New research from Teagasc has shown that an Italian ryegrass population in Ireland is resistant to the herbicide glyphosate.

This is the first case of proven resistance to glyphosate, according to the organisation.

Glyphosate cannot control the weed in this single site. This is the first confirmed case of resistance to the herbicide and could signal the start of a major issue for farmers across the country.

Dr Vijaya Bhaskar, weed researcher at Teagasc crops research department in Oak Park, said: “The suspect Italian ryegrass population was tested with different glyphosate products at four rates.

"Of concern was the fact that all products were ineffective, even at the maximum allowed rate. Molecular analysis revealed a known target-site mutation at the EPSPS Pro-106-Ser position, which has been associated with glyphosate resistance.”

Over 50 weed species worldwide are resistant to glyphosate. Italian ryegrass was also found to be resistant to the herbicide in Italy and the UK.

Higher rates

Higher rates of product have been needed to control some populations in recent years, but this development means the herbicide did not control the weed. This finding comes from the EVOLVE project, which has been collecting grass weed seeds from tillage farmers across the country for a number of years.

While the research has been carried out on tillage farms, the development is a concern for the wider agricultural sector. Approximately 60% of glyphosate sold to farmers is used on grassland ahead of reseeding.

Resistance to other herbicides

Research had already shown that 78% of Italian ryegrass tested by Teagasc was resistant to ACCase (Falcon, Axial) and ALS (Pacifica and Broadway Star) herbicides.

However, this new research has shown a population resistant to glyphosate.

Dr Bhaskar has been extremely active in testing problem grass weeds and broadleaved weeds for resistance. Research published in the Irish Farmers Journal last year showed that glyphosate applied at 540g/ha did not achieve 100% control of blackgrass and Italian ryegrass populations that are already resistant to ACCase and ALS herbicides.

This work showed differences in the types of glyphosate products used and the varying control levels achieved.

Teagasc showed that higher rates of generic glyphosate, which are copies of the original but may be formulated differently, are needed to control grass weeds compared with Roundup, the original producer of glyphosate.

At that time, 540g/ha of Roundup was needed to control 100% of a sensitive population, which does not have resistance issues, compared with 720g/ha of generic glyphosate.

Resistance in the UK

In January 2025, the UK weed resistance action group confirmed the first case of resistance to the herbicide glyphosate in a UK weed species.

The Agriculture and Horticulture Development Board (AHDB) reported that: “Resistance at the maximum label rate of glyphosate was identified in tests on a single field population of Italian rye-grass (Lolium multiflorum)."