(Cyperus iria)

ALS inhibitors (B/2)

United States, Louisiana
Rice Flatsedge (Cyperus iria) is a monocot weed in the Cyperaceae family.  In Louisiana this weed first evolved resistance to Group B/2 herbicides in 2013 and infests Rice.   Group B/2 herbicides are known as ALS inhibitors (Inhibition of Acetolactate Synthase ).  Research has shown that these particular biotypes are resistant to halosulfuron-methyl and they may be cross-resistant to other Group B/2 herbicides.

The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

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QUIK STATS (last updated Dec 19, 2013 )

Common NameRice Flatsedge
SpeciesCyperus iria
GroupALS inhibitors (B/2)
LocationUnited States, Louisiana
Contributors - (Alphabetically)Daniel Stephenson 
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Confirmation Tests

Greenhouse, and Laboratory trials comparing a known susceptible Rice Flatsedge biotype with this Rice Flatsedge biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetic studies on Group B/2 resistant Rice Flatsedge have not been reported to the site.  There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature
Mechanism of Resistance

The mechanism of resistance for this biotype is either unknown or has not been entered in the database.  If you know anything about the mechanism of resistance for this biotype then please update the database.
Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes.  If you have any information pertaining to the fitness of Group B/2 resistant Rice Flatsedge from Louisiana please update the database.
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Weed Scientist/specialist
Louisiana State University Agricultural Center
Dean Lee Research And Extension Center
8105 Tom Bowman Drive
Alexandria, 71302, Louisiana
United States
Email Daniel Stephenson
Web   : Web Site Link


The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Louisiana have been instrumental in providing you this information. Particular thanks is given to Daniel Stephenson for providing detailed information.
Herbicide Resistant Rice Flatsedge Globally
(Cyperus iria)
Herbicide Resistant Rice Flatsedge Globally
(Cyperus iria)
Drag a column header and drop it here to group by that column
Herbicide Resistant Rice Flatsedge Globally
(Cyperus iria)
#CountryFirstYearSituationActive IngredientsSite of Action
1 BrazilBrazil 2014 Rice bispyribac-sodium, imazapic, imazethapyr, penoxsulam, and pyrazosulfuron-ethyl 5 ALS inhibitors (B/2)
266Cyperus iriaRice Flatsedge13040
2 United States (Arkansas) United StatesArkansas2010 Rice bispyribac-sodium, halosulfuron-methyl, imazamox, imazethapyr, and penoxsulam 45 ALS inhibitors (B/2)
266Cyperus iriaRice Flatsedge5560
3 United States (Louisiana) United StatesLouisiana2013 Rice halosulfuron-methyl 45 ALS inhibitors (B/2)
266Cyperus iriaRice Flatsedge7785
4 VenezuelaVenezuela 2015 Rice pyrazosulfuron-ethyl 80 ALS inhibitors (B/2)
266Cyperus iriaRice Flatsedge11015
Literature about Similar Cases

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PERMISSION MUST BE OBTAINED FIRST if you intend to base a significant portion of a scientific paper on data derived from this site. Citation:
Heap, I.  The International Herbicide-Resistant Weed Database.  Online.  Saturday, April 10, 2021 .  Available Copyright © 1993- 2021 All rights reserved. Fair use of this material is encouraged. Proper citation is requested.