Skip to main content
ARS Home » Southeast Area » Tifton, Georgia » Crop Protection and Management Research » Research » Publications at this Location » Publication #191445
Title: GLYPHOSATE-RESISTANT PALMER AMARANTH (AMARANTHUS PALMERI) CONFIRMED IN GEORGIA

Author
item CULPEPPER, A - UNIVERSITY OF GEORGIA
item GREY, T - UNIVERSITY OF GEORGIA
item VENCILL, W - UNIVERSITY OF GEORGIA
item KICHLER, J - UNIVERSITY OF GEORGIA
item Webster, Theodore
item BROWN, S - UNIVERSITY OF GEORGIA
item YORK, A - NC STATE UNIVERSITY
item DAVIS, J - UNIVERSITY OF GEORGIA
item HANNA, W - UNIVERSITY OF GEORGIA

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2006
Publication Date: 7/1/2006
Citation: Culpepper, A.S., Grey, T.L., Vencill, W.K., Kichler, J.M., Webster, T.M., Brown, S.M., York, A.C., Davis, J.W., Hanna, W.W. 2006. Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) confirmed in Georgia. Weed Science. 54:620-626.

Interpretive Summary: Palmer amaranth is among the five most troublesome weeds in cotton, peanut, and soybean throughout the Southeast US. A glyphosate-resistant Palmer amaranth biotype was confirmed in central Georgia. In the field, glyphosate applied at three times the normal use rate controlled this biotype only 17%. The biotype was controlled 82% by glyphosate at 12-times the normal use rate. In the greenhouse, I50 values (rate necessary for 50% inhibition) for visible control and shoot fresh weight, expressed as percent of the non-treated, were 8 and 6.2 times greater, respectively, with the resistant biotype compared with a known glyphosate-susceptible biotype. Data suggests resistance is due to an altered target site. This is the world’s first confirmed case of glyphosate resistance in an Amaranthus species and has significant ramifications for future weed management. Rapid spread by pollen is expected in this dioecious species (forced outcrossing). Moreover, resistance to other herbicides, such as dinitroanilines and acetolactate synthase inhibitors, limits the options to control glyphosate-resistant Palmer amaranth. Other Amaranthus species are prevalent in all regions of North America, especially in the midwestern and southern US where glyphosate-resistant crops have been broadly adopted. Glyphosate resistance in this particular Palmer amaranth population will likely spread to other adjacent Amaranthus species by outcrossing, limited only by movement of viable pollen in the atmosphere.

Technical Abstract: Palmer amaranth is among the three most troublesome weeds in Georgia cotton, peanut, and soybean and is among the top five most troublesome weeds in most other southeastern states. A glyphosate-resistant Palmer amaranth biotype was confirmed in central Georgia. In the field, potassium salt of glyphosate applied to 5- to 13-cm tall Palmer amaranth at three times the normal use rate of 0.84 kg ae ha-1 controlled this biotype only 17%. The biotype was controlled 82% by glyphosate at 12 times the normal use rate. In the greenhouse, I50 values (rate necessary for 50% inhibition) for visible control and shoot fresh weight, expressed as percent of the non-treated, were 8 and 6.2 times greater, respectively, with the resistant biotype compared with a known glyphosate-susceptible biotype. Glyphosate absorption and translocation and the number of chromosomes did not differ between biotypes. Shikimate was detected in leaf tissue of the susceptible biotype treated with glyphosate but not in the resistant biotype. This suggests resistance is due to an altered target site. This is the world’s first confirmed case of glyphosate resistance in an Amaranthus species and it is a significant finding with serious ramifications for future weed management. Rapid spread by pollen is expected in this dioecious species (forced outcrossing). Moreover, resistance to other herbicides, such as dinitroanilines and acetolactate synthase inhibitors, limits the options to control glyphosate-resistant Palmer amaranth.