Structures and uses of auxintype herbicides

The auxin-type herbicides currently in use are categorised into five groups (Table 7.1). These are the phenoxyalkanoic acids, benzoic acids (which includes natural auxin), pyri-dines and the more recently announced quinoline carboxylic acids. All active auxins appear to possess a free carboxyl group, which suggests that a negatively charged group is essential for activity, although other common chemical characteristics are less obvious. The presence and position of halogens has a profound effect on both activity and selectivity since, for example, 2,3-D and 3,5-D have no auxin activity but 2,4-D does. Furthermore, the addition of an extra chlorine creates 2,4,5-T which can control woody plants that can tolerate 2,4 - D.

Table 7.2 provides a brief overview of the weed spectrum controlled by auxin-type herbicides. Although only some examples are included, Table 7.2 clearly illustrates the reasons for the development of these herbicides and the introduction of mixtures to give an even broader spectrum of weed control. Thus, typical MCPA-susceptible weeds include charlock, shepherd's purse and fat hen, mecoprop was introduced to obtain control of cleavers and chickweed, and dichlorprop was developed to control Polygonum spp. In the following decade clopyralid became available for the additional control of scentless mayweed and creeping thistle and, more recently, soil-applied quinmerac has been developed for further control of speedwells, red deadnettle and cleavers. Mixtures first appeared in the 1960s and dicamba, for example, proved a useful addition for the control of mayweed. More recently, the addition of clopyralid, benazolin and the hydroxybenzonitriles ioxynil and bromoxynil has widened even further the spectrum of weeds controlled, and so these herbicide 'cocktails' have become widely and routinely used, particularly in cereals.

New uses for auxin4ype herbicides continue to be reported. For example, in 2005 Brinkworth and colleagues reported that the mixture of two pyridine carboxylic acids, fluroxypyr and aminopyralid, was effective for the long-term selective control of annual and perennial broadleaf weeds in grassland.

Table 7.1 Structures of auxin-type herbicides.

(a) Phenoxyalkanoic Acids

O—C-

COOH

H

r2

Cl

R1

R2

R3

Common name

H

CH3

H

MCPA

H

Cl

H

2,4-D

H

Cl

Cl

2,4,5-T

CH3 CH3 CH3

Cl

H H Cl

mecoprop dichlorprop fenoprop

R

COOH

R

Common name

il ^^^

CH3

MCPB

Cl

2,4-DB

Cl

(b) Benzoic Acids

COOH

COOH

COOH

L5 3II

Y]/

OCH3 Cl

rr

OCH3 Cl

Cl

Cl NH2

Cl Cl

Cl

Cl

chloramben

dicamba

tricamba

2,3,6-TBA

(c) Aromatic Carboxymethyl Derivatives

Cl CH2COOH

(c) Aromatic Carboxymethyl Derivatives

Cl CH2COOH

CH2COOH

CH2COOH

s benazolin naphthylacetic acid (NAA)

indol-3yl-acetic acid (IAA)

(Continued)

Table 7.1 (Continued)

Table 7.1 (Continued)

The auxin-type herbicides have stood the test of time and remain a very effective means of weed control, more than 60 years after their introduction. Those in use today are generally of low persistence, are environmentally benign and are considered unlikely to result in major problems with weed resistance in the foreseeable future.

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