Downy Brome (Cheatgrass) (Bromus tectorum) is a monocot weed in the Poaceae family.  In France this weed first evolved resistance to Group C1/5 herbicides in 1981 and infests Corn (maize).   Group C1/5 herbicides are known as Photosystem II inhibitors (Inhibition of photosynthesis at photosystem II).  Research has shown that these particular biotypes are resistant to atrazine and they may be cross-resistant to other Group C1/5 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 Sep 15, 2000 )

Common NameDowny Brome (Cheatgrass)
SpeciesBromus tectorum
GroupPhotosystem II inhibitors (C1/5)
Situation(s)Corn (maize)
Contributors - (Alphabetically)Jacques Gasquez 
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Jacques Gasquez
Bromus tectorum was resistant on railways not very far from Paris Today the only resistant population ever tested has disappeared. So it is impossible to estimate the actual spread of this biotype.

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Jacques Gasquez
Disappeared from the field! R populations were identified but no further problems have been reported.

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Jacques Gasquez
De Prado, R, and Menendez, J. Management of herbicide-resistant grass weeds in Europe. In Second International Weed Control Congress - Copenhagen, 1996. pp 393 - 398.

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Confirmation Tests

Greenhouse trials comparing a known susceptible Downy Brome (Cheatgrass) biotype with this Downy Brome (Cheatgrass) 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 C1/5 resistant Downy Brome (Cheatgrass) 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

Triazine resistant weeds often exhibit a lower relative fitness when compared to susceptible biotypes.  The most common mutation conferring triazine resistance (Ser 264 to Gly mutation of the psbA gene) also causes a reduction in CO2 fixation, quantum yield, and seed and biomass production.  There is no record in this database referring specifically to fitness studies on Group C1/5 resistant Downy Brome (Cheatgrass) from France.
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Laboratoire De Malherbologie
702 Hickory Ct
Dijon, 21034, Cedex
Email Jacques Gasquez


The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in France have been instrumental in providing you this information. Particular thanks is given to Jacques Gasquez for providing detailed information.
Herbicide Resistant Downy Brome (Cheatgrass) Globally
(Bromus tectorum)
Herbicide Resistant Downy Brome (Cheatgrass) Globally
(Bromus tectorum)
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Herbicide Resistant Downy Brome (Cheatgrass) Globally
(Bromus tectorum)
#CountryFirstYearSituationActive IngredientsSite of Action
1 FranceFrance 1981 Corn (maize) atrazine 17 Photosystem II inhibitors (C1/5)
40Bromus tectorumDowny Brome (Cheatgrass)457
2 SpainSpain 1990 Wheat chlorotoluron 38 PSII inhibitor (Ureas and amides) (C2/7)
40Bromus tectorumDowny Brome (Cheatgrass)190
3 SpainSpain 1990 Orchards simazine 38 Photosystem II inhibitors (C1/5)
40Bromus tectorumDowny Brome (Cheatgrass)191
4 United States (Oregon) United StatesOregon1997 Kentucky bluegrass imazamox, primisulfuron-methyl, and sulfosulfuron 45 ALS inhibitors (B/2)
40Bromus tectorumDowny Brome (Cheatgrass)1112
5 United States (Oregon) United StatesOregon2005 Grass seed clethodim, fluazifop-P-butyl, quizalofop-P-ethyl, and sethoxydim 45 ACCase inhibitors (A/1)
40Bromus tectorumDowny Brome (Cheatgrass)5375
6 United States (Montana) United StatesMontana2016 Wheat imazamox, procarbazone-sodium, and pyroxsulam 45 ALS inhibitors (B/2)
40Bromus tectorumDowny Brome (Cheatgrass)15071
Literature about Similar Cases

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Heap, I.  The International Herbicide-Resistant Weed Database.  Online.  Monday, August 10, 2020 .  Available Copyright © 1993- 2020 All rights reserved. Fair use of this material is encouraged. Proper citation is requested.