High levels of weeds in grass reduce its nutritional value (chickweed has low water-soluble carbohydrates), restrict grazing areas (animals won’t graze near thistles), restrict valuable grass growth (docks proportionally reduce silage yields) and can poison animals (ragwort). Farmers should be aware of their obligation to take action against certain weeds listed in the Noxious Weeds Act. This is important for cross compliance.

In normal circumstances, low levels of weeds in pasture are of no consequence and their removal may not be cost effective. Certain grassland weeds have high levels of trace elements and, also, have environmental benefits as food sources to birds, invertebrates and small mammals. However, when weeds take over 10% to 20% of a sward, weed control becomes important and warranted.

Common weeds and tips for control

Most weeds can be controlled using herbicides in new and established crops. Table 1 lists the products available in the Irish market with associated information.

  • Rushes: The soft rush is the most common of the many rush species in this country. Improved drainage helps to reduce infested areas and is essential if any herbicide programme is to be successful. Having a vigorous grass sward and keeping animals away from sprayed areas helps to prevent new rush seeds from germinating. Soft rush can be controlled with MCPA or 2,4-D applied in June or July when growth conditions are good. Cutting and removing the rush about three weeks before spraying will give the best results. The use of a wetting agent can help improve the ability of the spray to stick to the slender rush ‘target’.
  • Dandelions: Dandelions are a perennial weed with a deep taproot. They primarily spread by seed and can reduce the overall value of the pasture if allowed to proliferate. In small amounts, MCPA or 2,4-D will keep them at bay but, where long-term control is required, the aminopyralid/fluroxypyr/dicamba/CMPP-based sprays are best if applied in the summer or early autumn.
  • Chickweed: Common chickweed is an annual weed (lives for one year) and only spreads by seed. However, it can germinate and set seed throughout the year, making it seem like a perennial weed. It is a low, ground-hugging weed and its fast growth allows it to become established quickly, especially after reseeding. Trials have shown that it can reduce both the yield and quality of silage.

    • The key to its management is to get the grass sward established rapidly to smother the chickweed (and other annuals). Late-sown and poorly established reseeds are most likely to have chickweed problems. This weed has become increasingly troublesome where pastures are even slightly poached. In old pastures, there are many excellent chemical options available but the ‘holes’ left after killing the chickweed are ideal for more seeds to germinate, thus continuing the problem.

  • Ragwort: Ragwort is potentially deadly to livestock and is listed as a noxious weed. Even though its poisonous properties are well known, ragwort remains the cause of animal deaths and sub-clinical sickness every year. Although largely unpalatable, ragwort may be eaten when green, particularly when other grazing is sparse. It becomes more palatable to animals when cut or sprayed as it releases sugars and its poisonous properties can last well into the winter in fodder.

    • The control strategy must be based on the fact that ragwort is a biennial plant (lives for two years) and that the dead sprayed plant can still harm livestock. Small amounts of ragwort can be pulled up or dug out effectively and removed to somewhere safe. For larger numbers, sprays such as MCPA, 2, 4-D, Dicamba and Forefront provide good control but measures must be taken to avoid stock eating any dying or dead ragwort plants.

  • Thistles: Creeping thistle is the most widespread and troublesome thistle. It spreads mainly by its creeping roots, which can be metres in length, but it is also spread by its wind-blown feathery seeds, mainly in July and August. To make matters worse, it can produce new plants from small fragments of its roots after cultivation, enabling a proliferation of this weed when reseeding is carried out. Yield losses of up to 15% have been recorded but most damage is caused when animal grazing around them is prevented.

    • The extensive root system of creeping thistle is its key mechanism for survival in your fields. Frequent topping can reduce the root reserves but will seldom eradicate the problem as root fragments can remain viable and dormant for years.

    Thistles emerge in the spring at different times, so topping is a useful tactic to even up the growth stages before spraying. Chemicals such as 2,4-D, MCPA and dicamba reduce top growth but do not translocate down to the roots.

    For more persistent control, use Thistlex, Pastor or Forefront, but follow-up sprays will be needed. To help get the best transfer of chemicals down to the roots, apply these products when the thistles are at the rosette stage before the seed stalk extends upwards.

