheritance patterns; and modeling the evolution of glyphosate resistance will be established.

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