Managing mixed grassweed populations with herbicide resistance

Grassweeds are an increasing problem in Irish cereal farming due to their competitiveness and ability to develop resistance. This can happen in as little as three years of consecutive use of ACCase or ALS herbicides.

In this article, I present a case study of a commercial farm with a mix of resistant grassweeds identified from a nationwide survey. The challenge is to examine existing farmer practices and develop an integrated weed management (IWM) plan at field level to control or eliminate these resistant species. There may also be other farms with a similar problem and there will be more in the future. This case study provides valuable learnings for growers, agronomists/advisers and industry, to help cope with this increasing challenge.

Multiple grassweeds with resistance: a case study

The following case study examines a farm involving three fields, each with a mix of resistant grassweeds. The farm profile, existing control tactics and weed status (ie resistance status and population pressure) are described, followed by suggestions regarding optimal field-specific IWM measures.

Farm profile

The farm is a plough-based operation in the east of the country, with over 80ha of tillage on a clay soil type. Typical crop rotations were winter cereals with a cereal break crop or alternate winter and spring cereals.

It has a mix of grassweeds, including bromes, spring wild oats, Italian ryegrass and blackgrass persisting for more than four years in most fields, but confined mostly to headlands. The grower rated the grassweed pressure on the farm as medium to high.

Tactics to-date

The grower was asked to select all grassweed control strategies that were implemented on the farm from a list of 20 IWM strategies (see Table 1). These were developed for effective grassweed management and resistance prevention. Only seven of the 20 IWM strategies were adopted on the whole farm.

The farmer relied heavily on chemical control, with little regard for cultural control. However, reduced rates of chemicals may also have hampered good control.

Sensitivity status

Glasshouse screens identified that the blackgrass was resistant to ACCase/ALS herbicides; Italian ryegrass was resistant to ALS herbicides only and; great brome had some form of tolerance or reduced sensitivity to ALS-Pacifica only (Table 2).

Sterile brome and spring wild oats were well controlled by all herbicides. Remember, plants were sprayed on time at the two- to four-leaf stage.

Pre-harvest weed pressure assessed over two years in three fields

Figure 1A shows winter wheat grown in the same field for two consecutive years and sprayed with ALS-Pacifica each year for Italian ryegrass control.

However, the ALS-resistant Italian ryegrass was virtually unaffected, resulting in higher population pressure as indicated by the weed score for both years, particularly on the headland. The use of pre-harvest and pre-sowing glyphosate totally eliminated the lower levels of scutch by 2021.

Figure 1B shows winter wheat treated with ALS-Pacifica in 2020 for blackgrass and Italian ryegrass. The ACCase/ALS-resistant blackgrass and ALS-resistant Italian ryegrass in the headland and in-field were virtually unaffected.

Moving to spring barley and using ACCase-Axial in 2021 almost eliminated ALS-resistant Italian ryegrass, but had no effect on ACCase/ALS-resistant blackgrass.

Figure 1C shows the most complex field of all due to multiple grassweed challenges, including ALS-tolerant great brome, ALS-resistant Italian ryegrass, ACCase/ALS-sensitive sterile brome and ACCase/ALS-sensitive spring wild oats.

The lack of herbicide options in a cereal break crop in 2020 caused a higher population pressure of all of these critical grassweeds.

Moving to winter wheat and using ACCase-Axial in 2021 almost eliminated ALS-resistant Italian ryegrass in the headland and in-field. This method, however, did not reduce the population pressure of bromes or spring wild oats, possibly because:

Looking at all current actions, coupled with the knowledge of grassweed resistance and population pressure, the following advice offers an effective field-specific IWM plan.

Field 1: ALS-resistant Italian ryegrass and scutch

This technique, followed by spring cropping, would effectively control remaining populations.

• Remember, Italian ryegrass has the ability to quickly develop resistance to alternative modes of action. Resistance risk is the same for products that have similar modes of action, therefore it is necessary to protect ACCase herbicides by using them solely for critical use.
• Use higher than normal seed rates to increase the competitiveness of the crop. In all cases, use certified seed to prevent further infestation.
• Full recommended rates and optimum timing: When used at the correct plant growth stage (ideally at the two- to three-leaf stage or latest timing before crop GS30), the recommended field rate – in this case the ACCase herbicides – will give good control of the target susceptible grassweeds, including Italian ryegrass.

