Herbicide resistance in black-grass

Regular use of black-grass herbicides is associated with resistant weed populations. Learn about the extent of the issue and ways to tackle it.

Resistant black-grass populations are a major threat to arable production. First identified in black-grass in 1982, resistance has now been detected in almost every county in England and at a low frequency in Wales and Scotland. 

• Resistance to mesosulfuron + iodosulfuron (introduced into the UK in 2003 as ‘Atlantis’) is present in over 400 farms in 26 counties. ALS target-site resistance was confirmed in many resistant populations, although enhanced metabolism resistance also poses a big threat

• Use of high-resistance-risk ALS- and ACCase-inhibiting herbicides, in mixtures and sequences with lower-risk modes of action, increases the overall level of weed control, but does not prevent resistance increasing

• Resistance can reduce the efficacy of all available pre-emergence herbicides, but usually only to a limited degree. Flufenacet appears the least affected herbicide. Resistance also appears to increase relatively slowly, compared with the post-emergence ACCase- and ALS-inhibiting herbicides

• Additionally, high-risk management could drive the selection of resistance to the non-selective herbicide glyphosate

• Non-chemical control methods can give useful, if modest, levels of control of black-grass. Greater use of non-chemical control methods reduces the dependency on herbicides and the risk of resistance

Predicting the impact of resistance on an individual field is difficult because:

• The proportion of plants affected is different
• The type of resistance varies
• Some herbicides are more affected than others

There are two broad forms of herbicide resistance in black-grass:

Target site resistance (TSR). Mutations in the proteins targeted by particular chemicals can make weeds less sensitive to them. This form of resistance is relatively well understood. It can be countered by the rotational use of herbicides with differing modes of action.

Metabolic or multiple herbicide resistance (MHR). Weeds become more tolerant of a broad range of herbicides, irrespective of their chemistry or mode of action. Generally, this is due to the weed being better able to detoxify crop protection agents. MHR is also termed non-target site resistance (NTSR). As MHR is poorly understood, the BGRI focused on it.

The AHDB-supported resistance action groups (RAGs) are informal, UK-based groups consisting of experts from the Crop Protection Association (CPA) member companies, other representatives from the agrochemical industry, a range of independent organisations, including public-sector research institutes, and the Chemicals Regulation Directorate (CRD). The groups are completely independent of AHDB and work to produce guidance on pesticide resistance issues. As part of its support to the resistance action groups, AHDB publishes their guidance, including that produced by the Weed Resistance Action Group (WRAG).