Palmer amaranth: a review

Authors: WARD, SARAH - Colorado State University; Webster, Theodore; STECKEL, LARRY - University Of Tennessee

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2012
Publication Date: 1/2/2013
Citation: Ward, S.M., Webster, T.M., Steckel, L.E. 2013. Palmer amaranth: A review. Weed Technology. 27:12-27.

Interpretive Summary: Palmer amaranth has rapidly become one of the most important weeds of agronomic crops in the southern US. One of the factors linked with this recent change in the importance of Palmer amaranth is the changing weed management practices. Adoption of conservation tillage and abandonment of deep cultivation favors Palmer amaranth with its small seeds that germinate at shallow soil depths. Increased dependence on herbicides with a limited array of herbicides, especially sole reliance on glyphosate in RoundUp Ready crops, also contributes to a favorable environment for a weed such as Palmer amaranth that has large genetically diverse populations and the potential for rapid evolution of resistance. Palmer amaranth is an opportunistic and competitive weedy species with high fecundity, rapid germination and growth, and a capacity for phenotypic and phenological plasticity that enables seed production under different conditions until late in the growing season. As a desert-adapted C4 plant, its US range to date appears to have been limited to the southern tier of states in part by temperature requirements for germination. The rapid evolution in Palmer amaranth of a novel form of glyphosate resistance due to multiple copies of the EPSPS gene suggests the genome of this weed may also be capable of other forms of rearrangement. Epigenetic responses to changing environments, based on alterations in genome architecture and gene expression rather than changes in the underlying DNA base sequence, could explain at least some of capacity for rapid adaptation. The introduction of stacked trait crop varieties with multiple herbicide tolerances will provide growers with more flexible options for chemical control in the short term. Given the adaptive capacity this weed has already demonstrated, the further evolution of multiple herbicide resistance in Palmer amaranth is highly likely. The development and implementation of management practices that integrate sustainable herbicide use with appropriate non-chemical methods for cropping systems impacted by Palmer amaranth should therefore be a priority.

Technical Abstract: In little over 20 years, Palmer amaranth has risen from relative obscurity to its current status as one of the most widespread, troublesome and economically damaging agronomic weeds in the southeastern U.S. Numerous factors have enabled Palmer amaranth to become such a dominant and difficult to control weed, including its rapid growth rate, high fecundity and genetic diversity, ability to tolerate adverse conditions, and its facility for evolving herbicide resistance. It is both a serious threat to several U.S. cropping systems, and a fascinating model weed. Adoption of conservation tillage and abandonment of deep cultivation favors Palmer amaranth with its small seeds that germinate at shallow soil depths. Increased dependence on herbicides with a limited array of herbicides, especially sole reliance on glyphosate in RoundUp Ready crops, also contributes to a favorable environment for a weed such as Palmer amaranth that has large genetically diverse populations and the potential for rapid evolution of resistance. As a desert-adapted C4 plant, its US range to date appears to have been limited to the southern tier of states in part by temperature requirements for germination. The rapid evolution in Palmer amaranth of a novel form of glyphosate resistance due to multiple copies of the EPSPS gene suggests the genome of this weed may also be capable of other forms of rearrangement. Given the adaptive capacity this weed has already demonstrated, the further evolution of multiple herbicide resistance in Palmer amaranth is highly likely. The development and implementation of management practices that integrate sustainable herbicide use with appropriate non-chemical methods for cropping systems impacted by Palmer amaranth should therefore be a priority. In this paper, we review the growing body of literature on Palmer amaranth to summarize the current state of knowledge on the biology, agricultural impacts and management of this weed, and we suggest future directions for research.