Herbicide resistance

Herbicide resistance in a range of arable grass weeds is becoming a serious problem throughout New Zealand.

Resistance of ryegrass usually springs to mind but other species such as annual poa and wild oats are also affected, and the problem is likely to continue to develop in other grass weed species.

When resistance occurs, it means that in affected paddocks commonly used herbicides are either less effective than before or in the worst case do not work at all.

How widespread is resistance?

The short answer is very widespread and includes several grass weeds: perennial ryegrass, annual ryegrass, wild oats and annual poa. A recent FAR survey found ryegrass resistance in 36% of Canterbury paddocks tested.

Ryegrass resistance was first reported in New Zealand to multiple modes of action in 2017. Perennial ryegrass resistant to Group B herbicides and annual ryegrass to Group A herbicides.

In addition, annual poa exhibiting resistance to Group A herbicides was found in 2019, with Bayer paddock inspections in 2020 also suggesting resistance to Group B herbicides was also developing.

How does resistance develop?

Resistance to herbicides is naturally occurring, it is not caused by the herbicide itself.

In any population there are a small number of individuals which are less sensitive or even totally resistance to the herbicide, even before the herbicide has been used. When a herbicide is applied, these individuals have a better chance of survival and are “selected for”. Over time they can come to dominate the population rendering that type of herbicide ineffective.

What drives resistance development?

Two main factors drive the development of herbicide resistance.

One is the frequent use of herbicides with the same mode of action, especially when alternative modes of action are not used in the same or subsequent season/crops.

The second is poor grass weed control, as the natural occurrence of resistant individuals is a numbers game. The more plants that set seed, the more seed that is produced then the greater the chance a resistant individual plant will be present in the paddock.

What types of resistance are there?

Target site resistance

Herbicides work by interfering with processes within grass weeds and they do this by binding to a site of action within the plant – called the target site. And so, as its name implies, target site resistance occurs when plants which naturally have a modified target site encounter a herbicide. Because these plants have a modified target site the herbicide cannot effectively control them. Over time these plants are selected for and the herbicide becomes ineffective.

This type of resistance is particularly worrying as often the change in the plant does not reduce the plant’s ability to compete and set seed. There is no fitness penalty. And this means there is no reason for the population to revert to a sensitive population.

Also, because the herbicide can no longer bind with the target site in the plant it typically doesn’t matter what dose you apply. It just won’t kill the plant.

Once you have target site resistance it is most often here to stay.

Metabolic (non-target site) resistance

This type of resistance describes ways that the plant can survive a herbicide application that does not involve changes to the herbicide binding site. This could be due to enhance degradation of the herbicide by the weed, reduce herbicide uptake or other mechanisms.

With metabolic resistance, if the herbicide is not used for a period of time it may regain its effectiveness, but this can be short lived because when it is next used it tends to encourage development of the original problem again. With this type of resistance, increasing the dose applied can increase effectiveness but this can be expensive, short lived and care should be taken not to exceed label registered use rates.

What can I do?

While it may not be possible to stop the onset of herbicide resistance on your farm there are several simple precautions you can take to delay development. Put simply, stack the odds in favour of the herbicide so it can effectively control your weeds.

First and foremost, consider non-chemical weed control options such as residue burning, increasing crop competition and strategic cultivation, to name a few.
Practice good farm hygiene. Avoid transfer of contaminated soil around your farm and from other farms. Clean machines thoroughly. Avoid sowing seed that is contaminated with weed seeds.
The smaller the better. Almost always herbicides are more effective when grass weeds are smaller. So, start early and control weeds as they germinate or when they are growing their first few true leaves.
Manage the seedbank. Many weed species produce large numbers of seed and some weed species can survive for a long time in the soil. It is recognised that to reduce the seed population in the soil, high control levels are required (e.g. 98% or more)
Create a firm, clod free seedbed with adequate moisture and minimal trash. Use cultivation techniques that keep weed seeds near the surface and that don’t disperse them through the soil profile.
Utilise herbicides with different modes of action to create diverse herbicide programmes. Consider your crop rotations to ensure you can do this. And keep records of what you use.
Apply herbicides according to the label recommendations. Take note of recommended weed sizes, dose rates and climatic restrictions

Herbicide mode of action (m.o.a.)

Group	Mode of Action (m.o.a.)	Chemical examples
A	Inhibition of acetyl CoA carboxylase (ACCase)	Pinoxaden Fenoxaprop
B	Inhibition of acetolactate synthase ALS	Iodosulfuron Pyroxsulam
C2	Inhibition of photosynthesis at photosystem II	Diuron Isoproturon
D	Photosystem-I-electron diversion	Paraquat
G	Inhibition of EPSP synthase	Glyphosate
H	Inhibition of glutamine synthetase	Glufosinate-ammonium
K1	Microtubule assembly inhibition	Trifluralin Propyzamide
K3	Inhibition of cell division	Flufenacet Pyroxasulfone
N	Inhibition of lipid synthesis – not ACCase inhibition	Triallate Ethofumesate

Herbicides are classified in groups according to their mode of action. Although they may have different active ingredients (AIs), there is a risk that some AIs share cross resistance within the same group. This effectively means that whilst resistance may be selected for by one AI, other AIs within the group can be rendered ineffective.

Applying herbicides with different modes of action is one of the most effective ways to slow down the development of herbicide resistance. It is especially effective if herbicides can be applied in mixture. It is important to ensure all herbicides applied are effective against the target grass weeds at the dose applied.

Sakura is a group K3 herbicide which brings a different mode of action to certain grass weeds such as ryegrass species and wild oats and a higher level of control to others such as vulpia hairgrass. To find out more about Sakura read the Sakura User Guide

Note: This classification system using letters is due to be updated to fall into line with a global numbering system, however the basis for the groups and the principals will remain the same.