Why is the use of ACCase and ALS inhibitor herbicides being restricted?

Herbicide resistance management strategies have to date largely been communicated through advisory information. This has been effective in increasing the profile of resistance issues and promoting key messages to the industry. However, herbicide resistance is increasing in prevalence in the UK, with Enhanced Metabolism Resistance (EMR) present in over 1 million hectares cereals and Target Site Resistance (TSR) is becoming more prevalent, affecting several hundred thousand hectares.

PSD has the responsibility for applying the relevant legislation, as well as playing a leading role in the Defra policy on sustainable pesticide use. In response to the threat from herbicide resistance to the sustainability of UK agriculture PSD asked the ACP for advice on a number of strategies for the future use of high resistance risk herbicides used for the control of grass-weeds (ACP 316/2005).

The ACP advised that PSD should adopt a precautionary approach to resistance management and apply restrictions to the use of existing herbicides where there is a high resistance risk (ALS inhibitor and ACCase inhibitor herbicides). These restrictions would prevent the use of sequences or mixtures of herbicides that are considered to pose a very high risk of resistance.

These additional regulatory restrictions are intended to ensure that all growers use high resistance risk grass weed herbicides responsibly, both now and in the future. It will also bring older products in line with those subject to more modern risk assessment procedures, ensuring a consistent approach is adopted for all high resistance risk grass weed herbicides.

What will the restrictions mean for the use of herbicides containing ACCase inhibitors?

Effectively the new restrictions will preclude the use of two applications of a product containing the same ACCase inhibiting active substance e.g. two applications of any product containing clodinafop-propargyl. However, it would not preclude a sequence of two different ACCase inhibitors. Thus a sequence of a ‘fop’ followed by a ‘dim’ or a sequence of two different ‘fops’ would be possible. To avoid the build up of resistance products containing an ACCase inhibitor herbicide must not be applied more than twice to any crop.

When can a second ACCase inhibitor be used?

It is recognised that applying a second product containing an ACCase inhibitor to a crop will increase the risk of resistance development. On labels, therefore, it will state that a second ACCase inhibitor should only be used to control different weeds at a different timing. For example, in cereals one application could be made in the autumn to control black-grass, and a second in the spring to control either Italian rye-grass and/or wild oats.

What will the restrictions mean for the use of herbicides containing ALS inhibitors?

Most products containing ALS inhibitors are already restricted to one application per crop. The exceptions are where there is a recommendation for a split application e.g. propoxycarbazone-sodium, sulfosulfuron. If the approval holder can provide a valid resistance risk assessment and demonstrate that these active substances are applied at a dose which does not exert a selection pressure on high resistance risk grass weeds then this use could be re-instated. The approval holder should discuss this in more detail with PSD.

Will existing sequences of ALS inhibitors be affected?

No. There are currently no approved sequences of ALS inhibitor herbicides with claims for the control of grass weeds so the proposed restrictions will not affect the status quo.

Why are the restrictions different for ACCase and ALS inhibitor herbicides?

Greater restriction has been placed on the ALS inhibitor group of herbicides for two main reasons. Firstly there is already widespread resistance to the ACCase inhibitors whereas resistance to the ALS inhibitors is not as common. The ALS inhibitors are viewed as an important group of herbicides so preserving their activity is a priority, especially in light of the limited number of herbicide options available. Restrictions in their use have been informed by the lessons learnt from the development of ACCase resistance. Secondly there are some differences in the resistance profiles of the three sub-groups of activity within the ACCase inhibitors (‘fops’, ‘dims’ and dens’). This does not appear to be the case with ALS inhibitors with grass weed activity.

If a crop has been treated with a restricted herbicide and subsequently fails will another application be permitted in the re-drilled crop?

In this situation consideration will need to be given to the selection pressure applied to high resistance risk grass weeds present at the time of the first application. If an application is made and the crop subsequently fails but the field is sprayed off with glyphosate for example then any new weeds germinating will not have been selected for by the first application. This is a different situation to if the crop is re-drilled and grass weeds are present which have already been sprayed once.

What are HRAC codes?

The HRAC codes have been devised by the Herbicide Resistance Action Committee with the aim of creating a uniform classification of herbicide modes of action. Herbicides are classified alphabetically according to their target sites, modes of action, similarity of induced symptoms or chemical classes. The system itself is not based on resistance risk assessment but can be used by the farmer and/or adviser as a tool in developing a resistance management strategy. Details of the HRAC classification are available from the HRAC website http://www.plantprotection.org/HRAC/

If resistance is a concern resistance management strategies should be discussed with an adviser and priorities agreed for planning stubble management and stale seedbeds, cultivation choice and drilling date in conjunction with a herbicide strategy. The WRAG website (http://www.pesticides.gov.uk/rags.asp?id=714) provides supporting information including management guidelines, an herbicide-resistant grass-weeds audit and 10 facts about resistance

Will these restrictions ever be lifted?

If the approval holder for an affected product can provide a valid resistance risk assessment then certain uses may be re-instated. This might include data to demonstrate that the active substance involved is applied at a dose which does not exert a selection pressure on high resistance risk grass weeds The approval holder should initially discuss options in more detail with PSD. Ongoing research funded by Defra, PSD, BBSRC, levy boards and commercial companies will, of course, also continue to inform the regulatory process.

When will product labels be changed?

Amended Notices of Approvals for currently approved products containing relevant ACCase and ALS herbicides will be issued in July 2006. Labels for all new stocks of product must be amended as necessary within 12 months of the date of this Notice.

What if I have previously approved product in stock without the new label restrictions?

This should be used in accordance with the revised restrictions.