CHARACTERIZATION AND MITIGATION OF HERBICIDE-RESISTANT AND RECALCITRANT WEEDS
Location: Crop Production Systems Research Unit
Title: Multiple resistance to glyphosate and pyrithiobac in Palmer amaranth (Amaranthus palmeri) from Mississippi and response to flumiclorac
Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 21, 2011
Publication Date: April 1, 2012
Citation: Nandula, V.K., Reddy, K.N., Koger, C.H., Poston, D.H., Rimando, A.M., Duke, S.O., Bond, J.A., Ribeiro, D.N. 2012. Multiple resistance to glyphosate and pyrithiobac in Palmer amaranth (Amaranthus palmeri) from Mississippi and response to flumiclorac. Weed Science. 60:179-188.
Interpretive Summary: The rapid and widespread adoption of glyphosate-resistant crops, associated with the intense use of glyphosate and lack of rotation with non-glyphosate-resistant crops has resulted in reduced control of several populations of Palmer amaranth in Mississippi and several US states. Glyphosate-resistant Palmer amaranth populations could reduce yield and quality of row crops such as corn, cotton, rice, and soybean. Scientists from the Crop Production Systems Research Unit (USDA-ARS), Mississippi State University, and other institutions conducted studies to verify resistance to glyphosate, to explore alternative chemical control methods, and to understand why Palmer amaranth in Mississippi is resistant to glyphosate. Two Palmer amaranth biotypes were found to be 14- to17-fold resistant to glyphosate, compared to a glyphosate-sensitive biotype. The glyphosate-resistant biotypes were also resistant to pyrithiobac, whose herbicidal action is different from that of glyphosate. Flumiclorac, another non-glyphosate type-of-action herbicide, disrupted glyphosate uptake and movement in Palmer amaranth plants when applied in combination with glyphosate. The glyphosate-resistant Palmer amaranth biotypes absorbed and translocated less glyphosate compared to a sensitive biotype. These results provide valuable information to farmers, crop consultants, and other scientists on nature and level of glyphosate resistance in Palmer amaranth populations from Mississippi and on alternative methods of control.
Greenhouse and laboratory studies were conducted to confirm and quantify glyphosate resistance, to investigate interactions between flumiclorac and glyphosate mixtures on weed control, to determine patterns of absorption and translocation of glyphosate applied alone and in combination with flumiclorac, and to determine physiological mechanism of resistance to glyphosate in Palmer amaranth from Mississippi. The GR50 (herbicide dose required to cause a 50% reduction in plant growth) values for two glyphosate-resistant biotypes, C1B1 and T4B1, and a –susceptible biotype, GS, were 1.52, 1.3, and 0.09 kg ae ha-1 glyphosate. This indicated that the C1B1 and T4B1 biotypes were 17- and 14-fold resistant to glyphosate, respectively, compared to the GS biotype. The C1B1 and T4B1 biotypes were also resistant to pyrithiobac, an acetolactate synthase (ALS)-inhibitor, with GR50 values of 0.06 and 0.07 kg ai ha-1, respectively. This indicated that the C1B1 and T4B1 biotypes were 7- and 8-fold, respectively, less sensitive to pyrithiobac compared to the GS biotype that had a GR50 value of 0.009 kg ha-1. Flumiclorac was antagonistic to glyphosate by reducing glyphosate translocation. The C1B1 and T4B1 absorbed less glyphosate 48 h after treatment (HAT) compared to the GS biotype. The majority of the translocated glyphosate accumulated in the shoot above the treated leaf (that contains the apical meristem) in the GS biotype and in the shoot below the treated leaf in the resistant biotypes, C1B1 and T4B1, by 48 HAT. At 48 HAT, the roots of T4B1 biotype accumulated 30% glyphosate, which was appreciably higher than the 13% and 15% of glyphosate accumulated in the roots of C1B1 and GS biotypes, respectively. The C1B1 biotype accumulated negligible shikimate levels compared to the T4B1 and GS biotypes. Metabolism of glyphosate to AMPA was not detected in either of the resistant biotypes or the susceptible GS biotype. These results indicate that glyphosate resistance in the C1B1 and T4B1 population is partly due to reduced absorption and differential distribution of glyphosate and these biotypes are also resistant to an ALS-inhibiting herbicide.