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Part 1. Herbicide Resistance in Weeds

The phenomenon of herbicide resistance has attracted man concern over decades. By August 2004, there are 291 resistant biotypes, 174 species (104 dicots and 70 monocots) and over 270,000 fields showed resistance (WSSA, 2004).

The herbicide resistance in weeds is a result of selection for traits that allow weed species to survive specific management practices that would otherwise cause mortality. It is defined as “the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type” (WSSA, 1998).

Like other crop pesticides, herbicide resistant weeds are a symptom of intensive and extensive selection pressure targeting specific physiological or biochemical processes. This term is generally used to describe a characteristic of species (as intact plant or plant cells in culture) to withstand substantially higher concentrations of a herbicide than the wild type of the same plant species. The difference in herbicide resistance between phenotypes of the same species is qualitatively assessed in most cases by comparing the herbicide rates required to reduce survival (LD50), biomass (GR50 or ED50), or specific enzyme activity (I50) by 50% from untreated plants (Maxwell and Mortimer, 1994).

Pesticide resistance had already appeared as early as the 1950s, in insects and pathogens. However, it was until 1968 that the first herbicide-resistant weed was discovered, i.e. a biotype of Senecio vulgaris that was no longer controlled by the PS II inhibitor simazine (Ryan, 1970). In the next two decades, there have been numerous reports of weed biotypes exhibiting resistance to PS II inhibiting herbicides (Bandeen et al, 1982).

Herbicide resistance is being searched in many countries. Table 1 indicates the number of herbicide resistant weed biotypes (to different herbicidal groups; their mode of action) (WSSA, 2004). Table 2 shows herbicide resistant weeds by country and mode of action. A worldwide survey completed in 2003 indicated that the acetolactate synthase (ALS), PS II, ACCase inhibitors resistance is the most prevalent type of herbicide resistance found in weeds; they constitute 79, 64 and 30 resistant biotypes out of sum 272 that representing all resistant weed biotypes to herbicides. Meanwhile, wild oat represents the second most important herbicide-resistant species (Table 3). Table 4. represents herbicide-resistant wild oat - as one the main grass weeds -globally: country, year, sites, acres and mode of action (WSSA, 2004).

Table 1. Number of herbicide resistant weed biotypes (to different herbicidal groups; their mode of action) (WSSA, 2004).

Herbicide Resistant Weeds Summary Table
Herbicide Group	Mode of Action	HRAC Group*	Example Herbicide	Total
ALS inhibitors	Inhibition of acetolactate synthase ALS (acetohydroxyacid synthase AHAS)	B	Chlorsulfuron	79
Photosystem II inhibitors	Inhibition of photosynthesis at photosystem II	C1	Atrazine	64
ACCase inhibitors	Inhibition of acetyl CoA carboxylase (ACCase)	A	Diclofop-methyl	30
Synthetic Auxins	Synthetic auxins (action like indoleacetic acid)	O	2,4-D	23
Bipyridiliums	Photosystem-I-electron diversion	D	Paraquat	22
Ureas and amides	Inhibition of photosynthesis at photosystem II	C2	Chlorotoluron	20
Dinitroanilines and others	Microtubule assembly inhibition	K1	Trifluralin	10
Thiocarbamates and others	Inhibition of lipid synthesis - not ACCase inhibition	N	Triallate	7
Triazoles, ureas, isoxazolidiones	Bleaching: Inhibition of carotenoid biosynthesis (unknown target)	F3	Amitrole	4
Glycines	Inhibition of EPSP synthase	G	Glyphosate	4
Chloroacetamides and others	Inhibition of cell division (Inhibition of very long chain fatty acids)	K3	Butachlor	2
Nitriles and others	Inhibition of photosynthesis at photosystem II	C3	Bromoxynil	1
PPO inhibitors	Inhibition of protoporphyrinogen oxidase (PPO)	E	Oxyfluorfen	1
Carotenoid biosynthesis inhibitors	Bleaching: Inhibition of carotenoid biosynthesis at the phytoene desaturase step (PDS)	F1	Flurtamone	1
Mitosis inhibitors	Inhibition of mitosis / microtubule polymerization inhibitor	K2	Propham	1
Organoarsenicals	Unknown	Z	MSMA	1
Arylaminopropionic acids	Unknown	Z	Flamprop-methyl	1
Pyrazoliums	Unknown	Z	Difenzoquat	1
Total Number of Unique Herbicide Resistant Biotypes				272

* Herbicide Groups are according to “Herbicide Resistance Action Committee “HRAC”, WSSA).

Table 2. Herbicide resistant weeds by mode of action, in some countries (WSSA, 2004).

