20 February 1998

Resistance is

spotted with

easily

guide

Herbicide-resistant grass-

weeds are spreading.

Steve Moss of IACR

Rothamsted provides a

quick checklist of what to

look for in the field

HERBICIDE resistance was first identified in blackgrass in 1982. By 1996 it occurred on 750 farms in 30 counties across England.

Wild oats and Italian ryegrass have also developed resistance. The former were first found in 1993, and are currently found on fewer than 50 farms. Affected areas include Dorset, Essex, Kent, Lincs, Oxon, Suffolk and Wilts. Resistant Italian ryegrass occurs on about half that number of farms, and has been found in Cambs, Essex, Lincs, Oxon and Wilts.

It is difficult to confirm resistance solely on the basis of field observation, but some indicators are:

&#8226 The level of weed control of other susceptible species. If these have been controlled successfully, then resistance may explain why living weeds have survived.

&#8226 The presence of living plants next to dead plants of the same species (Fig 1). That may occur due to poor spraying technique or subsequent germination, but it can be a very good indicator for target site resistance in grass-weeds where "fop" or "dim" graminicides have been applied in broad-leaved crops.

&#8226 Herbicide history. Repeated annual use of the same herbicide, or herbicides with the same mode of action, favours selection for resistance.

Keep and refer to field records of herbicide use (Fig 2). Do not discard old records – a record of herbicide use over the last 10-20 years is of much more value than records for only the past 2 to 3 years.

&#8226 Cropping and cultural history. Many, but certainly not all, cases of resistance are associated with intensive winter cereals and non-inversion tillage (Fig 3).

&#8226 Occurrence of resistance in the vicinity.

&#8226 Development of dense patches of grass-weeds in areas where patches have not been noticed before (Fig 4). These may be quite small initially – perhaps less than 100sq m.

&#8226 If you are concerned that resistance may be developing, collect a seed sample in July (Fig 5) and have a resistance test carried out. More, hopefully quicker, diagnostic techniques are likely to become available during the next year.

For more information read the new WRAG "Guidelines for Preventing and Managing Herbicide-Resistant Grass Weeds." These are available free from Home-Grown Cereals Authority, Caledonia House, 223 Pentonville Road, Kings Cross, London N1 9NG. &#42