Herbicide resistance in tillage farming
Author: Jimmy Staples
Over the last decade, it has become increasingly difficult to manage some of the pests that we were previously able to control comfortably with plant protection products.
This can be attributed to:
- The loss of key active ingredients due to de-registration
- A lack of new active ingredients coming onto the market
- The development of resistance to the current active ingredients on the market
Loss of active ingredients
The loss of active ingredients is a worrying trend and according to the DAFM website, there were 17 active ingredients whose registration was revoked between December 31st 2018 and August 23rd 2020. The last date for use of many of these active ingredients has now passed with the loss of chlorothalonil (Bravo) and the neonicotinoid family of insecticides creating big challenges.
Resistance in the tillage field
This loss of key active ingredients is compounded further by the development of herbicide resistance. BYDV control can be challenging due to pyrethroid resistance within some species of aphids. Diseases like Septoria are constantly putting available chemistry to the test and resistance to herbicides is becoming increasingly common throughout the country.
We have both broad-leaved and grass weed resistance confirmed here in Ireland and it is suggested that this is under-reported. Here in Wexford, I have come across resistant populations of chickweed, corn marigold and wild oats and I am currently awaiting results from Teagasc in relation to populations of blackgrass and Italian ryegrass which may also be resistant.
Cultural control: Integrated Pest Management
The Enable Conservation Project has confirmed herbicide resistance in Italian ryegrass, blackgrass and wild oats. Blackgrass and Italian ryegrass populations have been found to be resistant to ALS herbicides (Alister Flex, Pacifica Plus, Broadway Star, Monolith), ACCase herbicides (Axial Pro, Falcon, Stratos Ultra) and in some instances, a combination of both. Where both ALS and ACCase resistance are present, it severely limits your crop herbicide options.
In these cases, Integrated Pest Management (IPM) strategies need to be to the fore and there are many options available. These include but are not limited to:
- Use of rotations
- Cover crops
- Companion cropping
- Arable grass margins
- Using competitive cultivars
- Pushing back sowing dates
- Increasing seeding rates
- Stale seedbeds
- Cultivation type and depth
- Establishment system
- Rogueing
- Crop destruction
- Remote sensing
- Monitoring and evaluation
A successful IPM strategy will consist of a number of these measures chosen specifically to meet the challenges of an individual’s farm and should be used along with the targeted application of herbicides. These should be implemented in a structured fashion over the course of the whole rotation. Achieving effective control of many of these weeds will not be possible in one, two or even three years.
Herbicide resistance management strategy
Prevention of course is better than a cure and there is simple, yet effective measures that we can take to try and ensure that herbicide resistance does not evolve on farm. Most of these measures will revolve around the use of and application of herbicides.
When spraying, consider the following as part of a herbicide resistance management strategy:
- Has the target weed(s) been identified correctly?
- What is the target growth stage and when is the weed going to be at this stage?
- What volume of water do I need to use to ensure sufficient coverage?
- Do I have issues with the pH in the water I am using?
- What rate of herbicide do I need to achieve control of the weed(s) at the target growth stage?
- Try to keep tank mixes as simple as possible, so as not to stress the crop and ensure that all components of the mix work effectively.
Some things are outside of our control. The last two springs have been incredibly challenging with weather conditions either too wet or too cold and the variations in day and night-time temperatures meant spraying days were very limited. I came across a number of cases of wild oats and sterile brome which were not controlled by herbicides in both 2020 and 2021. Samples were sent for resistance testing and results have shown that they were not resistant to the products used. A number of factors contributed to the lack of control, namely, big variations in day and night-time temperatures, shading from the crop and the use of lower rates of herbicide than was required.
It is important to understand the weeds on your farm as this will help you to decide on management strategies. For instance, it can take from five to ten years for resistance to develop in a population of wild oats as opposed to a population of blackgrass where resistance can develop in just three generations.
If you do suspect herbicide resistance on your farm, please get in contact with your advisor as the earlier the problem is diagnosed and action is taken, the greater the chance of a positive outcome.
Further Information
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