Read time: 6 minutes

How can nozzle choice contribute to herbicide resistance?

with David Johnson, technical extension specialist, FMC

Low-dose herbicide applications can easily occur if a significant percentage of the droplets from the spray nozzles are so fine that they are ‘lost’ before they reach their target.

Frequent exposure to the same herbicide, particularly at lower-than-label rates, allows plants with some genetic tolerance to survive, set seed and dominate the local weed population.

David Johnson, technical extension specialist with FMC in the northern region, says there are several less obvious sources of fine spray droplets to consider when setting up to spray.

“One source is the effect of the tank mix itself on spray quality out of the nozzle,” he says. “The nozzle selection guides only provide the expected spray quality range when spraying water through the nozzle – adding other products to the water can dramatically change the resulting spray quality.”

“Legislation designed to reduce spray drift of 2,4-D requires growers to use Very Coarse or coarser nozzles. We conducted extensive tests of a common fallow weed control mix of glyphosate and 2,4-D amine plus ammonium sulfate in the CPAS wind tunnel with Dr Andrew Hewitt to measure the effect of this mix on spray quality,” says David. “With this tank mix, over 50 per cent of nozzle types produced a spray quality range that would be expected from a nozzle rated one or two categories finer. For example, most Very Coarse nozzles actually produced the amount of fines expected from a Medium nozzle – according to the spray guides.”

David suggests that growers consider using nozzles rated coarser than the minimum suggested on the herbicide label, particularly for high-risk products such as 2,4-D.

Spray efficacy is an essential component of the WeedSmart Big 6 for reducing herbicide resistance risk. The potential for low dosing of weeds increases selection pressure for herbicide resistance.

What are fine droplets, and what affects them?

In brief: Fine drops are defined as all droplets at application that are too small to travel directly to the ground under their own weight.

The details: Nozzle selection, tank mix effects on that nozzle, boom height, travel speed and volume all need careful consideration to minimise the production of fine droplets.

All spray nozzles produce a portion of fine droplets. The coarser the nozzle category, the lower the percentage of fines produced.

Avoid minimum or borderline set-up specifications for nozzle selection and pressure. Choose the lowest pressure at which the nozzle provides the target spray quality (while still operating within its optimum pressure range) and use nozzles one or two categories coarser than the guide suggests.

While suspension concentrate formulations contain thickeners that reduce the production of fines, soluble liquid formulations and tank mixes that include wetting agents are more likely to increase the production of fine droplets and therefore a coarser nozzle would be beneficial.

Lowering the boom height from 1 m to 0.5 m automatically halves the amount of fine droplets. As well as delivering more product to the target weeds, lowering the boom also vastly reduces the downwind no-spray-effect distance.

Slowing the ground speed of the spray rig allows the droplets to take a more vertical path to ground, and there is less stubble capture and less evaporation, increasing droplet survival.

Increase the water volume to gain better coverage with larger droplets.

Should I use coarser nozzles for all herbicides?

In brief: Not necessarily, but you should always use the coarsest spray quality that works.

The details: All labels carry a minimum spray quality recommendation. For example, if the label says to use Coarse as a minimum spray quality, then Very Coarse or even Ultra-Coarse may be just as effective.

Growers can do a strip trial using a section of Very Coarse (or coarser) nozzles on their boom and their usual nozzles on the rest of the boom for one run of the paddock. If the efficacy in that strip is good, then switch fully to that coarser nozzle for the next season.

Following this advice can result in very acceptable efficacy with ultra-coarse spray qualities, even on very small weeds. If you can spray effectively with fewer fines, make the necessary sprayer set-up changes permanently.

What is the impact of low herbicide dosing?

In brief: Low herbicide dosing can occur in the field of application and in adjacent areas. In addition to losses to drift, poor water quality also reduces the effective dose.

The details: If you apply a tank mix at a rate of 55 L/ha with a medium spray quality at a boom height of 1 m, only 33 L of the spray will hit the ground directly. The target weeds will be exposed to a low rate, reducing the efficacy of the spray job and allowing more weeds to survive and set seed. Some of these survivor weeds may have resistance to the herbicide and so resistant populations can develop faster.

Likewise, the other 22 L of the spray that has drifted will have a similar effect on plants in adjacent areas. This is one reason why herbicide resistance often builds up along fence lines and other non-crop areas. These non-crop areas generally do not have other weed control tactics, such as competition, stopping seed set or seed destruction technologies applied.

Pay attention to sprayer set-up, environmental conditions, water quality and tank mix components to ensure the intended dose is delivered to the weeds. The most expensive spray job is one that doesn’t work.

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