Remember, post-emergent herbicides should be sprayed on the dry leaves of small, actively growing plants for maximum efficacy.

• Crop walking before and after herbicide applications helps determine herbicide performance. As different fields present different challenges, using weed maps will help monitor areas and record specific field information each year to carry out the necessary interventions (eg spot spraying) required within fields.
• Hand rogueing: Walking fields to remove small infestations will prevent seed return and prevent a small patch from spreading.
• Pre-harvest glyphosate: Thisbis primarily targeted to kill scutch, which propagates mainly by rhizomes. But this technique will not destroy seeds that have already set, so glyphosate must be used pre-sowing to eliminate populations of plants that emerge from newly shed seeds.
• Machinery hygiene: It is essential to clean combines and other machinery before coming into your fields and again before moving from field to field, as each field has a unique challenge.

It is important to practice all IWM measures to reduce the seed bank, prevent seed return and ultimately eliminate the ALS-resistant Italian ryegrass populations from that field.

Field 2: ALS-resistant Italian ryegrass and ACCase/ALS-resistant blackgrass

The cultural/non-chemical IWM plan for field one would effectively eliminate the lower levels of ALS-resistant Italian ryegrass populations found in this field. But this tactic will not work for ACCase/ALS-resistant blackgrass.

In any case, adopt a zero-tolerance approach to eliminate resistant blackgrass rather than managing these populations.

Once target-site resistance is present in a population, it does not disappear, even if the herbicide selection pressure is removed.

UK trials have shown that herbicide mixtures, rotation, or even non-chemical IWM, will only have a small impact on well-developed resistant blackgrass. Therefore, preventing seed return is critical. A strong recommendation is to put this field into grass leys/fallows for a minimum of five years to eliminate the soil seedbank of blackgrass.

If reseeding to grass is not an option, then a whole range of things should be considered. These include:

• Whole cropping or spraying patches with glyphosate (at least 3l/ha).
• Stale seedbed technique for year one (ie a year without a crop) then heading into winter oilseed rape to use metazachlor-based products (eg Katamaran) followed by propyzamide (eg Kerb) and clethodim (Centurion Max) for blackgrass control.

Other effective techniques that must be used simultaneously include:

• Good deep ploughing followed by a few years of minimum cultivation.
• Always use certified seed.
• Use competitive varieties and higher seeding rates.
• Multi-year spring cropping.
• Use of pre-emergence herbicides (eg pendimethalin, flufenacet or tri-allate).
• Spot spray patches with glyphosate.
• Hand rogueing.
• Machinery hygiene.

These are all critical to help control ACCase/ALS-resistant blackgrass populations.

In particular, this field should be harvested last to prevent resistant blackgrass from spreading to other fields.

Field 3: A mix of sensitive sterile brome and spring wild oats, plus ALS-resistant Italian ryegrass and ALS-tolerant great brome

The control options here are also quite challenging and require a lot of cultural and rotational elements to help supplement the limited chemical arsenal.

• Stale seedbed and pre-sowing glyphosate use (at least 1.5l/ha): Would be an effective option to eliminate the emerged plants of bromes and Italian ryegrass.
• Good-quality ploughing: Consistent depth, uniform furrow width and the complete burial of the surface seedbank of bromes and Italian ryegrass to a depth from which they will not emerge is critical. Remember, spring wild oats can proliferate across all establishment systems.
• Include non-cereal break crops (eg beans-SW-SB-WOSR-WW-WB) in the rotation.

Non-cereal break crops provide an opportunity to use ACCase-Falcon or Stratos Ultra to control all critical grassweeds found in this field.

In winter cereals, use pre-emergence herbicides (eg pendimethalin- or flufenacet-based products) to control bromes and Italian ryegrass. Remember, autumn-applied pre-emergence herbicides are unlikely to provide spring wild oat control.

• ACCase-Axial in wheat and barley crops (spring/winter) offers spring wild oat and Italian ryegrass control.
• ALS herbicides (eg Broadway Star) in winter wheat can be used for bromes and spring wild oat control, but only after eliminating the few remaining ALS-resistant Italian ryegrass plants using cultural/non-chemical IWM.

Alternatively, mowing field margins to prevent seed return can be helpful, but only where there are other grasses present to compete with the brome.

As with the other fields, hand rogue or spot spray small infestations. Machinery hygiene is always a basic requirement.