Grassweed Control

Good weed control is vital if farmers are to produce high yielding, good quality crops. Weeds compete for nutrients, light and water and if left uncontrolled will cause problems not just in the current crop but also in subsequent crops

There are a number of agronomic factors which can increase the grass weed challenges on farm.

These include:

• Continuous cereal cropping or insufficient crop diversification
• Earlier sowing of autumn-sown cereal crops
• Lack of harvest and cultivation machinery hygiene
• Lack of headland and field margin management
• High-dependence on chemical control
• Herbicide application at inappropriate timings
• Using reduced rates of herbicides
• Use of herbicides with the same mode of action for successive cropping seasons on the same field

Heavy reliance on herbicides for weed control has never more unsustainable. The loss of key active ingredients and the limited number of new products being brought to the market is compounded by the rise of herbicide resistance found in a number of the key grass weeds which affect arable crops. This makes the use of IPM even more important, the grass weed challenge must be managed across the length of the rotation.

• Integrated weed management
• Post harvest
• Establishment method
• Sowing
• In crop

Integrated weed management

One key way to manage grass weeds and to minimise the selection pressure for resistance, is to diversify or integrate weed management practices beyond using herbicides. Integrated weed management (IWM) combines non-herbicide or cultural techniques, which should be implemented first, with herbicide used when needed.

Components of Integrated Weed Management

IWM also targets weed seed bank reduction and prevents weed movement to different areas. IWM encourages farmers to: keep fields free of specific weeds; reduce the pressure on herbicides and prevent seed return. Correct weed identification coupled with an understanding of their agro-ecological traits is of paramount importance in devising effective IWM programmes.

• Establishing Arable Grass Margins
• Integrated Weed management to tackle grass weed problems - Vijaya Bhaska (pdf)

Post harvest

Stale seedbed

• Shallow (up to 5 cm) to encourage germination of weeds.
  • For Soft Brome, Rye brome and blackgrass, leave uncultivated for a minimum of three (3) weeks to encourage dormancy break and/or increased predation.
  • Glyphosate (at the 1 - 2 leaf stage) after weed establishment.
• Multiple stale seedbeds & glyphosate (as above).

Fallow

• Fallow fields left idle after harvest for an entire growing season.
• Glyphosate is applied to desiccate any regrowth.
• No cultivation is carried out until the following crop is due to be drilled.
• It will suit species that need light to break dormancy, g., meadow grasses, soft and rye bromes, and wild oats.
• Increased predation of seeds by insects, birds and small mammals are to be expected.

Rotation

• Plant autumn non-cereal break crops - allowing change to different herbicide mode of action.
• Plant different species types in the autumn and spring - allowing change to different herbicide mode of action.
• Sow spring non-cereal break crop - allowing multiple stale seedbed/cover crops in the autumn and change to different herbicide mode of action.
• Sow spring cereal crop - allowing stale seedbed/cover crop establishment.
• Sow short-term ley (3 to 5 years) to allow short lived weeds seeds to disappear thereby facilitating a return to annual cropping with a lower weed burden.

Establishment method

Ploughing

Ploughing should be completed to a depth of 15-20 cm to ensure optimal burial of surface trash, proving highly effective in managing sterile brome, great brome, and rat's tail fescue.

However, annual ploughing poses challenges as it mixes seeds throughout the soil profile, making weed control more challenging, particularly for wild oats, Italian ryegrass, blackgrass, and canary grass.

Non Plough based Systems

A common feature of all non-plough based systems is that they rely on dry soils for success. Delayed autumn planting is generally not a practical option in a wetter climate, thereby giving an advantage to grass weeds which prefer to germinate in early autumn.

Min-till

The cultivation can be deep (15 cm or greater) or shallow (up to 7 cm). It is usually deep to help drainage through the seedbed in wet years. The system can allow increased workloads and reduced labour input. Deep cultivations will mix weed seeds through the profile and can lead to higher levels of sterile brome (and other weeds) if not managed carefully.   

