(Oryza sativa var. sylvatica)

ALS inhibitors (B/2)

Malaysia, Pulau Pinang
Red Rice (Oryza sativa var. sylvatica) is a monocot weed in the Poaceae family.  In Malaysia this weed first evolved resistance to Group B/2 herbicides in 2018 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 imazapic, and imazapyr 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 Jul 16, 2018 )

Common NameRed Rice
SpeciesOryza sativa var. sylvatica
GroupALS inhibitors (B/2)
Herbicidesimazapic, and imazapyr
LocationMalaysia, Pulau Pinang
Contributors - (Alphabetically)Nilda R. Burgos, and Masilamany Dilipkumar  
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Masilamany Dilipkumar

CROSS-RESISTANCE TO IMAZAPIC AND IMAZAPYR IN A WEEDY RICE (Oryza sativa) BIOTYPE FOUND IN MALAYSIA Biótipos de Arroz Vermelho (Oryza sativa) com Resistência Cruzada a Imazapic DILIPKUMAR, M.* e Imazapyr Encontrados na Malásia BURGOS, N.R. CHUAH, T.S.  ISMAIL, S.                                                                                                                                   

 ABSTRACT - The Clearfield® rice production system is an effective management tool for weedy rice and other weeds in the direct-seeded rice culture. However, ifarmers cultivating the Clearfield rice disregard stewardship recommendations, the industry could face a problem of herbicide-resistant weedy rice which would occur through the selection of outcrosses. This study aimed to confirm imidazolinoneresistant weedy rice in Malaysia. The resistant weedy rice (R-WR) was found to be 67 fold more resistant to OnDuty® (premix of imazapic and imazapyr) than the susceptible weedy rice (S-WR) based on the GR50 values (rate that causes 50% inhibition of shoot growth). The Clearfield® rice cultivar was 32-fold more tolerant to OnDuty® than the S-WR. Furthermore, the R-WR was 54 and 89 fold more resistant to imazapic and imazapyr applied separately than the S-WR, respectively. The Clearfield® rice was 140- and 40-fold more tolerant to imazapic and imazapyr, respectively than the S-WR. The R-WR biotype was susceptible to non-selective herbicides glyphosate and glufosinate, as well as the selective graminicide quizalofop. Oxadiazon controlled the R-WR biotype, but pretilachlor was ineffective. The present study documented the first case of weedy rice that was cross-resistant to imazapic and imazapyr in Malaysian Clearfield® rice field.


* Corresponding author:
Received: August 8, 2017
Approved: August 15, 2017
Planta Daninha 2018; v36:e018182239
1 Rice Research Centre, Malaysian Agricultural Research and Development Institute (MARDI), MARDI Seberang Perai,
13200 Kepala Batas, Pulau Pinang, Malaysia; 2 University of Arkansas, 1366 W. Altheimer Drive, Fayetteville, AR 72704, USA;
3 University of Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia; 4 Universiti Kebangsaan Malaysia, 43600
UKM Bangi, Selangor, Malaysia.
Doi: 10.1590/S0100-83582018360100058
ISSN 0100-8358 (print)
1806-9681 (online)

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Confirmation Tests

Greenhouse trials comparing a known susceptible Red Rice biotype with this Red Rice 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 Red Rice 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 Red Rice from Malaysia please update the database.
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Professor - Weed Physiology
University Of Arkansas, Fayetteville
Crop, Soil
And Environmental Sciences
1366 W. Altheimer Dr.
Fayetteville, 72704, Arkansas
United States
Email Nilda R. Burgos
Web   : Web Site Link
Senior Research Officer (Weed Science)
Malaysian Agricultural Research and Development Institute (MARDI)
Inbred Rice Program
Rice Research Center
Seberang Perai
Kepala Batas, 13200, Pulau Pinang
Email Masilamany Dilipkumar
Web   : Web Site Link


The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Malaysia have been instrumental in providing you this information. Particular thanks is given to Nilda R. Burgos, and Masilamany Dilipkumar for providing detailed information.
Herbicide Resistant Red Rice Globally
(Oryza sativa var. sylvatica)
Herbicide Resistant Red Rice Globally
(Oryza sativa var. sylvatica)
Drag a column header and drop it here to group by that column
Herbicide Resistant Red Rice Globally
(Oryza sativa var. sylvatica)
#CountryFirstYearSituationActive IngredientsSite of Action
1 BrazilBrazil 2006 Rice imazapic, and imazethapyr 5 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice5372
2 ColombiaColombia 2018 Rice imazamox, and imazapyr 10 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice18170
3 Costa RicaCosta Rica 2010 Rice imazapic, and imazapyr 11 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice7885
4 GreeceGreece 2013 Rice imazamox, and imazethapyr 19 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice13054
5 ItalyItaly 2010 Rice imazamox 24 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice5614
6 MalaysiaMalaysia 2018 Rice imazapic, and imazapyr 28 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice17145
7 TurkeyTurkey 2020 Rice imazamox 71 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice19217
8 United States (Arkansas) United StatesArkansas2002 Rice imazamox, imazapyr, imazaquin, imazethapyr, and pyrithiobac-sodium 45 ALS inhibitors (B/2)
242Oryza sativa var. sylvaticaRed Rice13046
Literature about Similar Cases

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