Hancock Watch


Modes of Actions of Forestry Herbicides

Aug 06: Hancock pollutes Geelong Drinking Water with Hexazinone for 18 months (and counting)

(also see Factors Affecting Herbicide Performance)

There are 14 different Modes of Action groups with letters A to N.

Within these groups there are one of more chemical groups.

Some groups are not represented by herbicides registered for forestry or forest nursery use.

Groups are listed by resistance risk with Groups A & B being high risk, C to H moderate, and I to N low.

Group A

Arylphoxypropionates - ‘fops’.

Inhibit lipid synthesis, carbohydrates transport and chlorophyll manufacture and arrest growing points

Post-emergence grass specific (foliar active translocated).

General forestry registration for haloxyfop-R-methyl (Verdict 520, 520 g a.l/L) at 200 - 800 mL/ha.

Fluazifop-p-butyl is registered for tropical pines - P.elliotii, P.carribea etc. Usually applied with oil or wetter, safe over trees

Cyclohexanediones - ‘dims’.

Also grass specific foliar active post-emergence (translocated).

General forestry registration for clethodim (Select Herbicide 240 g a.l./L) at 500 mL/ha.

Combinations of fops and dims are often more efficacious than alone. Both fops and dims are EC formulations.

Group B

Inhibitors of enzyme acetolactate synthase - ALS inhibitors

Sulfonyl ureas (SU’s), translocated, soil and foliar active ie absorbed by foliage and roots

Pre- and post-emergence

Application rates are low, in forestry usually <100g/ha of products DF or WG formulations

For forestry, metsulfuron methyl (BrushOff) and sulfometuron methyl (Oust)

Metsulfuron methyl is used for woody weed control and is often mixed with glyphosate (eg. in Trounce and Cut-Out).

Requires organosilicone surfactant (Pulse Penetrant) for foliar application.

Used pre-planting at low rates (6-9 g a.I./ha) with glyphosate on ex-pasture sites to give excellent Sorrel and Clover control.

Has limited residual effect; allow 1-2 days/g plant-back period

Sulfometuron methyl

Residual, but has severe foliar effect on eucalypts - label warnings about mixing with EC grass herbicides.

Under permit for forestry use nationally at 30-100 g/ha, including aerial.

SU herbicides become more residual in alkaline soils and in cold climates

Root pruners - very mobile, organic matter in soils essential, care needed in some soils


Metsosulam (Headway Herbicide 714 g a.i./kg) being registered at 7-10 g/ha.

Niche herbicide for control of Wild Radish and Wild Turnip etc.

A few weeks of residual activity, but mainly foliar translocated Also should not be used with EC formulations

Group C

Inhibitors of Photosynthesis and Photosystem II

9 chemical groups, forestry/nursery interest in 4.

Triazines - atrazine (Gesaprim) and simazine (Simagranz/Gesatop) and triazinone - hexazinone (Velpar/Velmac)

Ureas - methabenzthiazuron (Tribunil, pine nursery only)

Uracils - terbacil (Eucmix products)

Triazines - chemical structure basis is an aromatic heterocyclic ring with two substituent secondary amine groups

Primarily soil active/root absorption and translocated but some also have foliar activity

Photosynthesis inhibition affects plant sugar production

Selective pre-emergence herbicides, control pasture weeds - grasses and broadleaves.

Atrazine in pine, simazine in eucalypts High use rates eg. Atrazine 4.5-8 kg a.i./ha, simazine up to 6 kg a.li/ha

Atrazine is controversial; subjected to extensive research into its fate in plantations (Forest Herbicides Research Management Group/NRA review of atrazine).

Hexazinone - essential for 2R Radiata pine plantations.Woody weed and pasture weed control.

Photosynthesis inhibition affects plant sugar production.

Hexazinone - Velpar DF and Velmac granular products Rates of 1.5 kg a.i/ha (mixed with atrazine in eg. Forest Mix granules) to 2-3.8 kg a.i/ha as spray (Velpar DF) or granules (Velmac G and CR)

Hexazinone is both soil and foliar active (granules soil only) - deadly to eucalypts!

Terbacil (Eucmix Preplant and Eucmix GR, mixed with sulfometuron methyl) - mainly soil active and controls pasture grasses and broadleaves.