HERBICIDE RESISTANT WEEDS BY COUNTRY AND MODE OF ACTION – click on a country for details.
#	Country Click for details	Total	A ACCase Inhibitors	B ALS inhibitor	C1 Triazine	C2 Urea/ Amide	D Bypiri- dilium	K1 Dinitro- aniline	O Synthetic Auxin	Other
1.	Argentina	1	0	1	0	0	0	0	0	0
2.	Australia	38	8	15	3	0	4	2	0	6
3.	Austria	2	0	0	2	0	0	0	0	0
4.	Belgium	18	2	1	7	1	3	1	0	3
5.	Bolivia	2	2	0	0	0	0	0	0	0
6.	Brazil	9	1	6	0	0	0	0	2	0
7.	Bulgaria	4	0	0	2	1	0	1	0	0
8.	Canada	39	2	14	11	3	2	1	4	2
9.	Chile	5	4	0	0	0	0	0	0	1
10.	China	5	0	0	1	2	0	0	0	2
11.	Colombia	3	1	0	0	1	0	0	1	0
12.	Costa Rica	5	1	3	0	1	0	0	0	0
13.	Czech Republic	13	0	1	12	0	0	0	0	0
14.	Denmark	1	0	1	0	0	0	0	0	0
15.	Ecuador	1	0	0	0	1	0	0	0	0
16.	Egypt	1	0	0	0	0	1	0	0	0
17.	El Salvador	1	0	0	0	1	0	0	0	0
18.	Ethiopia	1	1	0	0	0	0	0	0	0
19.	Fiji	1	0	0	0	0	1	0	0	0
20.	France	30	5	1	22	1	0	0	1	0
21.	Germany	17	1	0	13	3	0	0	0	0
22.	Greece	5	1	1	2	1	0	0	0	0
23.	Guatemala	1	0	0	0	1	0	0	0	0
24.	Honduras	1	0	0	0	1	0	0	0	0
25.	Hungary	1	0	0	0	0	0	0	1	0
26.	India	1	0	0	0	1	0	0	0	0
27.	Indonesia	1	0	0	0	0	0	0	1	0
28.	Ireland	1	0	1	0	0	0	0	0	0
29.	Israel	19	2	5	11	1	0	0	0	0
30.	Italy	12	3	4	4	1	0	0	0	0
31.	Japan	16	0	9	1	0	6	0	0	0
32.	Kenya	1	0	0	0	0	1	0	0	0
33.	Malaysia	14	1	3	0	0	6	0	3	1
34.	Mexico	2	2	0	0	0	0	0	0	0
35.	New Zealand	9	0	1	3	0	0	0	4	1
36.	Nicaragua	1	1	0	0	0	0	0	0	0
37.	Norway	4	0	0	3	1	0	0	0	0
38.	Panama	1	0	0	0	1	0	0	0	0
39.	Paraguay	1	0	1	0	0	0	0	0	0
40.	Philippines	1	0	0	0	0	0	0	1	0
41.	Poland	9	0	1	8	0	0	0	0	0
42.	Portugal	1	0	1	0	0	0	0	0	0
43.	Saudi Arabia	1	1	0	0	0	0	0	0	0
44.	Slovenia	1	0	0	1	0	0	0	0	0
45.	South Africa	7	2	2	1	0	1	0	0	1
46.	South Korea	5	0	5	0	0	0	0	0	0
47.	Spain	26	1	3	18	3	0	0	1	0
48.	Sri Lanka	2	0	0	0	1	1	0	0	0
49.	Sweden	2	0	1	0	0	0	0	1	0
50.	Switzerland	14	0	0	11	3	0	0	0	0
51.	Taiwan	1	0	0	0	0	1	0	0	0
52.	Thailand	4	1	0	0	1	0	0	1	1
53.	The Netherlands	7	1	0	5	1	0	0	0	0
54.	Tunisia	1	1	0	0	0	0	0	0	0
55.	Turkey	2	2	0	0	0	0	0	0	0
56.	United Kingdom	23	4	3	8	2	2	1	2	1
57.	USA	95	12	34	18	4	3	6	7	11
58.	Venezuela	1	0	0	0	1	0	0	0	0
59.	Yugoslavia	3	0	0	3	0	0	0	0	0

Table 3. Most important herbicide – resistant species biotypes* (WSSA, 2004).

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Common Name Scientific Name

1. Rigid ryegrass Lolium rigidum

2. Wild oat Avena fatua

3. Redroot pigweed Amaranthus retroflexus

4. Common lambsquarters Chenopodium album

5. Green foxtail Setaria viridis

6. Barnyardgrass Echinochloa crus-galli

7. Goosegrass Elucine indica

8. Kochia Kochia scoparia

9. Horseweed Conyza canadensis

10. Smooth pigweed Amaranthus hybridus

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*Weed species No. 2,4,5,6,7,10 are common in Egypt (Boulos and el-Hadidi, 1984).

