How water quality affects pesticides
Poor water quality can:
- increase chemical breakdown in the spray water (hydrolysis)
- reduce herbicides activity (inactivation)
- precipitate the pesticides out of solution
- result in spray line blockages.
Hydrolysis
Chemical breakdown on contact with water (hydrolysis) is dependent on:
- pesticide type – some are more prone than others
- formulation – sensitive products often have protective adjuvants added to reduce problems in the paddock
- pH – as a general rule pesticides are more stable at a neutral pH of 7.0. Sulphonyl urea herbicides will break down more quickly in acidic (low pH) conditions, and organophosphorus insecticides break down more quickly in alkaline (high pH) conditions
- time – from mixing until the spray droplet has evaporated
- temperature – the rate of breakdown increases as temperature increases.
Inactivation
Inactivation occurs when the herbicide binds (adsorbs) to contaminants – such as clay and silt – and reduces activity of the herbicide. Inactivation can also occur when mixing trace elements with some herbicides in the tank.
Quality
When water quality is less than optimal, apply the spray as soon as possible; do not leave the mix standing in the spray tank. If in doubt, get your water tested.
Read the label prior to mixing and applying any pesticide; the effect of variation in water quality on a pesticide will be outlined on the product label, usually in the section on tank mixing.
Water quality factors that affect chemical performance and possible remedies
Factor | Effect |
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Water pH –acidity or alkalinity of the water
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Muddy water |
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Water temperature |
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Hard water |
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Saline water |
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Organic matter |
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Water quality affecting herbicides
Herbicide | Hard | Saline | Muddy | Alkaline | Acidic |
---|---|---|---|---|---|
2,4-D or MCPA Amine | Test | OK | OK | No | OK |
2,4-D or MCPA ester | Test | No | OK | OK | OK |
Chlorsulfuron (e.g. Glean®) | No | No | OK | OK | Better? |
Clethodim (e.g. Select®) | OK | OK | OK | OK? | OK |
Clodinafop (e.g. Topik®) | OK | OK | OK | No | OK |
Clopyralid (e.g. Lontrel®) | Test | OK | OK | No | Test |
Dicamba amine | No | OK | OK | No | OK |
Diclofop (e.g. Hoegrass®) | OK | OK | OK | OK? | OK |
Diflufenican (e.g. Brodal®) | Test | OK | OK | No | Test |
Diflufenican + MCPA (e.g. Tigrex®) | No | OK? | OK | No | OK |
Diquat + paraquat (e.g. Spray.Seed®) | OK | OK | No | Test | OK |
Diuron | Test | Test | OK | Test | Test |
Diuron + 2,4-Damine | Test | Test | OK | No | Test |
Diuron + MCPA amine | Test | Test | OK | No | Test |
Fenoxaprop (e.g. Foxtrot®) | No | Test | No | No | OK |
Fluazifop (e.g. Fusilade®) | OK | OK | OK | OK? | OK |
Fluroxypyr (e.g. Starane®) | OK(1) | No | OK | OK | OK |
Glyphosate (e.g. Roundup®) | No | OK | No | No | Better |
Haloxyfop (e.g. Verdict®) | OK | OK | OK | OK? | OK |
Imazamox (e.g. Raptor) | OK | OK | OK | OK | OK |
Imazamox + imazapyr (e.g. Intervix) | OK | OK | OK | OK | OK |
Imazapic + imazapyr (e.g. OnDuty® | OK | OK | OK | OK | OK |
Imazapic + Imazapyr + MCPA | OK | OK | OK | OK | OK |
Imazethapyr (e.g. Spinnaker®) | OK | OK | OK | OK | OK |
Propaquizafop (e.g. Correct®) | OK | OK | OK | OK? | OK |
Quizalofop (e.g. Targa®) | OK | OK | OK | OK? | OK |
Sethoxydim (e.g. Sertin®) | OK | OK | OK | OK? | OK |
Simazine | OK | No | OK | OK? | OK |
Triasulfuron (e.g. Logran®) | No | No | OK | No | No |
Trifluralin (e.g. Treflan) | OK | OK | OK | OK | OK |
Water quality affecting insecticides
Insecticide | Hard | Saline | Muddy | Alkaline | Acidic |
---|---|---|---|---|---|
Chlorpyrifos EC (e.g. Lorsban®) | Test | Better | OK | No | OK |
Cypermethrin EC | Test | Better | OK | No | OK |
Dimethoate EC | Test | Better | OK | No | Better |
Phosmet (e.g. Imidan®) | Test | Test | OK | No | OK |
Trichlorfon (e.g. Dipterex®) |
|
|
| No | OK |
Lambda-cyhalothrin (e.g. Karate® Zeon) | OK | OK | OK | Use immediately | OK |
Water pH
Acidity or alkalinity affects the rate of hydrolysis or breakdown of chemicals in water. The pH scale is a measure of the acidity or alkalinity of water, and is based around a pH of 7 being neutral, pH of greater than 7 being alkaline, and pH of less than 7 being acid.
In technical terms, pH = –log10[H+] or the negative log of the hydrogen ion concentration in water.
Alkaline hydrolysis is dependent on:
- type of pesticide – some chemicals are more prone to hydrolysis than others
- formulation – sensitive products often have protective adjuvants added to reduce field problems
- pH – the higher the pH the greater the speed of breakdown
- time – from mixing until the spray droplet has evaporated
- temperature – the rate of hydrolysis increases as temperature increases.
Organophosphate, carbonate and pyrethroid insecticides are often more prone to alkaline hydrolysis than other insecticides, herbicides or fungicides. Some insecticides (carbofuran, dimethoate, methyl parathion, monocrotophos, phorate, trichlorfon) and glyphosate herbicide provide better control in acid water of pH 4–6. Copper, lime and lime sulphur sprays should be applied in alkaline water to reduce crop damage.
Bacillus thuringiensis (Bt) are also sensitive to alkaline water, but good formulations contain a buffering agent.
Half-life of common pesticides
Some pesticides have a very short half-life; the time it takes for half of the chemical to be degraded. Spray as soon as possible after mixing, and clean out as soon as possible after spraying.