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Grass weeds and control

Problem grass weeds

There are many problematic grass weeds-hyperlink in Ireland. Wild oats, bromes, blackgrass and canary grass were prioritised by the project Operational Group for specific focus. Other grass weeds such as annual meadow grass and scutch are also important and may be looked at through the project.

There are a number of ways to control weeds such as cultural control and using herbicides. This project will demonstrate how to optimise non chemical control methods and also increase the industries knowledge on herbicide resistance and how to prevent it.

The following outlines the main points to be considered with each of the weeds

Brome grasses

Sterile brome is the most widespread weed, found with a range of fertile soils throughout Ireland.

  • About 90 % of sterile and great brome germinates from Aug to Dec; it flowers from May to Jul and shed seeds from Jul to Aug.
  • Other bromes, especially soft (relic of Irish grassland), meadow and rye germinate in the autumn, but also in early spring.
  • Sterile and great brome requires vernalisation to produce seeds and these readily germinate in darkness, while soft, meadow and rye brome seeds require a period of post-harvest ripening and light to induce germination.
  • Sterile brome
    - Can produce and average of over 200 seeds but this can range from 5- 300 seeds per plant.
    - Seeds have poor persistent in the soil.
    - Seed emergence is reduced with increasing seed burial depth (> 10 cm).
    - 5 plants per m2 can cause yield reduction of 5 %.

Black-grass

Blackgrass is become more widespread in the last three years.

  • Similar to sterile brome, about 80 % of black-grass germination occurs from Aug to Oct, and it sets seeds from May to Aug.
  • Light induce germination.
  • Plants can produce 1000 seeds but this can be much higher (up to 6000 seeds per plant) in the absence of competition.
  • There is a 70% seed decline per year.
  • Deeper burial (> 5 cm) reduce seed emergence of freshly-shed seeds.
  • 8 to 12 plants per m² can cause yield losses of 2 to 5 %.

Spring wild oats

This is the most common type of wild oat in Ireland.

  • Predominately spring germinating, but odd seedling germinate from September to May.
  • Have the ability to survive in soil for several years and thus unaffected by seed burial depth.
  • Plants set seeds from Jun to Oct and light promotes germination.
  • Most seeds emerge from the top 10 cm soil, but some emerge from greater depths, up to 15 to 25 cm.
  • A single well-tillered plant has the ability to produce up to 2,000 seeds.
  • A population of 1 plant per m2 has the potential to cause a yield loss of 1%.

Lesser canary grass

This weed has become quite common in both winter and spring crops in fields from Cork to Louth.

  • It has a protracted emergence pattern.
  • Detailed information about growth habit and biology is sparse for canary grass.
  • It has a prolific seed return.
  • Severe infections can lodge crops and make combining very difficult.

Agronomic factors increasing grass weed pressure

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

Bromes

There are 5 brome species present in tillage farms in Ireland. The species found are sterile brome, great brome, soft brome, meadow brome and rye brome. Sterile brome is the most common; it is found predominantly along the headlands at ins and outs and along hedge rows. However, they can be found throughout a field after spreading through machinery activity or other means. Hand rogueing pre-harvest or licking plants with Glyphosate is effective at controlling mild infections. Brome seeds can become viable immediately after flowering or just a few days due to its reproductive system.

Anisantha species

Sterile/ Barren Brome (Bromus sterilis)

Scientific classification:

Kingdom

Plantae

Family

Poaceae

Genus

Bromus

Species

B.sterilis

Description

Sterile brome is native and widespread in Ireland. It’s not hard to recognise as its familiar purple, drooping heads are seen towering above cereal crops and around hedges each June and July. It’s an annual grass weed which means a plant must germinate from a seed every year. Its natural habitat includes verges, field headlands and waste ground. It grows freely on waste or cultivated land on well-drained soils. It is competitive in winter wheat and winter oilseed Rape. It can cause a yield reduction in wheat of 2.4% with just 3 plants/m2. It is increasing problematic on arable land that is in continuous cereals especially winter barley where herbicides options are minimal. Brome has no auricles.

