MSU Extension Vegetables

Background

Late-season weed management is essential to consider when developing a weed control plan for your operation. Despite one’s best efforts, weeds can often escape early-season control. This can be a result of poor timing, missing the plants with cultivation, spray applications or flaming, or due to herbicide resistance. If allowed to reach reproductive maturity, escaped weeds can cause management problems in future years due to replenishment of the weed seedbank. These larger weeds can also harbor crop pests and diseases, interfere with harvest by obstructing equipment, or degrade final crop quality through contamination from weed residues.

Management practices for escaped weeds are notoriously limited due to the difficulty of controlling weeds when they have reached a significant size. Weed wipers use an applicator made of an absorbent material, such as sponge or a rope wick, that is saturated with herbicide and used to contact weeds growing above the crop canopy, killing the weeds but leaving the crop unaffected. The downsides of this method include a lack of herbicide options effective on larger weeds, limited efficacy on herbicide-resistant populations (depending on what product is being applied), and its unsuitability for use in organic systems.

Another option is sending in hand weeding crews to manually weed the fields, which is particularly common for managing escaped weeds in Michigan vegetable production. While this method does tend to be effective, the labor is expensive, time-consuming and hard monotonous work for employees.

The major manufacturer and supplier of electrical weeding equipment in the United States is The Weed Zapper, a Missouri-based company that began production in 2017. Electrical weed control technology has also been developed by the Brazilian company Zasso and the European companies AgXtend, Rootwave and Crop.Zone. While electrical weeders from foreign companies are rare in the U.S., familiarizing yourself with the technology that has been developed overseas shows the versatility of this technology in different cropping systems and gives a clue as to the future of the equipment in modern agriculture.

Pro-environmental attributes of the equipment are that it does not disturb the soil and does not require the use of any chemical herbicides. Electrical weeding provides systemic control of even larger plants, making it an effective option for controlling weed seed bank inputs by terminating weeds at or prior to reproductive maturity. Similar to mechanical weeding and certain herbicides, it is non-selective and therefore caution must be taken to prevent crop injury. This typically restricts in-season use to crops with low-canopy growth habits that don’t come into the path of the electrode. While this limits the application of electrical weeding, there still remains many crops where growers might benefit from integrating this equipment into their weed control plan.

For more information on how electrical weeding works, check out the factsheet “Basics of Electrical Weed Control” and the video “Introduction to Electrical Weed Control” from Michigan State University Extension.

The remainder of the article will focus on recent research in field and vegetable crops where electrical weeding has been shown to have practical utility.

Field crops

The low canopy of soybean makes it possible for electrical weeding to be used for managing escaped weeds. Recent research at the University of Missouri investigated the effects of the Weed Zapper on crop injury, weed control, and weed seed viability. In the first study, electrocution was applied once or twice at a tractor speed of 2 or 4 miles per hour when weeds reached a certain average height (30 cm or 60 cm) or growth stage (flowering, pollination, or seed set). In a second study, electrocution was applied throughout soybean reproductive growth stages to determine impact on late-season waterhemp control, as well as soybean injury and yield.

Weed control

Visual symptoms of electrocution included severe necrosis and wilting that appeared anywhere from three to 24 hours after treatment. This trial found that electrocution was most effective at later plant growth stages, most likely due to more weeds being above the canopy and contacting the electrode. Grass species were observed to have less visual control from electrocution and better recovery when treated at an earlier growth stage. There were a minimal number of weeds that survived the treatment—the following species all had some that survived: barnyardgrass, giant and yellow foxtail, and waterhemp. Waterhemp was most likely to survive during the pollination and flowering stages, which may be due to which parts of the plant were electrocuted and the plant’s ability to compensate growth at axillary buds.

The factors studied that most highly correlated with control efficacy were plant height and amount of plant moisture at the time of treatment. Higher control was achieved as plant height increased, while higher plant moisture led to lower control. Researchers also observed that a second pass of electrocution increased weed control anywhere from 4% to 15% for all weed species evaluated, other than yellow foxtail. Overall, operating speed did not seem to matter.

