Ian BurkeBy Ian Burke

Is there anything new for defeating herbicide resistance? That depends on your perspective, your willpower, and most definitely your wallet.

From a weed science perspective, herbicide resistance is the outward expression of a broken tool, but one of quite a few tools that we use to manage weeds in general. We find tools to manage weeds, and integrate them to produce a more sustainable system overall. It just happens that herbicides are a very inexpensive and efficient tool. Replacing an inexpensive and efficient tool with another new tool is a very difficult challenge.

Another new herbicide, by itself, is not a way to defeat resistance – it simply forestalls the reckoning. The Aussies call it ‘hitting the wall’ – when you hit the wall, selective herbicides are no longer effective and the cropping system as practiced can continue no further.

As dire as that sounds, we have a relatively resilient cropping system. Winter wheat, if kept relatively weed free early in the growing season, is very competitive. If moisture is good, and cultural practices are optimized, the wheat might appear weedy but yield might be only moderately reduced.

There are also ‘new’ rotational crops like canola and peas suitable for every rainfall zone and cropping system. Our cultural weed management tools have never been more diverse. Crop rotation isn’t a new idea, but certainly many wheat-fallow systems are newly able to rotate.

Occasionally (rarely) all the right things happen. In the case of herbicides, a number of new products appeared on the market just as there was an increased adoption of reduced tillage practices in high rainfall zones in eastern Washington. The new herbicides, combined with reduced tillage, significantly reduced and in a few cases, eradicated wild oat populations in the early 2000s.

More often we see successive series of new herbicide technologies introduced in such a way that each is obsolete just as the new one comes to market. Or worse, there is no additional cultural input deployed. Downy brome management fits that description – first Maverick (or Outrider), then Olympus, Osprey, Powerflex and Beyond were introduced over an 8 year period. There are areas in Columbia and Walla Walla counties where downy brome is resistant to all five herbicides!

Weeds have an assigned value – the cost of herbicides. But in the absence of a herbicide to manage them, what is the value of weeds? Instead of managing them, perhaps eradication (as happened with wild oat) should be the goal for many of our troublesome weeds.

Australian farmers have implemented a zero tolerance strategy for weeds by making harvest weed seed management an integral part of their management scheme. They are wind-rowing and burning, collecting all their chaff and burning or using direct bale systems. They don’t let their weeds go to seed!

With weed infestations at such low levels, critical decisions can be made in the knowledge that the weeds won’t be a problem. For example, in a dry fall (like ours this year), an Aussie grower with low weed pressure probably wouldn’t worry if the pre-emergence herbicides had been activated – if they even bothered to use them. Such a decision and lack of stress is only possible when weed populations are so low as to be insignificant. It’s a paradigm change in perspective among farmers that results in the elimination of weeds as a significant pest.

There are a number of effective tools that can be deployed to eradicate troublesome populations. Hand weeding, although very expensive, is an effective tool for troublesome weeds like cereal ryegrass, jointed goatgrass, and even downy brome and Italian ryegrass. Farmers have found that hand weeding often yields a return in investment over time compared to less effective herbicide systems – control is complete where applied.

The next obvious technological revolution will be to employ robots in place of humans for hand weeding. Such tools are on the horizon, and should fundamentally change weed management. Instead of tolerating the survivors of a herbicide application, those survivors might be completely removed autonomously. Or simply controlled by the robots themselves.

Harvest weed seed control, either by direct bailing or using a Harrington Seed Destructor could be added to the system to reduce seed deposition. We are also discovering that we can stimulate seed germination with plant growth regulators.

The process will be a test of willpower – a single failure can reset the seed bank, and our weed seed is very persistent, often requiring years of complete control for there to be an apparent reduction in germination. The eradication of weed populations through depletion of the weed seed bank can, however, fundamentally change the cost to manage weeds over time, and represent a significant investment in the future of our wheat production system.

I would argue that what is needed in the “new” department is a change of perspective about how we value weeds–not a new tool to manage them.