Herbicide Resistance is Certainly Manageable!

“Doing the same thing over and over and expecting a different result” is often quoted as Einstein’s definition of insanity. It could easily be argued that is exactly the practice that farmers and agronomists have found themselves following in recent years. This is because herbicides in the past were highly effective, cheap and easy to use. But reliance on herbicides alone has contributed to the widespread herbicide resistance problems that we are seeing today. If you look at the problem simply, herbicide resistance is nature’s way of telling us herbicides alone are not sustainable and introducing more diverse weed control methods is required to disrupt the weed’s life cycle.

How farmers and agronomists were dealing with the challenge of herbicide resistant weeds and discussing how to manage herbicide resistant weeds. Farmers and agronomists are actively looking for better ways of dealing with herbicide resistance, with the momentum moving to more cultural controls of weeds rather than relying on synthetic chemistry. This is particularly important since no new herbicidal mode of action has been discovered for over 20 years, and even if a new mode of action was discovered today it would take many years to work its way through the regulatory process before reaching the market.

There is need for effective communication when it comes to talking about herbicide resistance.

This starts with effectively communicating new research on herbicide resistance in a format that farmers and agronomists can understand, right through to ‘farmer to farmer’ discussion groups where sharing and finding solutions as a collective is really working.

To put it simply: herbicide resistance is a problem that is not going to go away, but it is certainly manageable!

Herbicides and resistance
The accepted definition of herbicide resistance is the one given by the Global Herbicide Resistance Committee, which is:

“Herbicide resistance is the ability of a weed biotype to survive an herbicide application, where under normal circumstances that herbicide applied at the recommended rate would kill the weed. In contrast, plant tolerance to a particular herbicide is the inherent ability of that plant species to survive and reproduce after treatment with that herbicide.”

Herbicide resistance is broadly broken down into two groups: target site resistance and non-target site resistance. Herbicides work by binding onto enzymes and inhibit the metabolic process within the plant, leading to death. In target site resistance the binding site within the plant has been altered by mutations meaning that the herbicide can no longer bind to the enzyme and will lead to the plant surviving. Target site resistance is a total resistance and affects herbicides that are in the same chemical group. All other forms of herbicide resistance fall into non-target site resistance, and this most commonly comes into a group known as enhanced metabolism resistance, which is where the plant can detoxify the herbicide faster than it reaches the target site within the plant.

Herbicide resistance
Although herbicide resistance is predominantly in grass weeds, the three broadleaf weeds should not be ignored since herbicide resistance in the broadleaf weeds is almost entirely down to relying on the same mode of action at reduced rates year on year.

Resistance testing
The benefits of herbicide resistance testing are obvious. Without testing for resistance to herbicides then you can never be 100% sure that you have resistance. Inadequate control of weeds from a herbicide can come from a number of factors involving poor sprayer set up, and can include: water volume, coarseness of spray, sprayer speed and boom height. Climatic factors also come into play, such as: temperature, soil moisture and speed of weed growth and growth stage.

These are just a few examples of what can lead to poor control from a herbicide and only by properly testing for herbicide resistance can you discount external factors and actually confirm herbicide resistance. Some of the benefits from testing are that you can find out which herbicides should still give a good level of control, thereby preventing unnecessary herbicide applications.

Another good reason is to monitor the resistance situation within a field, or as a quick reference when taking on a new block of land. Some of the reasons given for not resistance testing include: cost, difficulty or time required to collect a sample.

So What?
The overwhelming message I’ve heard time and time again is: ‘Diversity, diversity, diversity’ - the more things you can change the better! The fascinating thing about nature is its ability to take advantage of things staying the same, so if you keep changing your farming practice by constantly ‘mixing it up’ then it cannot adapt quickly enough to the new environment that has been created.

Cultural diversity is probably where there will be more immediate gains. Some of the more implementable cultural controls probably include:

• Make a plan, but be flexible
If you have made a plan then you have taken the first step in ‘owning the problem’ since herbicide resistance does not go away if ignored. It will only get worse. Also by thinking about fields in isolation you can work your way back to the root of the problem and think of the most appropriate solutions. Be prepared to change the plan if necessary but be patient at the same time because reducing a weed population can be a slow process.

• Soil health and cultivations
Healthy soils that are well drained and have higher organic matter contents give you so many more options with cultivations, cropping, etc. Go back to basics ensuring drainage schemes are working and if mole ploughing would be appropriate or would help.

Consider what cultivations are appropriate or if they are appropriate at all! Ploughing is a great method of weed control, but it cannot be done on too regular a basis otherwise you just end up mixing all the seeds throughout the soil profile. Ploughing is also a very skilled job and modern ploughs often don’t do a really good job of inversion. Other cultivations should be kept as shallow as possible with all drilling operations being as low-disturbance as possible.

On land clear of weeds, or in years of good weed control, no-till should be maximised so as not to bring fresh weeds to the surface.

• Attention to detail
Factor the implications of weed control into all your farming decisions. Time spent rouging weeds is very well spent if the population is low enough to allow, also taking the time cleaning down cultivator and combines in field gateways is often time well spent.

Finally, don’t undervalue what a difference really good communication can make and despite this new ‘posttruth’ age we now live in it’s reassuring to know that farmers still trust farmers.