Managing herbicide-resistant wild oats and less-sensitive bromes

In Ireland, wild oats and bromes are the two most frequently occurring grassweed species in cereal crops.

Due to the traditional level of spring cereal cropping, fields generally have spring wild oats, but occasionally fields can have a mixed population of spring and winter wild oats.

Among the five types of bromes, sterile brome is the most common and is found mainly at the base of ditches, headlands, field margins as well as within crop fields. Other types like great, soft, meadow and rye bromes are also found locally in many fields.

Both wild oats and bromes are predominantly self-pollinating and so herbicide resistance is relatively slow to develop compared to outcrossing blackgrass or Italian ryegrass. In recent years, we have identified a number of fields where there is difficulty controlling wild oats and bromes effectively with herbicides.

In this article, I discuss the chemical options available and the type of resistance that is evolving to spring post-emergence ACCase/ALS grassweed herbicides. I also outline integrated weed management (IWM) options for wild oat and brome control. This knowledge is critical to improve our ability to control suspect populations, and to prevent resistance development.

Chemical options

There are a number of ways that we can tackle these grassweeds, both within and outside of the crop to be grown:

Remember, tank-mixed residual herbicides (eg Firebird + Defy) or stacked products (eg Firebird Met containing DFF + flufenacet + metribuzin) will be most effective and faster-acting than single actives in controlling critical grassweeds. Also, pre-emergence herbicides applied in autumn are unlikely to provide spring wild oats control.

Herbicide resistance and types

Our nationwide grassweed survey has shown that fields with high infestations of wild oats or bromes are more likely to develop herbicide resistance or be less sensitive to herbicides.

From the 100 field populations of wild oats screened to-date, herbicide resistance has been found in 15% of populations, with plant survival from full-rate herbicide applications varying from 25% to 100%. These samples were collected in Wexford, Kilkenny, Cork, Tipperary and Kildare.

Those identified as resistant fell into three different categories (Table 1):

These results suggest that herbicides from the same group can behave differently in the same field.

The main mechanism of ACCase resistance is target-site resistance (TSR), with non-target-site resistance (NTSR) being partial or at an early stage.

However, ALS herbicides were highly active on all populations tested, including ACCase-resistant populations, when used at the correct plant growth stage (three- to four-leaf).

With over 100 field populations of bromes tested, only a few populations survived the full-field rate of ALS-Pacifica Plus, with plant survival ranging from 10% to 24%.

There is, however, widespread tolerance among brome types (including sterile, great and soft), collected in the Louth/Wexford/Cork triangle, to half-rates of ACCase-Stratos Ultra and/or ALS-Pacifica Plus, with plant survival from 10% to 50%+, which may be due to repetitive use of reduced field rates.

Figure 2 shows four sterile brome populations and one great brome population (collected from the triangle mentioned previously) surviving both half- and full-rate of ALS-Pacifica Plus. A further 10 populations survived a half-rate of ACCase-Stratos Ultra, with another 20 populations surving half-rate of ALS-Pacifica.

It is possible that non-target-site resistance (NTSR) might be the cause of the low-level ALS-resistance.

Nevertheless, ACCase-Falcon (half- and full-rate) and Stratos Ultra (full-rate only) as well as ALS-Broadway Star (half- and full-rate) herbicides were highly effective on all brome types, including less sensitive ones, when used at the correct plant growth stage (two- to three-leaf).

Managing wild oat and brome populations should utilise chemical control along with cultural/non-chemical methods, rather than relying exclusively on herbicides for control.

Integrated weed management (IWM) options include a range of practices that ultimately disadvantage the weed species and prevent or reduce seed return.

Map infected areas in fields in May/June to guide further actions.

Hand rogue small early-stage infestations or spray off distinct patches before seeds set, especially on headlands for bromes, to prevent seed return.

If resistant wild oats are widespread across fields, whole crop (cutting, baling and removing affected straw) or spray off with glyphosate before seeds start shedding to prevent seed return.

As many sterile brome infestations originate from field margins, establishing a perennial cover or grass margin (Cocksfoot mix and/or Timothy) will provide competition for the grass weeds to slow their growth and reduce seed return.

Such field margins should be mown in the following May and September to encourage grass tillering and prevent the return of brome seeds.

High levels of machinery hygiene are essential to prevent field-to-field spread of weed seeds.

For wild oats, delaying post-harvest cultivations for as long as possible encourages natural losses (predation) of freshly-shed seeds.

Different post-harvest treatments apply to the different bromes, hence correct identification is key. This is critical in direct drill or strip-till situations.

Wild oats are somewhat different because they can proliferate across all crop establishment systems.

Ploughing helps preserve their dormancy as well as bringing previously buried seeds to the surface. However, spring crops established after direct drilling may reduce wild oat infestations, provided emerged plants are destroyed by glyphosate pre-sowing.

Remember, stale seedbeds in autumn, delayed autumn-drilling or pre-emergence herbicides applied in autumn will not guarantee successful spring wild oats control. However these measures increase crop competitiveness.

Use of other IWM measures (eg planting of more competitive crops/varieties, use of higher than normal seeding rate, etc) are also necessary to reduce the weed seed bank.

Ploughing to 15cm or more is key to preventing brome re-emergence from below their emergence depth.

Relative to other weeds, repeated ploughing may carry less risk of bringing up viable brome seeds.

Delayed sowing until the second half of October will help avoid the main autumn flush of bromes.

This technique, followed by multi-year spring cropping, would give effective control in fields with large infestations of sterile and great bromes, as it avoids the peak germination period in autumn.

Chemical control

The inclusion of non-cereal break crops like beans and oilseed rape provides an opportunity to use propyzamide (eg Kerb) followed by ACCase-Falcon, Stratos Ultra or Centurion Max at full-field rate to control sterile and great bromes. Avoid having to spray older or larger brome plants that are well past the optimum stage for control, and avoid spraying under stressful conditions. Always conduct resistance testing to determine if your wild oat population is susceptible to ACCase graminicides.

Farms where wild oat populations have confirmed or suspected resistance to ACCase modes of action:

Farms where brome populations have less sensitivity to ALS or ACCase modes of action:

*Dr Vijaya Bhaskar AV is a research officer specialising in grass weeds at Teagasc Oak Park.