Seeds statistics;

  • Seed longevity >1 -5 years
  • Seed decline > 90% per year
  • Germination depth > Once the seed is covered up to 5cm deep
  • Flowers per plant > 4 - 10
  • Seeds per flower > 200
  • Seeds per plant > up to 2000

Key identification features

Sterile brome from a young plant has a hairy stem and leaves, it has a dense covering of hairs on the stem and leaf surface. It is easy to spot in barley crops due to its twisted slender leaf appearance; it is more difficult to distinguish from wheat crops due to the twisted leaf and slender leaf structure of the wheat plant. The key feature to look for is the hairs on the stem and leaf, a magnify glass is sometimes required to see the hairs especially on young plants. It helps to roll the leaf over your finger and hold it up to the light to see the surface of the leaf hairs.

It must be noted that all bromes have hairs so furthermore identification may be required, if possible finding the seed that the plant emerged from can be a good indicator but it is sometimes hard to differentiate between the different bromes.

The ligule is like a barcode for grass weeds, the ligule of each plant differs allowing for accurate identification. To find the ligule of a plant is quite simple, it can be seen at the stem where the leaf has the left the stem.

Great brome (Bromus diandrus)

Scientific classification

Kingdom > Plantae

Family > Poaceae

Genus > Bromus

Species > B.diandrus

Description

Great brome has been found in some areas of Ireland but is not as dominant as sterile brome. The reason partially is the fact that it is been mistaken for sterile brome in some cases. Similar to sterile, great brome is found in field margins, waste ground and roadsides. It usually prefers a sandy soil and dunes.

Cultural weed control

Cultural control includes non chemical control of weeds such as rotation, cultivation practices, clean seed, and is a vital part of Integrated Weed Management practice. A major part of an integrated weed control practice is to; stop seed set, prevent seed return, deplete the seedbank, and kill weed seedlings.

Some of the cultural control practices include:

  • Use of competitive varieties
  • Non-plough techniques
  • Rotation
  • Machine hygiene
  • Hand rogueing for small infestations
  • Stale seedbed technique - retaining spring wild oats seeds on the soil surface as long as possible in the autumn encourage germination, allows seed to ripen and avoid the development of dormancy. 

Specific cultural weed control practices for Sterile brome and black-grass are

  • Delayed sowing of winter cereals, use of competitive varieties and hybrids, higher seed rates, time of nitrogen application and hand rogueing for small infestation
  • Spring cropping
  • Inclusion of non-cereal break crops with different herbicide chemistry
  • Rotational ploughing
  • Grass ley or fallows minimum of 2-3 years to disrupt the weed seed cycle
  • Whole cropping, if market for forage
  • Establishment of perennial cover or grass margin in the field boundary, especially for sterile brome.
  • Machine hygiene
  • Stale seedbed technique - shallow cultivation immediately after harvest encourages germination of sterile brome and spray with pre-drilling glyphosate. Alternatively, a good straw cover where it is chopped and spread uniformly may provide adequate darkness and moisture triggering rapid seedling growth of sterile brome. For black-grass, in dry soil conditions after harvest, leaving soil uncultivated allow natural predation and germination of freshly shed seeds. However, in moist soil condition after harvest, shallow cultivation can provide better conditions for black-grass seed germination.
  • Use of cover crops between harvest and seeding of next crop to provide a short break in the rotation.

Current herbicide options for cereals in Irish tillage farms

Utilising all the tools available to a farmer includes using herbicides. Due to; the limited number of active ingredients available, the loss of actives due to EU legislation and the increase of herbicide resistance, this project aims to help farmers minimise the use of herbicides, while at the same time, where a herbicide application is necessary, optimise the effectiveness of the herbicide.

Examples of typical herbicide approaches for the control of grass weeds

Black-grass in winter cereals – In winter wheat or winter barley, pre-emergence or early post-emergence herbicide based on Flufenacet or Pendimethalin products. In wheat only, the initial herbicide can be followed by spring post-emergence with Meso/Iodosulfuron or Mesosulfuron + Propoxycarbazone). In oats, there are no real herbicide options.

