Grass weeds, herbicide resistance and control practices: current status and challenges ahead

Vijaya Bhaskar presented his work at the recent Teagasc National Tillage Conference.

This research aimed too quantify the extent of grass-weed challenges on Irish tillage farms, a nationwide survey was conducted in 2020 to 2021.

The survey included 62 growers with plough-based tillage and 41 with non-inversion tillage from the main tillage areas.

There were three components to the survey:
a) assessment of the amount and distribution of grass weeds before cereal harvest in each of the two years;
b) analysis of collected samples for herbicide resistance, and
c) recording weed control measures used on each farm. The key results from this survey are:

• Among 103 farms we surveyed, bromes (on 62% of farms) and spring wild oats (56%) were the most prevalent with 8% of the bromes resistant to the ALS type herbicides normally used for control, and 10% of the wild oats resistant to commonly used ACCase herbicide types.   Italian ryegrass (13%) and black-grass (16%) were found on fewer farms but with higher resistance risks; 56% of Italian ryegrass populations and 43% of black grass populations were resistant to ACCase and/or ALS herbicides. 
• Over 85% of those using non-inversion tillage used crop rotation, compared to about 56% of those who ploughed. Herbicide use was similar in both systems.
• In spite of 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 in Ireland exploring 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 grass-weed challenges are increasing.  In particular:

• Increasing cases of black grass and Italian ryegrass, and the apparent speed at which resistance is developing in both these species, pose a significant threat to crop production
• Non-inversion tillage particularly when coupled with earlier sowing increases the grass-weed threat.
• There is a need to develop effective cultural/non-chemical weed control tactics to protect existing effective herbicides.

This survey has generated a useful baseline, from which future change can be measured. This will help us monitor the challenge of difficult-to-control weeds and the effectiveness of IWM measures.