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New strategies to maintain autumn grass weed control in cereals and oilseed

Effective grass-weed control is essential if rotations comprising mainly autumn sown crops are to be maintained.  HGCA Research Review 70 has identified that loss of key herbicides under the current or revised 91/414/EEC and/or the Water Framework Directive (WFD) could have significant agronomic and financial impacts on the production of cereals and oilseed rape. Estimates suggest that losses could be in excess of £500 million per annum as a consequence of less effective control of black-grass. In addition, there are increased levels of grass weed resistance to herbicides, detection of some key herbicides in water and no new modes of action anticipated in, at least, the medium term.  This means that the immediate problem the industry faces is one of retaining cost-effective grass weed control using only a few remaining herbicides, which individually do not provide sufficient control. 

An HGCA funded research project being carried out by TAG and SAC is evaluating new strategies to maintain autumn grass-weed (black-grass, sterile brome and annual meadow-grass) control in cereal and oilseed rape crops. The research is also seeking to avoid sustained reliance on approaches that are likely to exacerbate resistance and pollution issues. To this end a series of around 40 field experiments are being undertaken over a four year period. This independent research will utilise a wide array of actives and assess novel combinations using the full range of options available from all manufacturers. In order to maximise flexibility, as well as robustness and efficacy, research will seek to develop a series of approaches that are not just restricted to the most commonly used active ingredients.

The control of black-grass in cereal crops

• Research from the first two years of this project has shown that stacking (i.e. several products applied together) and sequencing (i.e. several products applied in close succession) of products is most likely to be effective at low to moderate black-grass populations in winter wheat or barley (Figure 1). This research is identifying promising approaches that will help to preserve and manage herbicide control strategies.
  
• Table 1 outlines some of the broad responses in the level of control achieved during the last two seasons of research across a range of products and programmes.  In all these scenarios a commercial standard treatment of Crystal (flufenacet + pendimethalin) followed by Atlantis (mesosulfuron-methyl + iodosulfuron-methyl-sodium) gave in excess of 95% control; however, resistance to Atlantis is increasing and programmes not including this product need to be developed.  Where a range of stack and sequence approaches not including Atlantis were used, control in excess of 95% could only be achieved at lower black-grass populations.  This will have implications on the requirements for other grass weed control management practices in scenarios where black-grass populations are likely to be higher.
  
• Within these approaches several of the more effective programmes have demonstrated a strong reliance on a limited number of key active ingredients.  There is some suggestion that these actives themselves may also be under some threat unless they are appropriately managed e.g. pendimethalin (at risk of revocation through the revision of 91/414/EEC) and flufenacet (laboratory studies suggesting potential for resistance development).
  
• The most successful stacking and sequencing strategies for the control of black-grass, in all scenarios, needed 3-5 active ingredients (and all featured flufenacet). 
  
• In general there was little difference between stacking or sequencing of a given series of products; this is valuable as it will allow flexibly without compromising efficacy (Figure 2).
  

Table 1.  The reduction in black-grass head numbers achieved in winter wheat, across a series of experiments, from a range of products and programmes (e.g. single herbicides, mixes, stacks or sequences that did not include Atlantis) during the first two years of research project 3341. 

The control of annual meadow grass in cereal crops

• Results to date have demonstrated that a range of products and programmes have the potential to be satisfactory replacements for IPU and trifluralin.
  
• Experiments carried out within research project 3341 in both Norfolk and Midlothian have indicated that reductions in ground cover providing in excess of 90% control of annual meadow grass can be achieved through a range of approaches;
  
  - Pre-emergence approaches; while specific levels of control varied in relation to product and experiment, strategies based on flufenacet (e.g. Crystal or Liberator), prosulfocarb (Defy) or pendimethalin (e.g. Stomp or Flight) were generally very effective.  Research to date suggests that many of these approaches can also demonstrate valuable levels of dose flexibility such that applications of a half full dose, or less in some situations, can provide effective control.
  