    In contrast, the spear thistle only spreads by seed but it produces a lot of seeds which are designed to fly long distances in the wind. Each plant lives for two years (biennial), producing a flattened rosette of leaves in year one and then the familiar, tree-like structure in year two. Once controlled in a reseed, it is rarely a problem in grazed fields except after poaching or other sward damage.

    Topping is not effective at controlling growth in year one but it can be carried out in the second year before the seed is set. However, the plant may re-grow in the following year and require topping once again. Chemical control options are the same as for creeping thistle.

  • Docks: Docks are well adapted to survive in our grazing swards. They are perennial plants (live for many years), have a large root system, produce many hardy seeds and are opportunistic in terms of where they germinate. Light on the soil surface is a major factor in enabling a dock seed to germinate, as moisture and temperature are usually adequate. Germination is most likely in open swards or after cutting. In silage fields, a possible over-supply of potassium for grass growth can favour the higher needs of the dock.

    • This excess potassium helps the dock root thrive and survive repeated cuttings. Research in Johnstown Castle has shown that dock seeds do not survive being ensiled (due to a low pH) but they can survive the passage through an animal’s intestine, so the origin of the fodder and how it was preserved may also be important for the spread of docks.

    Chemical control

    Best control of docks in an established sward will be achieved in good growing conditions, when docks are actively growing and when nutrients are actively being transported in the plant. If seed stalks are visible on the plant, or if the dock has diseased leaves or is under pest attack, it is better to cut/top or graze and allow re-growth of the docks before applying a chemical. Do not apply chemicals in a period of drought as the chemical will not be taken up by the plant leaves in sufficient quantities.

    Use the highest water rates on the manufacturer’s label for best effects. Dock seeds readily germinate and establish in reseeded leys due to a lack of competition from the establishing grass seedlings.

    Dock seedlings are far less likely to become established in established grass swards due competition from the sward itself.

    A well-targeted post emergence spray, following reseeding, can give effective control of docks that will persist for at least four years and should be a considered an essential component of the reseeding operation (Figure 1). The efficacy of this spray is due to it being applied onto susceptible dock seedlings and, thereafter, the competitive grass ley prevents new dock seedlings from establishing.

    Where docks are established in mature grassland, Forefront offers the best control option. Some regeneration from rootstocks can occur and repeat applications may be necessary.

    Where clover is important, the clover-safe options for post-emergence application following reseeding offer greater and longer-lasting dock control than the clover-safe options applied to established swards.

  • It is important to know some basic details about your target weed to help with successful control.
  • The use of herbicides to control weeds must be complemented by other non-chemical actions that further help to control the same problem.
  • Farmers applying more than 20 litres of chemical themselves must register with the Pesticide Control Service.
  • Farmers using any pesticide must keep a record which accounts for the product used, where it was used and the amount.

    • The simple answer is ‘yes’ – the new Sustainable Use Directive (SUD) applies to grassland.

    All grassland farmers who apply pesticides, eg weed killers for docks, must be registered as pesticide users by 26 November 2015. Online registration will be possible on the PCS website (www.pcs.agriculture.gov.ie/SUD.htm) this year.

    Farmers who spray herbicides on grassland must show they are using Integrated Pest Management (IPM) techniques such as drainage for rush prevention, fertility to encourage a vigorous sward, topping to reduce rushes/thistles and to improve control. These suggestions are to serve as a prompt reminder as to what IPM means for grass farmers.

    IPM techniques simply involve reducing seed return, while encouraging the grassland to be dense and competitive.

    Farmers who wish to register must have completed an appropriate Pesticide Application course – full list on PCS website. Proof of your training will be requested in a cross-compliance inspection.

    Sprayers: All sprayers with a boom width greater than 3m and all blast and orchard sprayers must be tested at least once by 26 November 2016. Spray contractors must be registered also.

    Spray records: You must keep records of pesticide use as per the example in Table 2. These forms can be accessed on the PCS website or you can keep these details in your own format. Again, these will be inspected during a cross compliance inspection. Table 2 shows an example of the details required by professional users in 2014 for DAFM records.