A comprehensive scientific paper outlining occurrences of grass weeds and herbicide resistance status in Ireland has just been published by Vijaya Bhaskar and his colleagues at Teagasc Oak Park, Co. Carlow.
The survey found that grass weeds have increased on Irish tillage farms due to increases in winter cereals and non-inversion tillage adoption, and also new weed introductions.
The loss of herbicides due to tighter regulation and increased herbicide resistance have also contributed.
Grass weeds and herbicide resistance research
The Teagasc research comprised a survey of growers from the east and south in 2020 and again in 2021.
The work included 62 growers with plough-based tillage and 41 with non-inversion tillage.
Grower demographics, their concerns about grass weeds and the use of different weed control strategies were recorded by questionnaire.
Grass weeds were quantified by careful assessment using a 24m x 24m grid sampling technique within a 4ha area within growers’ fields, prior to harvest in each of the two years.
Seed samples were collected for herbicide resistance testing.
Research results
The key results from the survey confirm that non-inversion tillage was associated with larger farms and younger growers with more formal education.
Most of the growers surveyed were aware of herbicide-resistant grass weeds (>80%) and most (>90%) used integrated weed management (IWM) practices to some extent.
Across the 103 farms, bromes (on 62% of farms) and spring wild oats (56%) were the most prevalent grass weeds.
Significantly, 8% of the bromes were resistant to the ALS-type herbicides normally used for control and 10% of the wild oats were resistant to commonly used ACCase herbicide types.
Italian ryegrass (13%) and blackgrass (16%) were found on fewer farms but with higher resistance risks. The research work also confirmed that 56% of Italian ryegrass populations and 43% of blackgrass populations resistant to ACCase and/or ALS herbicides.
Crop rotation and covers
Over 85% of those growers using non-inversion tillage used crop rotation compared to about 56% of those who ploughed.
Cover crops were used by 71% of those who practiced non-inversion tillage compared to 19% of those who ploughed. Herbicide use was similar in both systems.
In spite of greater adoption of IWM practices, non-inversion tillage farms had significantly higher population pressure based on weed scores for bromes and Italian ryegrass compared to those who ploughed.
According to the Teagsc research team, this research paints quite a worrying picture for the control of grass weeds in cereals.
Specifically, it confirms that blackgrass is now a serious threat to Irish crop production. In addition, bromes, wild oats and Italian ryegrass create an increasing challenge particularly when coupled with herbicide resistance evolution.
Non-inversion tillage, particularly when coupled with earlier sowing, increases the grass weed threat.
While IWM is being practiced to a limited extent, considering the weed and herbicide resistance challenges, there is an urgent need to develop effective IWM for all crop production systems, according to Teagasc.
The survey, in addition to identifying these challenges, has also set a useful baseline, from which future change can be measured. This will help research scientists monitor the challenge of difficult-to-control weeds and the effectiveness of integrated weed control measures.