Factors affecting herbicide performance

Controlling stressed weeds

When a weed has been subjected to stress it will not be adequately controlled by herbicide rates sufficient for unstressed weeds, even when there has been sufficient rainfall to make the weed appear healthy. Additives such as ammonium sulphate, wetters and oils can help improve the control of stressed weeds by 10-20%, but can also be unpredictable. Performance enhancements are specific to some herbicides or formulations, so always check the label.

Stressed weeds are harder to kill than healthy, actively-growing weeds. Plants can be stressed and not show any distinct visual signs. Stress can be caused by:

• lack of moisture, for example, dry conditions, physical or chemical impediments to root growth
• lack of oxygen due to waterlogging
• extremes of temperature, for example, cold (frost) and heat
• nutrient deficiencies
• insect pests, for example aphids and wireworms
• disease
• a sublethal dose of herbicide from previous applications or residues
• mechanical damage, that is, tillage or slashing.

Moisture stress, that is, drought conditions, is one of the most common plant stresses. Translocation and respiration slow dramatically when plants are moisture stressed, restricting the movement of herbicides to their sites of action. When herbicides are applied to stressed crops and pastures, herbicide breakdown via metabolic processes (within the plant) can be slowed, leading to crop or pasture damage.

Plants growing under dry conditions may also have a layer of dirt and dust on their leaves that can create a physical barrier to penetration by herbicides.

Weeds that have suffered moisture stress may have limited leaf development but extensive root systems developed to assist in the search for moisture. This means the above-ground plant biomass does not adequately reflect the true weed size or growth stage.

Plants experiencing high temperatures, low humidity and low soil moisture conditions (for example, plants growing following a summer rainfall) tend to have a thicker cuticle (the protective cover of the leaf) with more waxy deposits on the surface. This change in leaf structure, reduces absorption of foliar-applied herbicides.

The timing and amount of rainfall not only determines the moisture status of the plant but also removes dust from the leaves and modifies the leaf cuticle. Recent rainfall will therefore improve herbicide absorption.

Predicting a dose that will kill stressed weeds is difficult but if in doubt never ‘cut’ rates and consider the use of a double knockdown or a tillage operation as a follow-up.

Seasonal environmental conditions determine overall herbicide performance and conditions on the day of spraying determine the variation around this level. Herbicides perform best when weeds are actively growing. So weather that favours plant growth will also favour herbicide performance.

Cold weather will slow the growth of both crops and weeds. This can mean that the amount of herbicide absorbed and translocated is reduced resulting in less than the required lethal dose reaching the target site. This problem is exacerbated in dry soil where growth will already be limited by an alternative stress.

The ability of the crop (non-target species) to detoxify/metabolise the herbicide into non-harmful compounds may be reduced by cold weather. It is this reduction in the ability to metabolise the herbicide that may result in crop damage. This has been observed in particular with the sulfonyl urea herbicides.

There are numerous herbicides, such as diflufenican and carfentrazone-ethyl, that damage growth of existing crops. New growth is, however, not affected and plants are able to ‘grow away’ from the herbicide damage. In dry cold conditions plant growth is slowed considerably and the crop may not grow out of the damage as readily.

It is necessary to leave 10 days between spraying phenoxies and grass selectives to avoid antagonism. If you have had a series of frosts during this period, growth of both weeds and crops will have slowed. It is worth waiting a few additional days before spraying the grass selective to ensure that the crops and weeds have grown out of the effect of the phenoxy (and vice versa).

Tank mixing and compatibility of herbicides

Care needs to be taken when tank mixing as some herbicides are incompatible. Tank-mixing can minimise cost; each application has a cost in labour, fuel and wear on the spray unit and secondly, make best use of suitable spray weather, provide synergism - some herbicides enhance the performance of others, providing better weed control than when either herbicide is used alone, or when used in separate applications.

Warning: Some herbicides can be either physically or chemically incompatible and cannot be tank-mixed. Incompatibility may lead to application problems, have poorer weed control and/or cause crop damage. Always seek professional advice before trying a new mix and always check for product specific mixing directions on the label.

Formulation incompatibility

Formulation incompatibility is usually a problem of emulsifiable and flowable mixes. The solvent and/or emulsifier in the emulsifiable product can strip the dispersant from some flowable formulations. As a result, the active ingredient from the flowable formulation will flocculate and settle out. Certain combinations of simazine and trifluralin have done this in the past.

