Spray Drift

What is DRIFT?

There have been many theories produced over the years in regards to spray drift. The downfall of these theories is that they are only a theory, not fact! This taken into account, any person partaking in a spraying exercise, be it commercial or private sector, must look to achieve the highest possible efficacy out of the spraying system. To achieve this a number of variables need to be controlled and monitored.

Spray drift is a modern day issue that generates a large degree of attention due to its economic and environmental effects. Drift can cause huge financial losses in chemical and crops along with disastrous environmental effects. Let’s face the facts, driftable droplets are created within any nozzle design. To encompass how drift can be controlled, we must accept some factual data. It has been shown that any droplet below 50 microns in size becomes highly susceptible to drift and evaporation.

In the situation of spraying, water is the carrier of chemical. This is intended to reach a plant/ground target then spread across the leaf tissue area. The water component will then evaporate leaving a chemical residue on the plant surface area. When a droplet below 50 microns is released into the atmosphere, it will evaporate leaving a floating residual core within the atmosphere before reaching the plant target. The aim of controlling drift is to reduce the actual numerical amount of droplets released into the atmosphere. 

The Physics of Spraying

There has been published data over the years that suggest a droplet below 120 microns becomes driftable and evaporative. This information has generally been supported by data that has been interpreted incorrectly. Most of this trial data has been gathered under controlled conditions. This is where a droplet of specific micron range is allowed to freefall over a given distance. The length of time this process takes is then calculated. When in reality a droplet released from a boomspray or aeroplane, is done so with an energised velocity. Paddock spraying also attracts atmospheric conditions such as wind turbulence, which affect plant droplet deposition.

Velocity Counts!

Let us consider this! As a droplet leaves the nozzle orifice it has an accumulated energised velocity. For example, a 100 micron droplet exiting a Lechler LU Multirange flat fan nozzle contains an energised velocity of 22.2m/sec. This equates to a travel time of 0.0225sec for a 100micron droplet leaving a boom height of 500mm.

To claim that a 100 micron droplet is driftable is false and this is supported when you look at the facts. Testing has shown that a 100 micron droplet can reach its target with an unenergised velocity within 2 seconds! This is excluding the fact that any turbulence created within the environment will enhance this result.

Look Around You- Atmospheric Conditions

Drift is also greatly influenced by atmospheric conditions such as wind turbulence. To take a leaf out of the book of Bill Hughes (founder of Australian Agricultural Machinery), he simplifies the atmospheric effects on droplets by giving an analogy of spraying physics. This analogy explains that droplets travelling at a given velocity over a surface area (i.e. tractor travelling over a paddock at speed), with a wind speed above the ground surface will create an effect similar to wave rolling into a beach. Droplets will turbulate, giving the appearance of a blanket of spray feeding into the ground.

Because a 100 micron droplet has less mass than that of 200 microns, it is able to be carried within the turbulence more effectively. But as a general rule, a droplet of 50 microns or less is highly susceptible to drift. Although it must be remembered that some of the droplets less than 50 microns may contact with the plant target area, but this amount is extremely variable and difficult to calculate. As these droplets have a high “potential” to become driftable and evaporative, then obviously a nozzle that is designed to minimise the creation of these droplets is highly favourable.

Temperature is critical when spraying, where as a general rule, optimum spraying temperature less than 30 OC is desirable. In favourable conditions where humidity, wind and plant stress levels are acceptable, 32OC may be the limitation. Although to achieve this range a fair amount of expertise and observation is needed.

The Volume Median Diameter (VMD) refers to a droplet spectrum of a nozzle. The VMD is determined by 50% of the volume that exist as droplets produced by the nozzle being above the VMD size and 50% of the volume of droplets produced being below the VMD size. Humidity can effect droplet VMD and cause a high risk drift scenario. Although the effects of humidity on droplet diameter is relative to other meteorological conditions. As a guide, a relative humidity above 50% is desirable. Low humidity causes rapid evaporation of droplets especially in higher temperature conditions. This will causes the relative mass of droplets to fall, increasing the drift potential per volume of water.

The Effects of Wind

There is a general conception that zero wind is ideal for spraying. This is one of the most INCORRECT statements ever made. Some wind needs to be maintained to create turbulence and force droplets to contact with plant/ground target areas. Remember these parameters:

Do Not Ever Spray Below 4km/hr. Do Not Spray Above 20km/hr

If these parameters are exceeded, then inversion, poor plant contact and wind drift is likely. 

Inversion Drift! What Is It?

