Farmers must commit to using different herbicide strategies each year, if they want to prevent weed resistance becoming an issue on their farms.
“The most obvious way of developing a herbicide resistance problem is for farmers to use the same chemicals, year-in, year-out,” Teagasc tillage specialist Ciaran Collins has said.
He made these comments against the backdrop of herbicide resistance becoming a major issue on Irish tillage farms.
A lot of the problem can be traced back to growers using below recommended herbicide application rates in the past, as a means of saving money.
Survey of weed resistance
Teagasc conducted a nationwide survey, looking at the grass weed challenge on Irish tillage farms during the 2020/2021 year.
The study included 62 growers with plough-based tillage and 41 with non-inversion tillage from the main tillage areas.
The results of the survey were made public at this year’s National Tillage Conference.
Bromes were identified on 62% of businesses surveyed. On 56% of the participating farms, spring wild oats were the most prevalent grass weed.
In addition, 8% of the bromes analysed were resistant to acetolactate synthase (ALS) type herbicides that would normally be used for control.
It was found that 10% of the wild oat populations sampled were resistant to the commonly used acetyl coA carboxylase (ACCase) herbicide types.
Italian ryegrass and blackgrass weeds were found on fewer farms.
However, the proportion of these weeds with higher resistance risks was much higher, relative to other grasses – 56% in the case of of Italian ryegrass and 43%, where blackgrass populations were resistant to ACCase and/or ALS herbicides.
The survey also confirmed that 85% of those growers using non-inversion tillage used crop rotation, compared to about 56% of those who ploughed. Herbicide usage was similar in both systems.
IWM
However, despite greater adoption of integrated weed management (IWM) practices, non-inversion tillage farms had significantly higher populations of bromes and Italian ryegrass, compared to those farms that ploughed.
This is the first multi-component survey to be carried out in Ireland.
It specifically explored the amount and type of grass weeds that occur in tillage fields, herbicide resistance and IWM used, across farms with different crop establishment systems.
The results indicate that grass weed challenges are on the rise.
Specifically, there are increasing cases of blackgrass and Italian ryegrass resistance to herbicides now being identified in Ireland.
Moreover, the apparent speed at which resistance is developing in both these species, poses a significant threat to crop production.
Non-inversion tillage particularly when coupled with earlier sowing, increases the grass weed threat.
As a consequence, there is a need to develop effective cultural/non-chemical weed control tactics to protect existing effective herbicides.