Strip-Till

This technique cultivates approximately one-third to a half of the seedbed. Strip-till planters have a narrow leading leg generally set to a depth of 8 – 12 cm, creating a drainage channel ahead of the seeding tine, which is 1.0 -2.5 cm. The system can mix seeds to different levels in the soil, but the mix is less aggressive than deep min-till systems. Bromes can be problematic as most weed seeds are left on the surface, so careful management is necessary.  

No-Till or direct drill systems

As the name suggests, the system employs no cultivation other than planting the next crop with a disc or tine machine. The system relies on rotations, cover crops and good soil management to ensure success. The system is relatively efficient and with lower costs compared to others cultivation systems but it relies heavily on glyphosate and dry soils for success. Grass weeds can build in these systems, with rat's tail fescues and some broad-leaved weeds becoming more problematic. 

Rotational ploughing (1 year in 4 to 5 years) – Non-plough based systems

This technique relies on excellent soil inversion for trash/weed seed burial. The lower weed burden in newly exposed soil (coming from depth) by the plough will have a lower seed burden of many weeds, due to rapid seed decline of many weeds e.g. bromes, blackgrass, rats tail fescue, etc.  After ploughing, continue to work the surface (using minimal soil disturbance) and avoid ploughing again for several years.  

Establish non-cereal break crop by min-till/no-till methods – Plough-based systems

Similar to rotational ploughing, the new seedbed should have a lower seed burden which can be combined with excellent grass weed control in a non-cereal break crop (alternative herbicide modes of action can be used in oilseed rape, beans, etc.). Following a break crop, the seed burden should be lower, allowing min-till/direct drill establishment.

Use narrow (normal) spaced rows (125 mm)

Crop competition helps with weed control; however, wide rows may allow inter-hoe machinery to be used to supplement herbicide control. Narrower spaced rows will increase competition and help suppress weeds, especially with wild oats and blackgrass; however, this practice should be used with other weed control practices for best 

Sowing

Delay autumn planting

Delaying planting in the autumn capitalises on the biological vulnerability of grass weeds to early-year germination, consequently minimising weed establishment. Opting to sow crops in the later slot for your region, particularly after mid-October, will decrease the germination and establishment of bromes, blackgrass, and canary grasses. However, prolonged delays in planting carry the risk of poor crop establishment, diminishing the competition against grass weeds.

Use competitive cultivars & varieties e.g. hybrids

Crop competition serves as a valuable means of suppressing grass weeds. A competitive crop exhibits vigorous growth, effectively capturing light and crowding out the grass weed. However, it's essential to note that this effect has limitations and cannot deliver adequate control when employed in isolation.

Increase seed rate by 15% to 20% above normal rates

Similar to competitive cultivars, elevating the seed rate has the potential to enhance crop competition, effectively crowding out grass weeds. Lower seed rates may result in sparser crops, providing space for grass weeds to compete. However, surpassing these recommended seed rate levels becomes cost-prohibitive, heightens the risk of lodging and disease incidence, and may lead to lower grain quality.

In crop

• Crop walking & monitoring
• Weed mapping – manually or precision
• Crop destruction – Total crop kill or removal for silage
• Hand Rogueing
• Mechanical Weeding
• Harvest
• Hygiene/Biosecurity
• Use certified seed

Crop walking & monitoring

Spotting and hand-rogueing individual grass weeds in a field represents the simplest method to prevent the development of a problematic population. Conducting crop walks from May to the end of June is crucial for all fields. Diligent observation during this period allows identification of potential resistance issues that may be emerging. It is essential to maintain detailed records noting which herbicides are effective each year. Reviewing these records before applying herbicides in the following year ensures informed decision-making.

Weed mapping – manually or precision

Mapping can be conducted during hand-rogueing. Sketch the weed populations onto a map, either online or on an old DAFM LPIS map. Note the weed type, density, and size/area in the field. This map proves valuable in subsequent years for implementing targeted actions in these areas, such as hand-rogueing, spot spraying, or reverting to grassland.

