THE microwave is moving out of the kitchen and into the paddock, where it could become the latest weapon in farmers' 10,000 year war against weeds.
When University of Melbourne electrical engineer Graham Brodie was working with the Wood Innovation CRC on wood-drying technologies, he noticed that microwaves exploded the cells in damp wood.
That got him thinking about the use of microwaves in weed control. He later assembled some prototypes, and found that the energy output of a household microwave oven is enough not only to fry weeds, but to sterilise seeds to a depth of about five centimetres in the soil.
In a time of rampant herbicide resistance, and with few new herbicide compounds in the research pipeline, the prospect of a new weed-fighting technology has drawn significant interest.
Dr Brodie said the concept has attracted some useful research and development funds. It is hoped to have an Australian designed and built weed microwaving machine on the market within the next three to five years.
Microwaves interact with water molecules, causing them to rotate and align with the direction of the waves.
The movement causes friction, and the friction causes heat - enough heat to cause steam. The buildup of steam within individual cells causes the cells to explode; much as an unshelled egg will explode if unwisely cooked in a microwave oven.
When Dr Brodie realised the same principle could be used to kill weeds, he had to devise a way of concentrating microwave energy onto the ground.
His prototypes have hooked up a two kilowatt microwave generator - the same power as used in household ovens - to a horn antenna, which resembles the speaker horn on an old phonograph. The antennae point at the ground.
Weeds under the antenna are killed in a second of microwave exposure, and seeds to a depth of five centimetres are rendered infertile - a strike against the weed seedbank that in itself presents a major advantage over chemical treatments.
Unfortunately, soil microbes are also killed. Dr Brodie said analysis of soil bacteria showed a big drop in numbers under microwaves, but not a complete wipeout, and there was rapid recolonisation.
Microwave's effects on earthworms and other organisms is yet to be determined.
The R&D program will also consider the issue of time. A second to kill a weed isn't long, but it's considerably longer than most broadacre farmers will want to linger over a weed in a 200 hectare paddock. Chemical spraying is often done at speeds of more than 20 km/h.
Dr Brodie thinks that increasingly sophisticated weedseeking technology, which allows a unit to fire only when it has sensed a weed, might help preserve paddock soil life.
And the great strides being made in automation may address the question of time: a robot working 24/7 can operate at a more leisurely pace than humans in need of sleep.
Practically, Dr Brodie expects the first microwave system to be made available as a tow-along unit, probably built as an entire system rather than as bolt-on modules.
But farm automation is becoming increasingly practical. In the project's five-year time frame, it may become viable to sell a robotic system alongside the standard unit.
Apart from providing a non-chemical approach to weed management, microwave weed killers could be used in all weathers, work regardless of plant species or herbicide resistance, and would need no withholding periods.
Meanwhile, laser researchers at the University of Leibniz in Hannover, Germany, are approaching the same challenge with the idea of bolting low-power lasers to weedseeking robots.
Lasering weeds has its own unique set of challenges. The laser's power has to be set exactly right or the light can actually speed up weed growth.
A tractor-mounted paddock system is also out of the question, because the laser has to be aimed with high accuracy at the plant stem. Machinery vibrations make in-field laser systems impractical.
Instead, the Laser Zentrum Hannover scientists are investigating mounting the system as a static unit within greenhouses.
More mind-bogglingly, they are also examining the possibility of taking the technology into the field by using swarms of laser-carrying multicopter drone aircraft to detect and kill weeds in the field.