Table 4. Herbicide resistant wild oat globally: country, year, sites, acres and mode of action (WSSA, 2004)

HERBICIDE RESISTANT WILD OAT GLOBALLY Avena fatua
#	Country Click for details	Year	Sites	Acres	Mode of Action
1.	Australia (Western Australia)	1985	11-50	1001-10000	ACCase inhibitors (A/1)
2.	Australia (South Australia)	1988	51-100	1001-10000	ACCase inhibitors (A/1)
3.	Australia (New South Wales )	1991	11-50	501-1000	ACCase inhibitors (A/1)
4.	Belgium	1996	1	1-5	ACCase inhibitors (A/1)
5.	Canada (Alberta) Multiple Resistance	1989	1001-10000	1000001-2000000	Thiocarbamates and others (N/8) Pyrazoliums (Z/8)
6.	Canada (Manitoba)	1990	1001-10000	>2000000	ACCase inhibitors (A/1)
7.	Canada (Saskatchewan)	1990	1001-10000	>2000000	ACCase inhibitors (A/1)
8.	Canada (Alberta)	1991	101-500	10001-100000	ACCase inhibitors (A/1)
9.	Canada (Manitoba) Multiple Resistance	1994	51-100	1001-10000	ACCase inhibitors (A/1) ALS inhibitors (B/2) Arylaminopropionic acids (Z/25)
10.	Canada (Saskatchewan)	1996	1001-10000	100001-1000000	ALS inhibitors (B/2)
11.	Canada (Saskatchewan)	1996	1001-10000	100001-1000000	Thiocarbamates and others (N/8)
12.	Canada (Saskatchewan) Multiple Resistance	1996	1001-10000	100001-1000000	ACCase inhibitors (A/1) ALS inhibitors (B/2) Thiocarbamates and others (N/8)
13.	Canada (Manitoba)	1997	1001-10000	100001-1000000	ALS inhibitors (B/2)
14.	Canada (Manitoba)	1997	501-1000	100001-1000000	Thiocarbamates and others (N/8)
15.	Canada (Manitoba) Multiple Resistance	1997	6-10	1001-10000	ACCase inhibitors (A/1) ALS inhibitors (B/2) Thiocarbamates and others (N/8) Arylaminopropionic acids (Z/25)
16.	Canada (Alberta)	1998	11-50	1001-10000	ALS inhibitors (B/2)
17.	Canada (Alberta) Multiple Resistance	1998	11-50	1001-10000	ACCase inhibitors (A/1) ALS inhibitors (B/2) Thiocarbamates and others (N/8)
18.	Chile	1998	11-50	10001-100000	ACCase inhibitors (A/1)
19.	France	1996	11-50	101-500	ACCase inhibitors (A/1)
20.	South Africa Multiple Resistance	1986	2-5	51-100	ACCase inhibitors (A/1) ALS inhibitors (B/2)
21.	United Kingdom Multiple Resistance	1994	11-50	51-100	ACCase inhibitors (A/1) ALS inhibitors (B/2) Arylaminopropionic acids (Z/25)
22.	USA (Oregon)	1990	101-500	1001-10000	ACCase inhibitors (A/1)
23.	USA (Montana) Multiple Resistance	1990	501-1000	10001-100000	Thiocarbamates and others (N/8) Pyrazoliums (Z/8)
24.	USA (Montana)	1990	51-100	1001-10000	ACCase inhibitors (A/1)
25.	USA (Oregon)	1990	1	11-50	Dinitroanilines and others (K1/3)
26.	USA (Minnesota)	1991	51-100	1001-10000	ACCase inhibitors (A/1)
27.	USA (North Dakota)	1991	101-500	1001-10000	ACCase inhibitors (A/1)
28.	USA (Washington)	1991	51-100	1001-10000	ACCase inhibitors (A/1)
29.	USA (Idaho)	1992	11-50	1001-10000	ACCase inhibitors (A/1)
30.	USA (Idaho) Multiple Resistance	1993	51-100	10001-100000	Thiocarbamates and others (N/8) Pyrazoliums (Z/8)
31.	USA (Montana)	1996	2-5	11-50	ALS inhibitors (B/2)
32.	USA (North Dakota)	1996	2-5	501-1000	ALS inhibitors (B/2)
33.	USA (California)	1996	11-50	1001-10000	Pyrazoliums (Z/8)
34.	USA (Colorado)	1997	6-10	101-500	ACCase inhibitors (A/1)

World survey on herbicide resistant weeds:

http://www.weedscience.org/in.asp

Herbicide resistance in plants:

http://www.weedscience.org/Summary/UspeciesMOA.asp?lstMOAID=2

http://www.weedscience.org/summary/CountrySummary.asp

http://www.weedscience.org/summary/MOASummary.asp

World distribution of herbicide (ACCase inhibitors) resistant weeds:

http://www.weedscience.org/ACCaseDist.GIF

Countries, number of locations, areas of in which herbicide resistance in wild oat appeared:

http://www.weedscience.org/Summary/USpeciesCountry.asp?lstWeedID=28&FmCommonName=Go

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