Weed seed viability

The study showed that when weeds had set seed at the time of treatment, a reduction of seed viability from 54% to 80% occurred across the species being studied, which included waterhemp, cocklebur, giant and common ragweed, horseweed, giant and yellow foxtail, and barnyard grass.

Crop injury/yield

In their second study, the researchers found a soybean yield reduction of 11% to 26% in all treatments, except for treatments at R4 and R6 growth stages which did not show a significant yield difference from the control. It is important to note that in this trial the researchers were trying to determine the extent of yield reduction and purposefully caused contact with the soybean plants. In a typical setting, farmers would allow for a height differential to avoid contacting soybean foliage with the electrode.

The research from Missouri indicates that electrical weeding can serve as an effective part of an integrated weed management program to help control late-season weeds in soybeans. Other potential uses for electrical weeding in field crops are control of bolters in sugarbeets and desiccation of potato vines prior to harvest. More research is required to evaluate the equipment’s performance in a number of niche use cases.

Vegetables

Numerous vegetable crops lend themselves to the use of electrical weed control due to their low canopy. These include, but are not limited to, carrots, snap beans, parsnips, celery, onions, bush squash and various brassicas (broccoli, cauliflower, cabbage, etc.). Electrical weeding appears to hold promise as an effective replacement to hand weeding as a form of late-season control for many of these specialty vegetables.

Recent research in Sushila Chaudhari’s lab at Michigan State University investigated electrical weeding equipment in terms of weed control, crop injury and economic viability compared with hand weeding, as well as its effects on weed seed germination, and soil microbial communities in conventional carrot and organic green bean production systems. Late-season weed control methods, included one hand-weeding event, one electrical weeder pass, two electrical weeder passes performed consecutively, one pass followed by another pass after a 14-day interval, two passes followed by one pass after a 14-day interval, and no late-season control. Major findings in the primary research areas are included below:

Weed control

Applying multiple passes spaced weeks apart was found to give improved control of redroot pigweed and common lambsquarters in the organic green bean system as more weeds emerged above the crop canopy throughout the growing season. Applying multiple passes on the same day was found to give comparatively higher control of redroot pigweed in the conventional carrot system as weed escapes were fewer but larger and therefore required repeated electrocutions for more effective management. There was also evidence of lower electrical weeder efficacy in areas of higher weed density.

Crop injury/yield

Lower electrode height may be needed to treat weeds where there is less of a height differential between the weed and the crop. This can result in higher foliar crop injury. Electrical weeding did not cause any internal damage to carrot root tissue. None of the electrical weeding treatments nor hand weeding led to a yield difference in either trial. This suggests that the main benefit of electrical weeding is that, by eliminating weeds before they produce mature seed, weed pressure can be reduced in later seasons.

Weed seed germination

Electrical weeding was found to reduce mature redroot pigweed seed germination by 10-14%. Additional electrical weeding passes did not significantly affect weed seed germination.

Root-zone soil biology

There was no strong evidence to suggest that electrical weeding had any significant effect on root-zone microbial communities in carrot and green bean fields, in terms of microbe population size and nitrogen cycling activity.

Economic viability

Electrical weeding cost acre^-1 was found to be 20 and 28 times less than hand weeding in carrots and green beans, respectively, even when factoring in the added ownership and operating cost of electrical weeding equipment. The lower cost was due to lower time required per acre, which speaks to the high cost of agricultural labor and time needed for hand weeding.

Conclusions

Electrical weeding is seeing greater adoption by growers who need better late-season weed management solutions. It is still early days regarding research and development of electrical weeders for various cropping systems. Recent academic research has indicated the utility of electrical weeding for eliminating escaped weeds in a way that causes minimal crop injury. The potential for electrical weeding to reduce seed germination and viability in electrocuted weeds can be used to improve integrated weed management for vegetable and field crop growers alike.