Chronology of herbicide resistance

The first reported case of herbicide resistance is generally held to be that of resistance to triazines in Senecio vulgaris in the USA. Ryan (1970) reported that resistance to simazine and atrazine was detected in this species in 1968. Undoubtedly, herbicide resistance had occurred prior to this date and some earlier reports do highlight poor control that can, in hindsight, be interpreted as possible detection of resistant biotypes. Since the first confirmed case of herbicide resistance, numbers of cases have continued to rise year upon year (Figure 12.6). By summer 2010, 346 resistant biotypes had been confirmed from 194 species (114 dicots and 80 monocots) (Heap, 2010) . Resistance has been found to many classes of herbicide and is most prevalent against ACCase inhibitors, triazines and ALS inhibitors. The current 'worst weeds' with respect to herbicide resistance are shown in Table 12.4 . Although only introduced to the market in 1982, ALS inhibition is the most widespread

Table 12.4 The most important herbicide- resistant weed species worldwide, as stated by Heap, 2009. Ranking is based upon number of cited references.

1

Rigid ryegrass

Lolium rigidum

2

Wild oat

Avena fatua

a

Redroot pigweed

Amaranthus retroflexus

4

Fat hen

Chenopodium album

5

Green foxtail

Setaria viridis

e

Barnyardgrass

Echinochloa crus-galli

i

Goose grass

Eleusine indica

e

Kochia

Kochia scoparia

9

Horseweed

Conyza canadensis

10

Smooth pigweed

Amaranthus hybridus

mode of action for which resistance is reported. This is probably a combined result of the extensive use of these compounds, the number of different active ingredients of this type on the market and the specificity of the site of action. Additionally, the natural occurrence of a number of mutations of the target enzyme (ALS) that are less sensitive to herbicide, but are still enzymically active, has undoubtedly added to the problem.

Resistance is likely to continue to increase, especially where the number of available herbicides with different modes of action is reduced, where production costs reduce herbicide choice and where genetically modified, herbicide-tolerant (GM-HT) crops encourage the repeated use of a single herbicide in cropping systems.

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