Electric Weed Control?

By Mindy Robinson

Coming to citrus orchards?

An ongoing study in hazelnut orchards offers possible electric weed control (EWC) solutions for other tree nut orchards, especially in light of increasing herbicide resistance.

Marcelo Moretti, Assistant Professor at Oregon State University, conducted two studies in 2021 utilizing EWC to manage Italian ryegrass in hazelnut orchards. One study focused on the necessary speed of operation to provide effective weed management. The second, multi-year study focuses on crop safety when using EWC.

Because hazelnuts are harvested from the ground, they are grown conventionally with mostly no cover crops and no tillage. Mowing suppresses the tops of weed plants but requires too many passes to be effective.

For the herbicides used in hazelnuts, growers are seeing resistance to all of the post-emergent herbicides such as paraquat and glufosinate, Moretti says. Thus, a non-chemical weed control alternative is needed.

“The end goal is to control or eliminate (weed) seed production in order to have the benefits of rye grass as a cover crop but be able to kill it when not needed,” Moretti says.

With today's growing herbicide resistance, commercial developers such as Zasso, RootWave, and Crop.Zone have developed EWC units focused on meeting the needs of different agricultural users.

HOW EWC WORKS

EWC kills weeds with thermal energy. A high-voltage electrode touches the foliage of the plant, allowing an electric current to pass through the plant; electric resistance in the plant converts the electrical energy to heat, killing the plant. The higher the plant's conductivity, the less energy it takes to kill the plant.

An EWC unit is typically tractor-mounted, and the unit's generator is connected to a PTO (Power Take Off). The generator is coupled to a transformer that increases the voltage. A module controller connects to the electrodes that are contained in sets of electrical fingers, which provide applicator contact to the plants and the soil. The applicators vary in shape and size, which is dictated by the size of the generator. A 30 kVA generator can treat about four feet at a time.

Along with equipment setup, the target plant and soil conditions play a role in the efficacy of the operation. Plant factors such as morphology, stage of development, water content, and plant density all affect EWC efficacy. Younger, herbaceous plants with high water content and particular root systems are more susceptible to EWC applications.

Soil factors include impedance, mineral composition, texture, moisture, temperature, and porosity. EWC is more effective in soils that have less conductivity, which forces the electricity to stay in the plants longer, creating more thermal energy. Optimum soils with low conductivity are lighter, sandy, dry, and warm.

STUDY RESULTS

Moretti's study on the necessary speed of operation for EWC using a Zasso unit shows that 9,000 volts at 2 mph efficiently kills Italian ryegrass. When soil conditions are wetter, the unit must be run at 5,000 volts to minimize the risks of damage to the EWC unit or the tractor. Although the 5,000-volt setup (at 1 mph) is less efficient, it does kill ryegrass.

Moretti's multi-year study on crop safety looks at variables such as plant variety, electric rates, and application — with and without suckers. The study will monitor tree growth, photosynthesis rate, and yield. Based on the 2021 results, EWC has not adversely affected crop safety.

Although not a part of the study, Moretti observed that yellow nutsedge, Canada thistle, field bindweed, and horsetail were all sensitive to EWC.

As the study progresses, cost factors also will be evaluated.

As expected, the most significant limitation of EWC is the potential for causing fires. In Oregon, EWC is not ideal when it is too dry (usually after July) or too wet.

EWC provides effective weed control at 2 mph, is initially safe to hazelnuts, and is safe to humans. It is compatible with existing production systems, as it does not damage irrigation lines.

The economics are to be determined but initially show to be about $50 per acre to implement — comparable to conventional herbicide application — after the initial equipment investment (Moretti purchased a Zasso unit for $50,000 in 2020 for this study). The benefits to the environment are that it leaves no chemical residue and does not require soil tillage.