Hazelnut (Corylus avellana L.) is an economically important tree nut crop with a world market value of over $2 billion. The vast majority of the 88,000 acres of US hazelnuts are grown in Oregon’s Willamette Valley, where environmental conditions are ideal. Weed management is key as orchard floors must be kept weed- and debris-free for mechanized harvest. Clean orchard floors prevent weed competition, maximize harvest efficiency, and support high yields by reducing competition with weeds.
Troublesome weeds such as Italian ryegrass (Lolium perenne L. spp. multiflorum; Figure 1) can thrive in hazelnut orchards. This annual winter weed (Poaceae) grows across the world and is widely distributed throughout North America.
Figure 1. Italian ryegrass growing in a hazelnut orchard, and seedling (inset).
In Oregon, Italian ryegrass management has been complicated by the development of resistance to four herbicide modes of action (groups 1, 9, 10, and 15). Many factors influence Italian ryegrass management in Oregon hazelnut orchards. Dormant trees are generally planted during the mild winters, while rainfall is abundant. Although the fields are weed-free at planting, weed species such as Italian ryegrass quickly emerge, interfering with hazelnut orchard establishment. Furthermore, hazelnut acreage is rapidly expanding in Oregon, with bearing orchards reaching 60,000 acres in 2021, double the area of 2014. New orchards replace crops, including wheat, tall fescue and perennial ryegrass fields, where herbicide-resistant weed populations have developed. Herbicide applications select for resistant individuals, that escape control, and may produce seed. Increasingly, chemical control options become limited.
At this time, herbicides are the primary means to control weeds in hazelnut orchards. Italian ryegrass resistance to glyphosate and post-emergent herbicides leaves growers with fewer chemical weed control options. Electricity has always sparked interest as a control method. Widespread adoption of this rarely used yet interesting control option has been limited due to safety concerns and the cost of the equipment.
More recently, the release of new technologies and equipment, such as those being tested at Oregon State University, has the potential to change this scenario. We believe this technology could diversify weed management programs and reduce herbicide resistance selection.
Electric weed control exposes weeds to electrical energy. This energy is converted into heat that ruptures cell membranes and ultimately destroys the xylem (water conducting vascular tissue) and phloem (vascular system that transports sugars and other nutrients) of the target plant. However, many soil and plant factors impact the efficacy of electric weed control. Mineral composition, texture, temperature, porosity, and moisture status affect soil resistivity and directly control how long the electrical pulse remains in the treated plant. In soils with high resistivity, such as sandy soils, the electrical energy is less likely to move from the treated plant into the soil. The weed receives a larger dose of energy, and the outcome is better control (Zhou et al. 2015). Oregon hazelnuts are typically grown in silt and silty loam soils (medium-to-low resistivity), we expect more energy to be required to provide similar control levels relative to silty sand or sandy soils. Plant factors such as weed density, infestation, and plant type can also impact efficacy.
The energy transmitted to a single plant (E) is proportional to electrode voltage (V) and contact time (Tc) (Vigneault and Benoit 2001). Further, the electrical resistance of the plant itself (Rp) will dictate how much energy moves throughout the plant.
Our research efforts are directed to evaluating the feasible use of electricity for herbicide-resistant weed control in Oregon orchards. An electric weeder (ZassoTM, Zug, Switzerland; Figure 2) adapted for use in hazelnut orchards has been tested for control of herbicide-resistant Italian ryegrass in orchards near Corvallis, Oregon. Two voltage settings (5 and 9 kV) and four application speeds (0.5; 1; 2; and 3 mph) were tested, and a nontreated control was included. Italian ryegrass plants were 1- to 2-ft tall at the time of treatment. Ryegrass shoot and inflorescence biomass were measured to provide control efficacy information.
Figure 2. Electric weeder that we have adapted and used in trials against Italian ryegrass and other troublesome weeds in Oregon hazelnut orchards. See how electrical charges are generated and delivered to weed plants.
Treatment with the electric weeder caused rapid and significant damage to Italian ryegrass plants (Figure 3). As expected, the best results were observed at lower speeds, as the treated plants remain in contact with the equipment paddles, which are the source of the energy discharge.
Figure 3. Electric weeder trials on a hazelnut orchard in Corvallis, Oregon. (A) Untreated control plot; (B) plot which was treated with 9 kV employed at 2 mph.
Italian ryegrass was well-controlled by treatments with 9 kV, and application speeds of up to 2 mph. The same level of weed control was achieved with 5 kV treatments, at an application speed of up to 0.5 mph (Figure 4). The best voltage x speed combination was 9 kV at 2 mph. This combination resulted in a 90% decrease in shoot and a 70% decrease in ryegrass inflorescence biomass compared to the untreated control plants. Conversely, 5 kV did not decrease ryegrass biomass accumulation to such low levels, even at application speeds of 0.25 mph. We also observed that Italian ryegrass seedlings were easily controlled at any voltage setting. This is likely due to their less developed root systems and overall biomass.
Figure 4. Italian ryegrass shoot biomass (blue bars) and inflorescence biomass (red bars) were measured at each treatment. At the highest voltage setting (9 kV), Italian ryegrass was well controlled at application speeds of up to 2 mph. A 5 kV application required slower application speeds (0.25 and 0.5 mph) to get the same level of weed control.
Our results suggest the possibility of integrating electric weed control into a more resilient weed management program designed to improve Italian ryegrass control in Oregon hazelnut orchards. We observed that grasses are more easily controlled with electricity than broadleaves are. Ongoing research efforts are focused on determining:
- what makes some weed species more prone to die from electrocution, and
- how much energy is lethal to various weed species?
We also observed damage to such troublesome species as yellow nutsedge (Cyperus esculentus L.) and horseweed (Conyza canadensis (L.) Cronq.). These species are currently under investigation, as are other factors that might affect electric weed control efficacy. One research question of special concern is how often electric weed control must be applied to deplete underground food reserves of weed species.
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Citations
Vigneault and D.L. Benoit. “Electrical weed control: theory and applications,” in Physical Control Methods in Plant Protection, Vincent and Fleurat-Lessard, published by Springer-Verlag, New York, 2001, p. 174.
Zhou, J. Wang, L. Cai, Y. Fan and Z. Zheng, “Laboratory Investigations on Factors Affecting Soil Electrical Resistivity and the Measurement,” in IEEE Transactions on Industry Applications, vol. 51, no. 6, pp. 5358-5365, Nov.-Dec. 2015, DOI: 10.1109/TIA.2015.2465931.