  - Post emergence approaches; a range of post emergence options have also provided effective control, for example products that could be used in an early post emergence situation (e.g. chlorotoluron) or products that could be used later in the season such as the sulfonyl urea based product Othello (diflufenican + iodosulfuron-methyl-sodium + mesosulfuron-methyl).
  
  - In addition to annual meadow grass control many of the approaches examined within this research would also provide useful control of a range of broadleaved weeds.

The control of sterile brome in cereal crops

• Experiments considering sterile brome control strategies are being undertaken in winter wheat and winter barley.  In addition to examining novel stacking and sequencing approaches with existing products, research has also considered new active ingredients and existing active ingredients (or combinations of active ingredients) that may not currently be approved for use in a specific crop / weed situation.
  
• Research within this programme has suggested that in high populations a pre or early post-emergence treatments alone gave poorer control compared to the use of a sequence, however, in lower populations research is suggesting that there may be more scope for stack and sequence approaches to provide effective control.  For example from winter wheat studies carried out in Scotland as part of research project 3341;
  
  - In a high population; where untreated sterile brome levels were in excess of 750 heads m^-2, Avadex (tri-allate) when used alone gave no control (in terms of a reduction in head numbers); while Atlantis (+ Biopower), used alone, gave a reduction of around 35%; however when the two products were used in sequence a 70% reduction in head number was achieved.
  
  - In a low population; where untreated sterile brome levels were only around 27 heads m-2 Liberator (flufenacet + diflufenican) reduced the head number to 7 heads m^-2; Liberator + Avadex to 4 heads m^-2; while a Liberator + Avadex followed by chlorotoluron sequence reduced this to less than 2 heads m^-2.
  
  
• In a similar manner to that outlined for black-grass in cereals, this could have implications on the requirements for other grass weed control management practices in scenarios where populations are likely to be higher.

The control of black-grass in oilseed rape crops

• The current choice of herbicides or mix partners for grass-weed control at the pre-emergence timing, where trifluralin was traditionally used, is becoming increasingly limited.  Equally, there is mounting concern over other active ingredients, such as metazachlor (e.g. Butisan), carbetamide (e.g. Crawler) and propyzamide (e.g. Kerb) that are being found in water.  Carbetamide and propyzamide are not affected by resistance in black-grass and both are of paramount importance for within crop and rotational control of the weed.
  
• Research within project 3341 has been addressing suitable replacement strategies to account for the loss of trifluralin while also investigating utilisation techniques for carbetamide and propyzamide that could maintain efficacy without exacerbating potential pollution risks.  Research is also investigating a series of novel products for use in oilseed rape; for example materials that have proven to be promising in other brassica crops.
  
• Results obtained over the last 2 years suggest that;
  
  - There is some promise with older chemistry, such as napropamide (e.g. Devrinol) and tri-allate (e.g. Avadex) that could be used to replace or support pre or early post emergence residual options. 
  - The performance of both carbetamide to propyzamide can be influenced by dose and timing and a reduced dose at a timing conducive to good efficacy may be as effective as a higher dose at a timing that is less favourable.
  - The more effective control programmes have generally required a sequence based approach utilising either carbetamide to propyzamide.
  - A selection of responses are as outlined in Figure 3.

Figure 1. The effect of treatment on the number of black-grass heads m^-2 in winter wheat
These treatments are a selection of those used within the full research programme

Timings - pre-em / peri-em / 1 leaf / 2-3 leaf
All products used at full label rate
The use of treatments within these experiments does not constitite specific guidance.

Figure 2. The effects of stacks or sequences on the number of black-grass heads m² in winter wheat
These treatments are a selection of those used within the full research programme

Timings - pre-em / peri-em / 1 leaf
All products used at full label rate
The use of treatments within htese experiments does not constitute specific guidance

Figure 3. The effect of treatment on the number of blackgrass plants m^-2 in winter oilseed rape.
These treatments are a selection of those used within the full research programme.

The use of treatments within htese experiments does not constitute specific guidance