Sterile brome in winter cereals – In winter wheat, pre-emergence or early post-emergence herbicide based on Flufenacet or Pendimethalin products followed by spring post-emergence with Meso/Iodosulfuron or Mesosulfuron + Propoxycarbazone or Pyroxsulam + Florasulam. In barley, options are more limited. There are no treatments for use in oats.

Wild oats or canary grass in winter/spring cereals – Use autumn herbicides only if they emerge in large populations. In general, leave until the spring emerges have appeared. Use Amidosulfuron + Mesosulfuron-methyl + Iodosulfuron-methyl-sodium or Fenoxaprop or Pinoxaden or Pyroxsulam + Florasulam. There are no treatments for use in oats.

Table 1 – Common Herbicides used for Grass weed control

Active ingredientsPopular productsHRAC MOA class & groupApplication(†)Weed susceptibilityCrop
WO BG SB CG
Diflufenican + Mesosulfuron-methyl + Iodosulfuron-methyl Alister Flex® PDS (F1),ALS (B) Pre   x (x)   WW
Diflufenican + Flufenacet Firebird® PDS (F1), VLCFAs (K3) Pre   x (x)   WW, WB
Flufenacet + Diflufenican + Flurtamone Vigon® VLCFAs (K3), PDS (F1) Pre   x (x)   WW, WB
Chlortoluron + Diflufenican + Pendimethalin Tower® Photosystem II (C2), PDS (F1) Pre   x (x)   WW, WB
Flufenacet + Picolinafen Pontus® VLCFAs (K3), PDS (F1) Pre     (x)   WW, WB
Pendimethalin Stomp Aqua® Microtubule inhibitors (K1) Pre   x (x) (x) W
Pendimethalin + Picolinafen Flight® Microtubule inhibitors (K1), PDS (F1) Pre     (x)   WW, WB
Pendimethalin+ Diflufenican Bulldog® Microtubule inhibitors (K1), PDS (F1) Pre   x (x)   W, B
Prosulfocarb Defy® Lipid synthesis (N) Pre   x (x)   WW, WB
Fenoxaprop Foxtrot® ACCase (A) Post x     x W, B
Pinoxaden Axial® ACCase (A) Post x x   (x) W, B
Pyroxsulam + Florasulam Broadway Star® ALS (B) Post x   x x WW
Amidosulfuron + Mesosulfuron-methyl + Iodosulfuron-methyl-sodium Pacifica Plus® ALS (B) Post x x x (x) WW
Mesosulfuron + Propoxycarbazone Monolith® ALS (B) Post x x x   WW
Sulfosulfuron Monitor® ALS (B) Post   x   (x) WW

MOA – mode of action;

Pre- pre-emergence, Post- post-emergence;

(x) - not specified on label;

WO – wild oats, BG – black-grass, SB – sterile brome, CG – canary grass;

WW – winter wheat, WB – winter barley, W- winter and spring wheat, B – winter and spring barley

Herbicide resistance

  • Herbicide resistance is defined as ‘the evolved ability of a weed population to survive a maximum dose rate of herbicide that was previously known to be lethal’.
  • Resistant weed populations may by-pass the herbicide action by two mechanisms: target-site resistance (TSR), where simple mutations prevent the herbicide from binding effectively to its site of action and non-target site resistance (NTSR), where complex multigenic changes, allow the weed to metabolically detoxify or degrade the herbicides to an extent where they are ineffective.
  • Resistance is further exacerbated by the lack of alternative herbicide types, forcing growers to repeatedly use the same active ingredients.
  • Herbicide resistance is a global problem with 500 unique resistance cases being reported, globally until 2019.
  • Herbicide Groups A (ACCase) and B (ALS) pose a very high risk of resistance.
  • NTSR has been reported to be the common type of resistance to glyphosate, and also plays a key role in resistance to ACCase and ALS inhibitors. Although, NTSR is slower to develop, NTSR resistant weeds are widespread in major tillage crops. For example, chlorotoluron and pendimethalin actives can affect NTSR to some degree.
  • Herbicide resistance can also occur through spread of resistant genes, in contrary to independent-endowing mutations.
  • Sensitivity tests are valuable to provide direction to correct management strategies