Inversion is a word often talked about, yet often ignored. Inversion is a combination air temperatures. Typically, a still morning is when inversion is likely. Inversion occurs when a blanket of warm air traps cooler air close to the ground. When these air currents do not mix, suspended particles from spraying then have no other choice but to float laterally. These particles may then float some several kilometres and settle later, with a down draft, on an undesired target area. This can have devastating effects.

To determine inversion conditions, use the simple task of the observation of smoke. Where smoke rises and dissipates, these are normal conditions. If smoke rises and condenses this indicates an inversion factor. Also, fog conditions are usually an obvious indication of inversion. These conditions should be avoided and the correct chemical selection such as a non-volatile chemical is vital to minimise inversion risk. There are various formulations available depending on the target. One example may be a 2,4-D formulation present as Amine instead of Ester.

Choose Your Chemical Wisely!

The selection of chemical is relatively important in being able to manage drift. Chemical usage should be based upon daily atmospheric conditions. For example 2,4-D Ester is highly volatile and a risk especially in inversion conditions. Other less volatile alternatives to ester may be amine, sodium salt or acid based formulations. Once again, if the operator sprays within the desired parameters, these inversion scenarios become unlikely. Where there is uncertainty, the operator should not commit to spraying until conditions are desirable.

Vapour Drift

Vapour drift occurs during inversion conditions. Poor nozzle selection can greatly influence the amount of droplets present as vapour. These very fine droplets can travel with wind to have devastating effects of drift. Choose a nozzle that eliminates the production of these extremely fine droplets.

Very fine droplets bellow 50 microns are at risk of vapour drift. This is influenced by chemical formulation, temperature, humidity and wind direction/currents. These factors have to be observed and judgement made by the operator to reduce the risk of vapour drift.

Leave Some Space!

Maintaining a Buffer Zone whilst spraying is rule that should become standard practice in every spraying program. Some crops can be up to 10000 times more sensitive to a particular chemical formula than others. It is important to consult with product manufacturers upon the precautions when using their product. In any case your spraying program should not allow any drift to occur outside the boundary the operator is spraying within.

It hard to be specific on how large a buffer zone is required, as this varies on the conditions you are spraying within. Wind speed, wind direction, temperature and humidity are only some of the variables. Commonsense would prevail that a 30 to 60 metre buffer in mild conditions would be sufficient with a non-volatile chemical. Spraying should never occur when the wind direction gives way to risk of spray being blown into the next paddock.

What Is A Drift Retardant?

There are a number of adjuvants on the market that are designed as drift retardants. The function of these products is to reduce the driftable aspect of droplets with the spray volume.

Retardants exist as additives usually mixed with the chemical/water mix during the filling process. These products usually exist as a liquid form. The majority of additives are often vegetable oil based or petroleum based.

Drift retardant work by coating the spray droplets. This then causes droplets to become greater in size and mass. Each droplet also gains an oil coated surface area. These factors allow greater protection against wind drift, evaporation and vapour production.

Losses associated with chemical waste through low plant contact is minimised with a drift retardant. Target collection becomes greater, therefore financial and economic losses are reduced.

Care must be taken with some adjuvants, especially when using venturi type nozzles. Always check nozzle patterns after adding these products as they may limit air bubble foration in venturi type nozzles.

Know Your Machine

Machine selection is very important in controlling drift. The farmer needs to quiz the manufacturer on machine design and nozzle selection. At Jetstream it is done for you and customised for your conditions. Boom stability and design is an often-overlooked necessity when a farmer purchases a spray rig. Obviously boom instability can cause severe height applications of spray, increasing drift potential. Stability needs to be matched to boom suspension and machine structural stability.

Maintaining an efficient spray height can decrease drift risks. An efficient spray height is one that will allow sufficient coverage of the target area whilst maintaining a minimal distance above the target area.

A spray height of 500-600mm is generally acceptable in most situations. Obviously this depends upon the topography of the paddock, crop height, nozzle selection and wind speeds. As height increases, so does wind velocity. This must be considered. Spraying a developed crop may require an increase in spray height. The operator must ensure that coverage is maintained to give adequate spray pattern overlap above the plant target. At the other end of the scale, when applying pre-emergence herbicides, the lowest practical boom height is needed. DIAGRAM (COVERAGE TWIN LINE COMPARISON GRAPH)

  

As shown above, a Jetstream Electric Tandem Sprayline is able to maintain the greatest efficacy from nozzle output. This allows for a dramatic decrease in boom spray height as opposed to other makes. Only 59mm is required with an ETS unit as opposed to 166mm with other makes.

