Non Chemical Weed Management

May 2023

A Lighter Touch uses non-chemical methods to control weeds in crops.

Non-chemical methods of controlling weeds in horticultural and arable crops are gaining favour as growers respond to ever increasing species of herbicide resistant weeds being identified, loss of existing herbicides due to legislative bans, lack of new modes of action since the 1980s, and consumer demand for safe food. The industry extension programme A Lighter Touch seeks to use the best technology available to shift the focus of crop protection and integrate biological and ecological processes into food production.


Using agroecology (the application of ecological principles to agricultural systems and practices) offers New Zealand growers the prospect of producing food sustainably and ethically without use of synthetic agricultural chemicals, meet the demands of sophisticated markets, and establish a high-value niche for our exports. “A Lighter Touch”, a Sustainable Food and Fibre Futures (SFFF) fund and industry-supported programme of trials and extension, has been under way for several years. Non-chemical weed management in crops is one important aspect of the programme, as sustainable production becomes a marketing virtue. 


Dr Charles “Merf” Merfield is an agroecologist with special expertise in biological and organic farming and is the Head of the Future Farming Centre at Lincoln’s BHU Organics Trust. An adviser to the programme, he says that in some respects non-chemical weed control is ‘back to the future’.  “Before the advent of chemical herbicides growers used inter-row cultivation, cover crops, and different crop species adjacent to each other rather than vast areas of monoculture. We’ve had about five generations of growers using chemicals since those times, and we need to look back, learn from those techniques and improve on them,” he says. “I use the metaphor of the four toolboxes – physics, chemistry, biology, and ecology. Most growers use only one – chemistry. What about the other three?”


“Every single arable farm should have a broadacre weeder on it. It's bonkers if these guys don't have these things in the shed. Spring tine weeders, the main type, were invented 50 years ago, and there's 40-odd manufacturers of these things. You can have any colour you like! We also need to decide how else to make the environment inhospitable to weeds. There’s a lot of bare ground underneath crops like maize, so what could we sow underneath that will suppress weeds and provide other benefits? Clover, for example, covers the ground, provides some nitrogen, and once you harvest you’ve already got a crop in the ground that you can graze.”


Other ground cover plants might provide an environment for beneficial insects, says Merf. Intercropping is another technique that gave way to monoculture cropping when herbicides were introduced. Electrothermal weed treatments are more recent and has huge potential as a control method. “But the first thing to do is to redefine what is a weed, because many farmers think anything that's not the crop is a weed. The agroecological view is there's a lot of other plants that are not causing any harm, and therefore they're not weeds.”


After that, it’s a matter of applying agroecology, using whatever tools are available. Conventional growers can learn a lot from the best organic growers who have been using non-chemical weed control for decades. Standard mechanical weeders are readily available but the most advanced weeders – electrothermal and level 3 robots – are being developed overseas and will take some time to get here, says Merf. “Companies like Case IH are investing millions in specialised equipment such as electrothermal weeders. Chemical giants, anticipating the challenges coming for herbicides, are joining in.”


“Giant inter-row hoes are now using GPS guidance or computer vision technology to speed down rows without the need for an operator. The next step after that is into robotics, using vision and convoluted neural networks to identify every single plant under their canopy, determine which is a crop plant and kill everything else. And they're using all kinds of methods; mechanically pushing the weed into the ground, using lasers, electro-thermals, focused blue light – so this is really cutting edge. These robots are essentially a replacement for all selective herbicides. And electrothermal is potentially a replacement for broad spectrum glyphosate use, such as killing off a field of pasture before direct drilling. It can also be used for desiccating wheat and oats prior to harvest in place of glyphosate.” 


The world is heading towards having enough non-chemical technologies to replace herbicides. However, New Zealand is a very small market and a price taker at the bottom of the world, says Merf. “We have one company, Weed Spider, that invented the Seed Spider 30 odd years ago as a precision, high density vegetable drill, but they are entirely focused on California. Basically, everybody in the weeding robot space is focused on California, which grows 80% of the vegetables for the United States. There are also developments going on in Asia and Europe, but it will be a while before we can access them down here.”


So, what can New Zealand growers do in the meantime? Learn from the best of organic growers and replicate what they do, says Merf. “For arable farmers it’s ‘steel in the field’. There are two broad classes – what you'd call broadacre or contiguous machines, like spring weeders and spoon weeders that can be used across the whole field surface, and you use them instead of herbicides. Then you can start moving into inter-row hoes that are working between the crop rows but also potentially in them as well, using different techniques.”


“That's a step up in capital costs, particularly if you're using guidance systems, but then with broadacre machines and the inter-row hoes you can almost eliminate the need for herbicide in arable crops. In New Zealand's climate there's organic, arable growers doing exceptional jobs in terms of weed management, so it's basically a case of replicating what they've been doing.”


At Hart’s Creek organic farm, Tim Chamberlain’s family has been farming organically for decades. In 1986 Tim’s father said Tim could try organic methods on the “hardest to irrigate” quarter of the farm “to see if it would work”. Today, they grow many crops on 180ha for a range of markets (to minimise risk); soya beans, a range of brassicas, pumpkins, carrots and onions, leaf wheat and barley, herbs for drying, dandelion for coffee substitutes, echinacea, fennel and clover. They also graze Wiltshire sheep and a few cattle.


Among his equipment for weed control in crops Tim has a computer-guided inter-row hoe, a 15m side spring tine weeder, flame weeders, and other machinery. “For all crops it’s often a matter of monitoring and doing what is needed. For example, the carrots and onions get pre-emergence flame weeding and then post emergence inter-row cultivation as necessary,” he says. “For the broadacre crops we use tine weeders often”


Merf also advises adopting a whole-system approach, “Making use of subsidiary crops that cover the ground and protect it; smother crops, which produce a whole lot of biomass for crimper rolling; living mulches; and trap crops. Subsidiary cropping encompasses a whole bunch of different techniques that are not well known here yet. Growers should be looking to their levy payer organisations, like FAR, for advice and guidance on what options are available and how to use them.”