Capture ripe seed sample and test for herbicide resistance

Determining the grass weed species is crucial for implementing the correct management practices. It is also essential to be aware of the herbicide resistance status of the weed for effective management. When conducting herbicide resistance testing, focus on live plants where the appropriate herbicide was applied correctly at the label-recommended rate. For herbicide resistance testing in grass weeds, collect two cups full of ripe seeds, provide the necessary crop management information, and send them to Teagasc Oak Park, Carlow.

Crop destruction – Total crop kill or removal for silage

Preventing weed seed set and return is crucial for reducing the weed seed burden in a field. Focus on areas with a moderate to heavy weed infection and take targeted actions, such as either using a total herbicide to destroy both the crop and weed or cutting and removing the crop and weed. This intervention needs to be carried out before the seeds ripen and fall to the ground. While this approach may seem drastic and expensive, the cost of crop removal can be weighed against the potential expense of future weed control if the population spreads across the entire field or farm.

Hand Rogueing

Preventing the return of weed seeds to crops is essential for reducing the weed population. Hand-rogueing is particularly effective for small weed populations, primarily aiming to hinder the initial spread of a grass weed. Ensure the entire plant is pulled before the weed seeds fall from the head, and transport the rouged plants to an area away from the field for safe disposal.

Mechanical Weeding

This can involve either removing the weed from the ground using a tined hoe or extracting the weed seed heads above the crop. The ground-level weed removal necessitates planting the crop in wide rows, utilising specialised machinery, and requiring dry conditions during the operation and for a few days afterward to allow the weed to dry out and perish. On the other hand, removing the heads of weeds above the crop can be effective but is constrained by both the height of the crop and the specific target weed.

Harvest

A combine fitted with an additional piece of equipment can address weed seed management (although not adequately tested in Ireland). There are three main approaches:

1. The chaff from the combine is concentrated into a narrow band, facilitating composting to destroy weed seeds contained in the chaff.
2. A set of high-speed rollers is used to squash/roll the seeds, effectively destroying them.
3. All chaff expelled from the grain separator is collected before reaching the ground, aiming to collect both chaff and weed seeds. This material is then dumped in a designated area outside the field and can be composted.

However, all of these method may be limited in crops with very high biomass and these techniques are mostly used in drier countries with low yields ( e.g. Australia).

Hygiene/Biosecurity

Ensure that purchased seeds, whether cereals, oilseed rape, cover crops, etc., entering the farm are certified by a competent authority. Whenever possible, request additional assurances or proof of the highest seed standards, particularly for cover crops. It is important to prioritise harvesting and baling fields with grass weed problems last. Thoroughly clean the combine/baler before entering or leaving badly infested fields.

Prevent the return of weed contaminated organic material

Thoroughly check the origin of straw in farm yard manures to be imported.  Ideally only use straw which came from your farm e.g. swap deal with the livestock farmer. 

Machine hygiene

Ensure that newly purchased machinery or contractor's machinery is thoroughly cleaned before entering your farm. Begin by blowing down all crop material from the head/main combine, including behind all safety guards, starting from the top. Open trap doors on stone traps, elevators, etc. Subsequently, run the machine with all sieves and fans opened for at least 10 minutes. Finally, give the combine another thorough blowdown.

For balers, ensure that the chambers are empty, then blow down all material both inside and outside. Again, run the machine for 10 minutes followed by another blowdown of the entire machine.

Use certified seed

Source certified seed whenever possible for main crops (wheat, barley, oats, etc.) or cover crops. Ensure that the seed's source is known, and be familiar with the national guidelines for certified seed if it is coming from outside Ireland. If you are unsure about the seed, do not bring it onto the farm. Continue to hand-rogue the crop for grass weeds regardless of the source of the seed.

Home-saved seed with known weed-free status

When selecting seed, choose only from a known grass weed-free field. Hand-rogue the area two or three times before harvest. Ensure the combine and trailers are thoroughly blown down and cleaned to prevent contamination from other fields. Continue hand-rogueing the crop for grass weeds, regardless of the source of the seed. 

Summary of the main control options for five common grass weeds