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.
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:
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.
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.
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.
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.