Herbicides are the most efficient and cost-effective way to control grass weeds. Their use will remain vital to crop production alongside cultural/non-chemical integrated weed management tools.

These actions are also necessary because grass weeds develop resistance to key ACCase (eg Axial, Falcon, Stratos Ultra) and ALS (eg Pacifica Plus, Broadway Star) modes of action.

Grass weed herbicide resistance is mainly influenced by the following factors:

In this article, I summarise the results of the efficacy of ACCase, ALS and glyphosate herbicides to assist an integrated weed management and resistance management plan tailored for targeted grass weeds and individual growers. The increasing populations of difficult-to-control grass weeds in tillage fields make this information critical.

Spring wild oats and sterile brome

These are the most widespread and competitive species in cereal crops.

With over 100 field populations of wild oats screened to date, ACCase resistance or cross-resistance has been detected in 15% of populations.

But there was no evidence of poor control from ALS herbicides at both rates (Figure 1) when used at correct plant growth (three- to four-leaf) stage.

Of the more than 100 field populations of brome tested, there were only a few populations with full ALS-resistance. But there is widespread tolerance to ALS-Pacifica and/or ACCase-Stratos Ultra at half rate, which may be a consequence of the frequent use of reduced field rates (Figure 2).

Remember, Axial does not offer brome control so there are no effective chemical options in winter barley. For this reason, it is necessary to use at least 1.5l/ha of glyphosate before sowing to reduce population size.

Additionally, ALS-Broadway Star and ACCase-Falcon were highly active on all populations at half or full rate when used at correct plant growth (two- to three-leaf) stage. This makes these actives important tools in eliminating less-sensitive bromes within crops.

As a rule, unless a field has a small population with high herbicide susceptibility, it is always recommended to use full rate of effective herbicides to control self-pollinating wild oats and bromes.

Growers are facing a significant challenge when managing blackgrass. Nearly 50% of field populations tested to date were multiple-resistant, showing resistance to ACCase-Falcon and/or Stratos Ultra, as well as to ALS-Pacifica or Monolith (Figure 3).

But ACCase-Centurion Max was found to offer above 90% control of resistant populations (Figure 4), when used at correct plant growth (two- to three-leaf) stage, making it an important tool to eliminate resistant blackgrass in non-cereal break crops (eg winter oilseed rape).

Full recommended field rate

As a rule, full recommended field rate of effective herbicides must always be used to control outcrossing blackgrass populations (whether sensitive or resistant).

In addition, at least 3.0 l/ha of glyphosate was necessary to effectively control both sensitive and resistant blackgrass (Figure 5).

Despite the fact that Italian ryegrass is only present on a small number of tillage farms, nearly 60% of populations tested had cross-resistance to all ALS herbicides, and a few of these populations also carried resistance to ACCase herbicides (Figure 6).

But ACCase-Stratos Ultra or Centurion Max at full rate, and glyphosate using at least 1.5 l/ha, were highly effective in eliminating resistant populations when used at correct plant growth (two- to three-leaf) stage.

As a rule, the full recommended field rate of effective herbicides must always be used to control outcrossing Italian ryegrass populations (whether sensitive or resistant).

We primarily rely on several pre-emergent herbicides (eg pendimethalin, flufenacet, prosulfocarb, etc.) to control annual meadow grass but our research found the first case of cross-resistance to ALS herbicide (Pacifica and Broadway Star) types in Ireland (Figure 7).

This is the second case of resistance in tillage situations in Europe (the first one being in France), so increased vigilance in identifying and mitigating resistant populations is essential. But ACCase-Centurion Max at full rate and glyphosate (at least 1.5l/ha) provided over 95% control, when used at correct plant growth (two- to three-leaf) stage (Figure 7).

Annual meadow-grass appears naturally tolerant to the most commonly used ACCase herbicide (eg Axial, Falcon) types.

This is a relatively new grass weed in Ireland and not usually listed on chemical labels. Initial screening indicates that this weed is naturally tolerant to ACCase herbicides (Figure 8). But ALS-Pacifica and Broadway Star and glyphosate were highly effective, when used at correct plant growth (two- to three-leaf) stage.

It is recommended to use ALS herbicides at full rate, and glyphosate at least 3.0 l/ha for maximum efficacy and for preserving existing chemical options.

This weed is not listed on chemical labels (ACCase-Foxtrot offers control of awned canary grass).

Initial screening found that ACCase, ALS and glyphosate herbicides effectively controlled populations but only when applied at full rate (Figure 9).

In addition to pre-emergent herbicide options (eg pendimethalin, flufenacet and prosulfocarb), initial screening found that all ACCase, ALS and glyphosate herbicides gave total control, specifically at full rates.

Overall, herbicide cross- and multiple-resistance has severely limited our ability to effectively control critical grass weeds.

Remember, these results were obtained under ‘optimal conditions’ in a controlled glasshouse rather than in the field, which could further affect product efficacy.

An integrated weed management approach will help preserve existing chemicals that are still effective (see Table 1). Some of the key strategies include: