Weeds, microwaves, and better crops: Killing weeds without herbicides

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Weeds pose a huge challenge for gardeners and farmers alike, impeding the growth of wanted plant species. Unfortunately, creating herbicides with complete selectivity for only unwanted, invasive species is very challenging, and the use of herbicides can also pose risks for the environment and wildlife. Dr Graham Brodie at the University of Melbourne has found an alternative way to make weeds disappear without the use of herbicides – by harnessing the power of microwave radiation.

Growing crops is a dangerous business. As well as an endless battle against pests and disease, providing optimal growing conditions for plants tends to encourage the growth of less welcome species: weeds.

Weeds are highly problematic as they compete with crops for soil nutrients and space, reducing food yields. As well as posing direct competition, weeds can also invite unwelcome guests such as plant diseases and insects that can wreak further havoc on crop production. Not all weeds are very friendly in themselves either. Species like boxthorns have sharp and dangerous spines, and others can be poisonous to both livestock and humans. Even seemingly benign plants, like wild garlic, can be problematic if they grow in pastures as the garlic contaminant affects the milk and produce from livestock and can render it unsellable.

Efficient and effective farming relies on as much control over the crop growing process as possible. While the ever-changing and hard-to-predict weather poses its own challenges, crop growers do their best to optimise soil conditions, spacing of plants, and planting times, all to maximise crop yields. This is a vital part of ensuring good security of food supplies.

“Efficient and effective farming relies on as much control over the crop growing process as possible.”

However, weeds like to disrupt all of this orderly planning and can have unpredictable, detrimental effects on the farming cycles. As a result, farmers devote a great deal of effort to getting rid of weeds. Sometimes this is a manual process, where the unwanted plants and roots are dug up from the soil by hand. Other times, techniques such as flaming are used, where the weeds are burnt out. This is not without its risks, particularly in countries and seasons where drought is a concern and it would be easy for a fire to spread out of control.

Some approaches are preventative, such as covering crops to make it difficult for unwanted plants to take root or preparation of the soil with mixtures that discourage weed growth. However, for large-scale farming operations where manual weeding becomes impossibly time consuming, by far the most common method is to use herbicides.

Dr Brodie with his unique applicator design. Microwaves penetrate the soil surface, only going to the necessary depths for weed killing.

Herbicides work in many different ways, sometimes by impairing the ability of the plant to generate its own food via photosynthesis or killing it by disrupting its cellular structure. A good herbicide is highly selective and will only kill particular plant species. However, this can be challenging to achieve in practice, with most herbicides proving fatal to nearly all plants at high enough concentrations.

While new herbicides are constantly being developed, Dr Graham Brodie at the University of Melbourne has decided to take a different approach to eliminating these pesky plants. Dr Brodie has demonstrated that microwaves can prove as deadly to weeds as any herbicide can, with the added bonus of avoiding many of the environmental and toxicological complications that are associated with extensive herbicide use.

Microwave magic

When cells absorb microwave radiation, the additional energy causes the cell walls to vibrate and eventually rupture. When this happens, it is fatal for the plant. Killing plants in this way can be very quick, efficient and does not suffer with the same long-term usage issues that herbicides face with the growing problem of herbicide resistance. The big challenge Dr Brodie encountered was finding a way to generate microwaves of sufficient intensity that was not highly energy inefficient.

Efficacy of microwave soil treatment on wheat grain yield, compared with other weed control options. Modified from “Microwave soil treatment improves weed management in Australian dryland wheat.” *Transactions of the ASABE,* 61(2), 671–680. Used with permission.

The trick in the development of his new equipment was the addition of an antenna to deliver the energy efficiently into a localised area. Dr Brodie’s unique design can be used not just to irradiate weeds that have already grown; the microwaves have also proved fatal to seeds germinating in the soil. This means with microwave radiation it is possible to disrupt the entire lifecycle of invasive weeds.

What makes microwaves highly effective for use in the field is that the soil is fairly transparent to microwaves so they can pass through without being absorbed. This means there is little energy loss of the microwaves as they pass through the soil and they do not cause any unwanted heating outside of the seed or plant that is the target. In many types of agricultural practice, most of the weed seed bank exists in the top two centimetres of soil. As the microwaves can penetrate up to five centimetres, this means the microwave radiation can reach all of the plants.

“When cells absorb microwave radiation, the additional energy causes the cell walls to vibrate and eventually rupture.”

Engineering feats

As well as the challenges of building and developing a microwave source that would be portable in the field, Dr Brodie also needed to ensure that, outside of the seed being targeted, the microwave radiation was not causing any unwanted heating of the soil that could be disruptive to crop growth. This is why he implemented a type of ‘slow-wave’ microwave technology that minimises radiation into the soil surface, only going to the necessary depths for weed killing.

Farming of the future? Dr Brodie’s slow-wave applicator is the first that has been used for agricultural applications.

Anything which contains water can absorb microwave radiation and one of the concerns with more traditional microwave technologies for weed management is that the amount of reflected radiation would harm the human operators of the device. The new device delivers an intense microwave field to kill the target weed, without radiating energy away, thus making it safe for the operator. It drastically reduces the total energy needed to kill weeds and their seeds, making the technology commercially viable.

The slow-wave comb applicators that Dr Brodie is using take advantage of the wave-like nature of microwave radiation. By engineering structures that ‘gather’ the microwaves into a confined region, it is possible to generate much higher local microwave intensities without increasing the overall energy cost of the generation.

While this technology has been used before in various microwave devices and for heating of very thin materials, this is the first time the slow-wave microwave applicator has been used for agricultural applications. Dr Brodie could also experiment with different microwave frequencies to find the optimum conditions for weed killing in soil. As moisture content in soils can vary dramatically, which in turn affects the microwave penetration, having the ability to optimise the machine’s conditions may be an advantage.

A greener future

Dr Brodie’s next big challenge is adapting the technology so it can be mounted on solar panel-powered autonomous vehicles. With advances in image processing technologies, robots that can distinguish different plant types or recognise crop planting arrangements are a possibility for fully automated weed management without an herbicide in sight.

As herbicide resistance becomes a growing problem and many of the financial costs associated with using herbicides continue to increase, Dr Brodie’s creation will have an important role to play in weed management, and zapping weeds with microwaves may become a regular part of farming in the future.

What are your next engineering challenges to overcome?
The next engineering challenge is to develop a microwave spot weeder for mounting on an autonomous vehicle or for domestic use.

References

Brodie, G, Pchelnikov, Y, Torgovnikov, G, (2020) Development of microwave slow-wave comb applicators for soil treatment at frequencies 2.45 and 0.922 GHz (theory, design, and experimental study). Agriculture (Switzerland), 10(12), 1–16. doi.org/10.3390/agriculture10120604

DOI
10.26904/RF-138-1821516625

Research Objectives

Dr Brodie has designed a new slow-wave applicator that might make microwave weed control commercially viable.

Funding

Grains Research and Development
Corporation
AgriFutures Australia
IPGroup

Collaborators

Professor Grigori Torgovnikov
Dr Dorin Gupta
Dr Muhammad Jamal Khan
Dr Peter Farrell

Bio

Dr Brodie is an electrical and electronic engineer with 40 years of industrial and academic experience. He designs industrial microwave systems, microwave sensors, overhead and underground electrical distribution networks, on-farm animal waste management systems, and on-farm renewable energy systems. He is best known for microwave weed control.