Structures and uses of graminicides

Graminicides may be categorised into six chemical families, namely the thiocarbamates, chloroacetamides, alaninopropionates, aryloxyphenoxypropionates (AOPPs), cyclohex-anediones (CHDs) and the phenylpyrazolines (Table 8.1).

The selective control of wild oats in cereals first became possible in the late 1950s with the introduction of the chlorophenylcarbamate, barban and the thiocarbamate, diallate.

Herbicides and Plant Physiology, Second Edition Andrew H. Cobb and John P.H. Reade © 2010 A.H. Cobb and J.P.H. Reade. ISBN: 978-1-405-12935-0

Table 8.1 Structures of graminicides.

(a) thiocarbamates

CsH7^

C3H7'

Common name

Cl Cl

Cl Cl

s diallate triallate

EPTC

(b) chloroacetamides

Common name

C2H5

C2H5 C2H5

C2H5

C — CH2OCH3 CHs acetochlor alachlor metazachlor metolachlor

Table 8.1 {Continued)

Table 8.1 {Continued)

R-i R2 R3 R4 Common name

Cl Cl H C2H3 benzoylprop-ethyl

H F Cl C3H7 flamprop-isopropyl

H F Cl CH3 flamprop-methyl

Table 8.1 {Continued)

Common name

C2H5

C4H0

propargyl

(CH3)2C=NOCH2CH2

fenoxaprop-ethyl fenthiaprop-ethyl cyhalofop-butyl clodinofop-propargyl propaquizafop

(e) cyclohexanediones

R2 O

R3 R4

Common name

CH3 CH3

C2H5S C CH2 H

C=O c3h7 ch2ch=ch2

C2H5 CH2CH=CH

C3H7 C2H5

alloxydim clethodim sethoxydim

Table 8.1 {Continued)

Table 8.1 {Continued)

However, barban has proven to be phytotoxic to certain barley cultivars, and the activity of the soil-applied diallate varies with soil type, condition and placement. Triallate has superseded diallate in cereals, since wheat and barley are more tolerant to this compound. These volatile herbicides must be rapidly incorporated into the top 2.5 cm of soil for maximum efficacy. Grasses which germinate from seed grow through this treated layer and are killed, although surface germinating seeds or those at greater depth are not controlled. No soil incorporation is required for the pre-emergent chloroacetamides, since these herbicides have a residual action in the soil, and so can kill annual broadleaf weeds and annual grasses for up to eight weeks after application.

The post-emergent alaninopropionates were introduced between 1969 and 1972 for the control of wild oats already present in cereals, and the introduction of the AOPPs and the CHDs in the mid-1970s has enabled the control of a broad spectrum of annual and perennial grasses in a wider range of crops. Furthermore, the development of the AOPPs (also termed the 'fops', such as diclofop, fluazifop and quizalofop) and the CHDs (also termed the 'dims', such as sethoxydim, tralkoxydim and clethodim) has produced active ingredients with widespread and successful grass-weed control at doses as low as 100-200 g ha-1. Recent commercialisation of pinoxaden, a phenylpyrazolin that acts at the same target site as fops and dims but is chemically distinct from these groups, has resulted in a third class of ACCase-inhibiting graminicides which are termed the 'dens'.

In the cases of the alaninopropionates and the AOPPs , the C2 of the propanoic acid group is a chiral centre so that both (R) and (S) enantiomers exist. Interestingly, only the (R) enantiomer is biologically active, so the removal of the inactive (S) half of an isomeric mix represents a doubling of activity. Thus, (R) isomers are commercially produced and used at half the dose rate of the racemic mixture.

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