Growers who have blackgrass have found the weed very difficult to control.

In previous articles in the Irish Farmers Journal over the past few weeks, we have outlined the cultural control methods and farmer experiences in trying to control blackgrass.

Where these growers continue to grow winter cereals and use herbicides with the same modes of action, they will inevitably select for resistant blackgrass.

The frightening fact is that these practices may result in the selection of herbicide-resistant populations in as little as three to four years.

The first few heads of blackgrass in your field are the critical ones to find and remove.

In this article, I will concentrate on the herbicides which give control of blackgrass and look at the management of herbicide-resistant blackgrass. Be in no doubt that the solution to blackgrass includes many different actions and herbicides cannot be solely relied upon.

Herbicide-susceptible blackgrass

Herbicide options for controlling herbicide-susceptible blackgrass (Table 1) include:

  • Glyphosate use before sowing the crop.
  • Residual herbicides, usually pendimethalin- or flufenacet-based products, used either pre-emergence or very early post-emergence of the crop and blackgrass.
  • Post-emergence herbicides, usually from the ACCase (eg Falcon) or ALS (eg Pacifica Plus) modes of action groups, used ideally when blackgrass is at the two- to three-leaf stage, or latest timing before crop growth stage GS30.
  • Resistance risk

    ACCase and ALS herbicides pose a very high resistance risk in blackgrass.

    The main mechanism of ACCase/ALS resistance in blackgrass is either target-site resistance (TSR), where the herbicide target enzyme is mutated to block the activity of herbicides from ACCase or ALS modes of action, or non-target-site resistance (NTSR), where the target plants are able to detoxify the ACCase or ALS herbicides before it reaches the target-site.

    In Ireland, we have a number of populations that have been in the country for many years that can still be effectively controlled with post-emergence herbicides.

    Glasshouse trials in 2020-21 showed that control lev els of 99% could still be achieved on 10 out of 18 blackgrass populations using high resistance risk herbicides, ACCase (Stratos Ultra) or ALS (Pacifica Plus), when applied at the correct plant growth stage (two- to three-leaf stage).

    Dealing with resistance

    In contrast, five populations (R2 to R6) collected across the country were resistant to both ACCase and ALS herbicides and three populations (R1, R7 and R8) were resistant to ACCase herbicides only (see Table 2).

    Target site resistance (TSR) (single or stacked resistance) is the main mechanism of ACCase/ALS resistance in blackgrass, although non-target-site resistance (NTSR) is also likely in some populations.

    Blackgrass plants (two- to three-leaf stage) of selected populations (R1 to R4) having single or double TSR were subsequently sprayed with herbicide rates ranging from 0.25 to eight times the recommended field rate of Falcon, Pacifica Plus or Roundup to determine resistance severity.

    Figure 1: Symptoms of susceptible (S2) and resistant (R1 to R4) populations of blackgrass following application of ACCase-Falcon (A), ALS-Pacifica Plus (B) and EPSPS-Roundup biactive (C) at dose rates from 0.25 to eight times the recommended label rates (highlighted in red).

  • Figure 1A shows that Falcon was virtually ineffective in all four populations.
  • Figure 1B shows that Pacifica Plus was virtually ineffective in three of the blackgrass populations.
  • Figure 1C shows that 1.5 times the recommended field rate of glyphosate was required to kill 99% of the treated plants, indicating that resistance to ACCase or ALS modes of action may influence the efficacy of glyphosate. Interestingly, even the ACCase/ALS-susceptible S2 population recorded a few survivors to recommended label rate of glyphosate.
  • The importance of knowing

    These findings highlight the importance of knowing the resistance or sensitivity status of the blackgrass population on your farm. In any case, all growers should adopt a zero-tolerance approach to prevent the introduction, establishment and spread of blackgrass, whether sensitive or resistant populations, having TSR or NTSR or both.

    Heads of blackgrass partially flowering having turned dark in colour. These could be a single plant or plants from seed shed in the previous year.

    Time to act

    Now is the right time to plan effective herbicide programmes to help manage both resistant blackgrass and minimise further resistance development. Key to all options is to map where the weeds occur. Actions include:

    1 General prevention and weed management strategies

    Herbicide programmes should be used following the implementation of cultural/non-chemical integrated weed management in combination with on-farm biosecurity measures.

    Chemical control options should start with the use of glyphosate on stubbles or stale seedbeds to kill emerged blackgrass prior to sowing.

    Use pre-emergence herbicides as they have a lower resistance risk.

    In winter oilseed rape, use a sequence of early pre-emergence treatment (metazachlor-based products) followed later by propyzamide (eg Kerb Flo) as resistance to propyzamide has not been found in blackgrass. Remember, spraying conditions, timing and tank mixes are key for efficacy.

    If additional weed flushes are expected, pre-emergence herbicides should be followed by post-emergence treatments applied to small actively growing plants. This should only be done in fields where weed pressure is low and resistance is not suspected.

  • In winter oilseed rape, use clethodim (eg Centurion Max) in sequence with propyzamide (eg Kerb Flo) as it may still offer control of small populations including ACCase-resistant or suspected ACCase-resistant strains. But note that clethodim resistance has been documented in other cross-pollinated species (eg ryegrass).
  • 2 Farms where blackgrass populations have resistance to both ACCase and ALS modes of action

    For multiple-herbicide resistant populations (eg R2 to R6 in Table 2), there are no chemical control options in any tillage crop.

    Long-term field trials in the UK have shown that herbicide mixtures or rotations, or even non-chemical weed management, will have little effect. This is because TRS does not disappear in a population, even if the herbicide selection pressure is removed. So a return to grass leys/fallows (a minimum of five years) would be needed to eliminate that soil weed seedbank.

    3 Farms where blackgrass populations have resistance to ACCase modes of action only

    For ACCase-resistant populations (eg R1, R7 and R8 in Table 2), adequate control may initially be achieved using ALS (eg Pacifica Plus) herbicides. But remember that blackgrass has the capability to quickly develop resistance to alternative modes of action.

  • If you find blackgrass in the growing crop, then prevent seed return by either hand rogueing (in small populations), burning off the crop/weed, or opt for a very early harvest (such as whole crop silage) before the plants have set seed. Practise all weed management techniques to reduce the seedbank and prevent seed return.
  • Key points

  • The total reliance on herbicides (pre- or post-) for blackgrass control is not sustainable and will result in problems on your farm.
  • Herbicide resistance is present in Irish populations and they are increasing and spreading to new regions.
  • Growers need to be extra vigilant when walking crops, as there is a likelihood that blackgrass may appear on your farm. Assume that any such population will be resistant and that it will not be controlled by a selective herbicide.
  • Get populations tested for herbicide resistance, as this is critical for developing aggressive blackgrass control strategies.