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Title: GLYPHOSATE-RESISTANT HORSEWEED (CONYZA CANADENSIS) IN MISSISSIPPI

Author
item Koger Iii, Clifford
item POSTON, D - DELTA RES & EXT CTR
item HAYES, R - UNIV OF TENNESSEE
item MONTGOMERY, R - MONSANTO COMPANY

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2004
Publication Date: 7/1/2004
Citation: Koger Iii, C.H., Poston, D.H., Hayes, R.M., Montgomery, R.F. 2004. Glyphosate-resistant horseweed (conyza canadensis) in mississippi. Weed Technology. 18:820-825.

Interpretive Summary: Glyphosate-resistant horseweed has been documented in several states of the southeastern United States. However, no populations of horseweed resistant to glyphosate have been documented in Mississippi to date. Seeds from mature plants that survived multiple glyphosate applications were collected from several fields in Mississippi and Tennessee in 2002. Plants originating from the collected seed were screened in greenhouse studies at the Southern Weed Science Research Unit, Stoneville, MS for potential resistance to glyphosate. All populations exhibited 8- to 12-fold level of resistance to glyphosate. These results demonstrate that the difficult-to-control biotypes were resistant to glyphosate, and that other management options other than glyphosate alone are warranted where populations of glyphosate-resistant horseweed persist.

Technical Abstract: Survival of horseweed in several glyphosate-tolerant-cotton and -soybean fields treated with glyphosate at recommended rates preplant and postemergence (POST) was observed in Mississippi and Tennessee in 2001 and 2002. Plants originating from seed collected from fields where horseweed escapes occurred in 2002 were grown in the greenhouse to the 5-leaf, 13- to 15-leaf, and 25- to 30-leaf growth stages and treated with the isopropylamine salt of glyphosate at 0, 0.035, 0.07, 0.14, 0.28, 0.56, 1.12, 2.24, 4.5, 9, and 18 kg ai/ha to determine if resistance to glyphosate existed in any biotype. All biotypes exhibited 8- to 12-fold level of resistance to glyphosate when compared with a susceptible biotype. Growth stage had little effect on level of glyphosate-resistance. One resistant biotype from Mississippi was 2- to 4-fold more resistant than other resistant biotypes. The glyphosate rate required to reduce biomass of glyphosate-resistant horseweed by 50% (GR50) increased from 0.24 to 3.0 kg/ha as plant size increased from the 5-leaf to 25- to 30-leaf growth stage. The GR50 rate for the susceptible biotype increased from 0.03 to 0.27 kg/ha glyphosate. These results demonstrate that the difficult-to-control biotypes were resistant to glyphosate, resistant biotypes could survive glyphosate rates of up to 16 kg/ha, and that plant size affected both resistant and susceptible biotypes in a similar manner.