There is mixed success with the use of shrouds fitting to a spray machine. Shrouds can reduce drift in some situations by 35-70%. This is all relative to spray height, winds and chemical formulation. With shrouds, the design somewhat limits the positive effects that wind turbulence may have upon droplet deposition. Coverage is also severely compromised with most designs. This protection system is only a secondary prevention against drift and if used should be done so in accordance with careful nozzle selection. If this is not practiced, a high volume of driftable droplets may still be at risk of escaping into the atmosphere.

Nozzle Design Does Help!

It is of the most concern to see technical editorial giving misleading information. At the end of the day, the consumer needs to be aware of all the “facts” to allow oneself to make an informed decision within their integrated management plan. One such false statement that I have seen printed time and time again is that water volume is proportional to drift. This is totally INCORRECT! Driftable droplets produced are proportional to nozzle type, design and in some cases, pressure. As I will explain, nozzle design is crucial to drift.

It is a fact that a nozzle producing a finer median of droplets gives better coverage. A well-designed nozzle will produce more of these finer droplets above the 50micron driftable barrier, which the Jetstream/Lechler nozzles achieve. It is a “myth” that these nozzles such as Lechler 30ltr/ha No2 produce more driftable droplets. An Ultra Low Volume Jetstream/Lechler 15ltr/ha No1 nozzle will produce only 2.8L of driftable and evaporative droplets per hectare as opposed to a 100ltr/ha No4 nozzle producing 4ltr/ha of driftable and evaporative droplets. All driftable and evaporative droplets are waste. This demonstrates that the numerical number of droplets (below 50microns) produced at a spraying rate of 100ltr/ha is 42.8% greater than that at 15ltr/ha.

Although the VMD is much higher within a No4 nozzle (230microns) as opposed to a No1 nozzle (120microns), this does not suggest that a No4 nozzle has a less numerical amount of driftable droplets (remember, below 50 microns is driftable).

Combined with a superior 120 degree fan angle, low volumes may be achieved whilst reducing drift to a minimum. With this technology it is not uncommon to achieve spraying rates as low as 15ltr/ha with maximum results. The key ingredient here is target contact. In fact, travelling at higher spraying speeds will aid in creating a positive atmospheric turbulence throughout the plant canopy. In doing so, plant coverage is optimized as the smaller droplets impinge on the outer and under leaf surface area.

Less droplets escape this turbulence at lower volumes as droplets below 140microns will adhere to finer grass-like weeds (any droplet above 300microns has a high runoff/waste value). Total plant kill rates of annual ryegrass in paddocks can be achieved at lower water volumes where ryegrass may be persistent at higher water volumes.

However, it is true that a poorly designed or incorrectly selected nozzle will not allow successful low spraying volumes to be achieved. Nozzles with a droplet spectrum that achieves a VMD of 120-140 microns are ideal to achieve optimum results in most conditions. Applying low volumes through any random nozzles will not necessarily work. A nozzle needs to be of at least 120 degree fan angle in most broadacre situations.

There are various nozzle designs available to the farmer. The majority of nozzles are designed to serve a specific purpose. With this in mind, it is vital to select the correct nozzle for each application to optimise plant collection, hence minimising drift.

Nozzle designs and brands are broad and various, so let us concentrate upon nozzles suitable for broadacre applications. 

Although there are numerous other types of nozzles available on the market eg cone type, none of these are as widely used as the discussed designs. It is vital that the user is fully aware of the performance of the nozzle that they are using.  

Tools of the Trade

It is advisable that every operator should own a wind speed indicator and basic weather station. This will allow a spraying program to be developed that allows spraying to occur within the atmospheric perameters. The basic information required is wind speed, temperature, wind direction and humidity. With this information at hand the spray rig operator is able to determine efficient spraying conditions, hence being able to optimise chemical efficiency and reduce drift potential. 

Drift Reduction Checklist

1. Assess the conditions you are spraying in: ie wind speed/direction, temperature, humidity, crop type, buffer zones and surroundings

2. Select a nozzle suitable for the spraying conditions eg 120 degree Air Inducted nozzle in windy conditions

3. Know the performance characteristics of your nozzle ie droplet spectrum, pressure perameters, coverage, efficiency and outputs

4. Select a pesticide/herbicide that is appropriate for the conditions and task, eg do not spray 2,4-D Ester in inversion conditions.

5. Utilise additives such as drift reduction Spray Oils to decrease drift potential.

6. Spray in accordance to atmospheric conditions. Leaving ample buffer zones near sensitive area. Utilise wind direction and velocity to give desired results. Spray at the lowest possible height whilst still maintaining coverage.

7. Always review conditions whilst spraying

REMEMBER- DRIFT CAN BE CONTROLLED BY USING COMMONSENSE AND STAYING WITHIN THE PARAMETERS