Manufacturers may change the emulsifiers and dispersants in their formulations from time to time in response to supply or price changes. A mix that was stable one year cannot be assumed to be compatible later. It should always be tested first, using the jar test described below.

Chemical incompatibility

Some products cannot be mixed because the two components react. This is usually a problem where the positive ion of one product reacts with the negative ion from another. The reaction between the ions reduces efficiency at best, but at worst will produce a tank full of gel. A striking example is 2,4-D amine and copper sulphate. Tank-mixing these produces a thick gelatinous precipitate which is extremely insoluble.

Compatibility jar test

Before making a sprayer-full of tank-mix for the first time, it is advisable to check the compatibility of the components, unless it is a recognised mix shown on the label. This can be done conveniently on a small scale using a medium size screw-top jar.

Instructions

1. Use water at the same temperature as the spray supply (preferably use the same water).
2. Take a 2 litre (L) jug or sealable jar
3. Add 500 millilitres (mL) water
4. Add flowable, granular or powdered products and mix.
5. Add emulsifiable concentrate (ester or oil soluble) products and mix.
6. Add water soluble products and mix.
7. Add another 25mL and mix.
8. Add other adjuvants and trace elements etcetera and mix.
9. Add wetting agents and mix.
10. Add spray oils and mix.
11. Add water to make up to 1L and mix.
12. Leave the mixture to stand for several hours and note any separation or layering. Any settling of flowable or powder products is normal, but any difficulty in resuspending any sediment should be noted.

Warning – physical compatibility does not imply biological compatibility.

Water quality for spraying herbicides/pesticides

Nearly all herbicides and other pesticides are applied to crops and pastures as a solution or suspension in water. Poor water quality can affect the effectiveness of herbicides.

In Western Australia alone, between 300 000-400 000 cubic metres of good quality water are required annually for herbicide application. For many farmers, supplying large volumes of clean, suitable water for spraying has been a problem.

The quality of the water used can cause problems due to:

• Muddy water - the silt or clay content can affect herbicide performance and cause blockages of the jets.
• pH - the acidity or alkalinity of the water can affect the rate of breakdown of chemicals in water.
• Hardness caused by calcium and/or magnesium salts. These ions can interfere with herbicides by the processes of inactivation, breakdown or precipitation.
• Salinity caused by sodium salts. These ions can also interfere with herbicides by the processes of inactivation, breakdown or precipitation.
• Temperature - very cold water can cause gelling, particularly with oil additives. High water temperatures can accelerate the process of hydrolysis.

General recommendations

• Check the herbicide label under 'restraints' to see what it says about water quality.
• Use rain water or scheme water wherever possible (if you can safely drink it, there is no concern about its pH).
• If supplies of good quality water are limited, review your application volumes if they are high (over 60 litres per hectare (L/ha)).
• If water quality is doubtful, then have it checked. Saline water is frequently clear and is satisfactory with some herbicides but not all (for example, phenoxys).
• Do not leave mixed herbicides standing in tanks for any longer than is necessary, especially if water quality is doubtful.
• Before using expensive additives, ensure that you really do have a problem requiring correction and that the additives will not adversely affect the performance of any proposed tank mixes.
• If good quality water is limiting, use dam water to apply herbicides such as simazine, atrazine, trifluralin, chlorsulfuron and triasulfuron. Salty water can be used to apply paraquat/diquat. Save the best water for more critical situations.

Sprayer decontamination

There is a risk of crop damage from traces of some herbicides left in the sprayer after treating the previous crop. Using the decontamination process described here will reduce the risk of damage. The potential contaminators are the sulfonylureas (Glean^®, Logran^®, Ally^®, Seige^® and Brushoff^®) and hormone-type herbicides such as 2,4-D, MCPA and picloram. 1% of the recommended rate of these herbicides can cause damage to lupins and pasture legumes.

Chlorine bleach cleaning

This method should be used to remove sulfonylureas and most other pesticides from sprayers.

Chlorine is a powerful oxidising agent and will decompose most organic molecules.

1. To each 100L of water in the tank, add 300mL of 4% bleach, 100mL of 12.5% or 6g of 65% pool chlorine. Agitate and circulate solution through all lines. The sprayer can be run briefly to fill the boom.
2. Let the unit stand for 15-30 minutes, then drain.
3. Repeat step 1. The solution may be left to soak overnight for an especially thorough cleaning.

