All blackgrass and Italian ryegrass must now be treated as having suspected resistance to herbicides, meaning they cannot be controlled by some chemicals.
Teagasc researcher Vijaya Bhaskar told the Grass Weeds Conference on Wednesday 8 November that there is now resistance to most common herbicides in wild oats and sterile brome, as well as blackgrass and Italian ryegrass.
He added that the best approach farmers can have to these weeds is zero tolerance.
Glyphosate, which is a non-selective herbicide, was even found to be ineffective at controlling some blackgrass when applied at 540g/ha to stubble fields.
Teagasc research showed 55% of blackgrass samples and 60% of samples of Italian ryegrass were resistant to commonly used herbicides.