The occurrence of herbicide-resistant grass-weeds in the United Kingdom and a new system for designating resistance in screening assays

Several organisations conduct screening tests for resistance in the UK. A c ompilation exercise was undertaken to determine how many individual farms h ad been identified as containing resistant grass-weeds. Resistant black-gra ss (Alopecurus myosuroides) has been found on 746 farms in 30 counties, res istant wild-oats (Avena spp.) on 65 farms in 19 counties and resistant Ital ian rye-grass (Lolium multiflorum) on 25 farms in 11 counties of England. N o resistant grass-weeds have so far been detected in Wales, Scotland or Nor thern Ireland. These results show that resistance occurs over a very wide g eographical area. At present, different organisations do not interpret screening results in t he same way in terms of designating degree of resistance. A new system is p roposed, which while based on the * rating system developed by IACR-Rothams ted/ADAS, requires the use of only a single susceptible standard. This new system is appropriate for wild-oats and Italian rye-grass as well as black- grass, and is applicable to a wider range of herbicides. This new system as signs samples to four categories, RRR, RR, R? or S (susceptible).