Alkaline cleaning

This method is recommended after use of acidic chemicals such as 2,4-D, MCPA, dicamba, picloram and triclopyr (e.g., Garlon^® ). The alkaline solution can dissolve these herbicides and if they were ester formulations, strong alkali is able to hydrolyse the molecules, converting them to a water soluble form that is more easily removed. Ester formulations are particularly bad contaminants, because they penetrate rubber and plastic seals and hoses. From there they can be gradually released into subsequent sprays and cause damage.

Water soluble amine formulations

1. Add either 800mL of household ammonia, 250grams (g) of sodium carbonate (washing soda), or 200g of sodium hydroxide (caustic soda) to each 100L of water in the tank. Agitate and circulate the solution through all lines. Run the sprayer briefly to fill the boom.
2. Let the unit stand for two to three hours, then drain.
3. Repeat step 1. The solution may be left to soak overnight for an especially thorough cleaning.

Ester formulations (emulsifiable concentrates)

Proceed as above, but only use sodium hydroxide, not ammonia or washing soda. Also add 200mL of wetting agent.

Precautions

• Chlorine bleach should never be mixed with ammonia. Chlorine reacts with ammonia, destroying the cleaning power of both. Volatile, acrid nitrosamines are formed, which can cause eye irritation.
• Caustic soda solution will corrode aluminium, brass and galvanised parts. These should be removed or isolated from cleaning solutions of caustic soda. Ammonia solutions at the concentrations recommended will not affect these metals.
• Sulfonylureas may be difficult to remove from damaged or cracked fibreglass tanks. Tanks should be repaired and cleaned thoroughly before the start of the spraying season.

Assessing herbicide performance

Herbicide failure can occur for reasons other than herbicide resistance, including application error, adverse environmental conditions and plant stress. Understanding how different herbicides work helps when assessing herbicide performance.

Factors to assess and improve herbicide performance

1. The rate at which plants die after spraying depends on the herbicide and rate applied as well as the weather conditions following application.
2. Assess the ‘claims’ made by the herbicide manufacturer. Some products registered for the control of weeds do not claim to kill the weed but rather, ‘suppress’ growth, reducing seed-set and competition against the crop.

Evaluate the likelihood of application error by asking:

• Has the target weed been accurately identified?
• What product was used and was it a correct choice for the target weed?
• Was the correct product rate used for the weed growth stages present?
• Were appropriate adjuvants used at the correct rates?
• Did the product reach the target? Certain herbicides may be intercepted and bound to other plant material (for example, stubble) or soil and thus not reach the target weed.
• Was the product measured accurately when making up the spray-tank mix?
• Was the quality of the water used satisfactory? The performance of some pesticides is affected by water quality characteristics such as hardness, pH, salinity and clay content.
• Was the water volume per hectare appropriate?
• Was the boomspray accurately calibrated?
• Were there equipment problems, for example, blocked nozzle, erratic pump performance?
• Were the correct nozzles, pressure settings, boom height and boom speed used to achieve the desired uniform coverage?
• Were label directions regarding environmental spray conditions observed?
• What else was added to the tank mix? Some pesticide mixtures can be mix together, but may be biologically incompatible. This can result in reduced weed control and/or increased crop damage. Performance may also be reduced if insufficient time has been left between separate applications of antagonistic products.
• Was the tank solution mixed properly and agitation adequate to keep it mixed?

Environmental conditions at time of spraying can influence herbicide performance. When assessing performance problems, records of the conditions at the time of spraying are important. Herbicide labels provide some guidance as to desired conditions or, alternatively, conditions to avoid when spraying weeds. Unfortunately, due to the nature of weather, the number of ‘ideal’ spray days during a season is limited. Important environmental conditions to record include:

• Time of day applied
• Presence of heavy dew
• Temperature at time of application and up to 10 days before or after application
• Clear skies versus heavy clouds or overcast conditions
• Rainfall events, for example, before application and before the rain-fast period of the post-emergent herbicide has elapsed. Heavy rain shortly after use of soil-applied herbicides can move them into the crop root zone, increasing crop damage
• Stressed weeds due to many factors, including, too dry or wet, frosts before or after application, poor nutrition, disease, insect attack and/or competition
• Soil pH affecting herbicide availability to weeds or the crop
• Whether the product leached or was otherwise destroyed so that uptake by target weed was limited.