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.
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.
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
Sulphonamides
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
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.
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
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)
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)
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
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
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.
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.
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.
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.