Group D

Inhibitors of tubulin formation

Mainly soil active Annual grasses and selected broadleaves

Not a major forestry group

Dintroanilines - pendimethalin (Stomp 330 g a.i./L) has eucalypt registration 9-12 L/ha; pre-emergence only by root contact, not translocated, very effective on Wireweed (Polygonum aviculare) and Rye Grass. Surflan (oryzamin) also registered for trees, but very expensive

Benzoic acids - chlorthal (Dacthal) in pine nurseries

Pyridines - thiazapyr (Visor) taken off market - cost

Group F

Inhibitors of cartenoid biosynthesis - disrupts photosynthesis in susceptible spp.

Triazoles - amitrole (Amitrole T 250 a.I./L) is the only important one in Group F

Foliar translocated, post-emergence control of grasses and broadleaves.

Major component of second season directed spray mixes in E.globulus and E.nitens

Trial work current with diffluence (nicotinanilide) and isoxaflutole (new sub-group)

Group G

Inhibitors of protoporphyrinogen oxidase

Dipenyl ethers - main one is oxyfluorfen (Goal 240 ga.i./L)

Soil active and contact - oxyfluorfen is a soil surface active herbicide and is effective only if applied to bare soil (or with glyphosate) - kills shoots on emergence.

Safe post-planting over pine and eucalypt at 0.72-0.96 kg a.i/ha

Trials have established mixes with hexazinone (pine) and simazine (eucalypts)

Group I

Disruptors of plant cell growth; auxin-like.

Main forestry sub-group is the pyridines - clopyralid (Lontrel Forestry Herbicide 750 g a.i/kg), triclopyr (Garlon 600 g a../L) and picloram (in Grazon, Access and Tordon TCH - timber control herbicide in mixes with triclopyr)

Triclopyr is foliar active, picloram is soil active and foliar

Clopyralid - Silver Wattle control in Radiata pine at 1.65 - 2.55 kg a.i./ha aerially in the past

Clopyralid - at low rates 150-200 g a.i./ha in pre-planting treatments on ex-pasture sites in eucalypts for residual control of thistles

Triclopyr/picloram for woody weed control/basal bark/stem injection/cut stump treatments in pine

Little effect on monocotyledons

Also includes phenoxy - 2,4-D, little used in forestry

Group M and N

Inhibitors of EPSP synthase

Main one is glyphosate (M) (Roundup, Roundup Biactive 360 a.i./L, Roundup PowerMax 540 g a.i./L)

Foliar active and translocated with no soil activity

Broad spectrum knockdown, used for pre-planting treatments often in combinations with eg. Metsulfuron methyl

Glufosinate (N) is mainly a contact herbicide with high selectivity for regen. Pine

Factors affecting herbicide performance 1

Soil moisture - soil residual herbicides require adequate soil moisture for root uptake.

Solubility is also related - to allow sufficient movement of the chemical, but very soluble herbicides have to be used carefully because of the risk of off-site movement.

Highly soluble herbicides are not suitable for some soil types eg.

Sandy soils low in organic matter, so that the chemical is excessively leached.

Underlying clay pans can also force lateral movement with potential off-site effects.

Factors affecting herbicide performance 2

Soil organic matter - adsorption/desorption; clay particles and sesquioxides can have similar retention effects.

Soil microbial populations - bacterial degradation is one of the major breakdown factors.

High organic matter content usually means enhanced microbial populations.

Soil temperature - relates to microbial population densities, rates of degradation by hydrolysis.

Factors affecting herbicide performance 3

Temperature effects can be deleterious eg. Hexazinone applied in dry or drying conditions can cause scorch (phototoxicity) to pine in later warming, moist conditions.

Soil pH - acid hydrolysis is a major degradation pathway for eg.The sulfonyl urea herbicides, but these are stabilised in anionic form in alkaline soils, so that rates of application need to be varied depending on pH.

UV degradation can be important for some foliar active herbicides.

Factors affecting herbicide performance 4

Soil half-life values are derived under standard laboratory conditions, but vary widely in different field conditions eg. Atrazine in an Australian (forestry) study had a half-life of 12 days in a soil from a Queensland site but 140 days similarly at a Tasmanian site. The major factor was temperature.

Volatilisation - although very soil fast, oxyfluorfen can be volatised in warm conditions

Soil fastness is usually related to the organic C exchange constant.