Herbicide resistance is the inherited ability of a blackgrass plant to survive a herbicide that would normally give effective control. There are two main types of resistance. These two mechanisms are independent and each can occur alone or in combination with the other. The commonest form of resistance is enhanced metabolism.

   Plants with enhanced metabolism detoxify herbicides and have cross-resistance to many different herbicides. The resistance is often partial rather than absolute. The cross-resistance can be to several herbicides with the same, or different, modes of action.

   The other form of resistance is target site resistance where the site of herbicide activity within the blackgrass is blocked. The resistance is absolute but only to the "fops" and "dims". Fortunately this form of resistance is less common but it is increasing.

   To date no blackgrass populations have been identified which are resistant to tri-allate, the active ingredient in Avadex.

   It belongs to herbicide group N, (as classified by the Herbicide Resistance Action Committee, HRAC) which has no blackgrass resistance problems.

Prevention and Management of Resistance

• Adopt a long term strategy combining chemical and cultural grass-weed control measures.
• Keep accurate field notes: detail cropping, cultivations, herbicide use, and level of control achieved each season.
• Do not assume resistance is the only reason for herbicide failure. Many other reasons exist.
• Monitor herbicide performance. If concerned about control levels, have a resistance test carried out, on a sample of the blackgrass seed.

Use cultural control measures. Don't rely only on chemical control.

• Crop rotation - spring cropping as well as winter; broad-leaved crops as well as cereals.
• Set-aside provides an excellent opportunity to control blackgrass.
• Delay drilling and use a stale seedbed to control blackgrass seedlings. Spray off emerged blackgrass with glyphosate.
• Crop competition: higher seed rates help smother the blackgrass.
• Prevent seed return and spread around the farm from combines and other farm machinery.

Use an appropriate herbicide strategy

• Never rely on only one herbicide.
• Never rely only on herbicides with the same mode of action. Combine products with different modes of action to achieve the most successful control. This strategy will slow resistance development and will aid effective management of resistant blackgrass populations.
• If resistance has developed, identify the herbicides to which your blackgrass is resistant. This will help determine the best herbicides to use to control your grass-weeds.
• Avadex should be used pre-emergence of weeds and crop. Use of Avadex at this time sensitises resistant blackgrass resulting in more effective control from sequential applications of other blackgrass herbicides, including those to which the blackgrass may be showing resistance.
• Follow Avadex with an early post-emergence blackgrass herbicide at the 1-2 leaf stage of the blackgrass, this is usually 4-6 weeks after Avadex application. Programmes based around Lexus* or Atlantis* are very effective when used in sequence after Avadex.
• The best control of blackgrass seedlings is typically achieved when the plants are at this very early stage.

   IPU and trifluralin may be used but levels of control are declining. However, do note that resistance to IPU typically develops much slower than most other herbicides. The proposed decreases in the maximum use rate of IPU will further reduce the in-field performance of this product.

• For effective control always use the recommended rates.

Acknowledgements:
   The resistant management guidance provided here is based on the "Revised Guidelines for Preventing and Managing Herbicide-resistant grass weeds", produced by the Weed Resistance Action Group (WRAG) and published by HGCA, from which copies are available free of charge.

*The notes on Avadex, Lexus and Atlantis have been developed jointly by Gowan, DuPont (UK) Ltd and Bayer Crop Science.

   Be careful to assess soil type accurately.

Pyramin DF	Burex 430SC	Agricola Lenacil Flowable	Magnum
Goltix WG	Venzar Flowable	Advizor*

*Sugar Beet crops only.