April 28, 2017
Handy Hints: Tackling weeds in grassland
As grass growth accelerates and spring progresses, it is the right time to assess if pernicious weed species are at problematic levels in pasture and mowing fields before it is too late treat them.
Understanding why weeds are there in the first place and how management can limit their abundance is arguably the best place to start when addressing weed issues, according to Ross Dilks, agronomist with Agrii.
He says: “The presence of weed species is usually a sign of a deficiency in the soil or management issues. For example, buttercups tend to occur where the soil is poorly drained and has a low pH.
“Docks will colonise where the sward has become poached and there are bare patches, and it is usually associated with poor drainage. Addressing the cause of the weed problems will reduce the need for expensive chemical control.”
- Test soil regularly: A broad spectrum analysis covering pH, macronutrients including phosphate, potash, magnesium and calcium, as well as micro elements such as boron, sulphur and manganese will highlight any deficiencies which could favour weed species
- Lime when necessary: Maintaining the optimum pH for grass growth will enable it to compete effectively with weeds, especially as some weed species thrive in low pH conditions
- Avoid poaching and soil compaction: Weed grasses, dock and buttercup species will rapidly colonise patches of bare soil caused by poaching and erosion and will grow well in poorly drained, wet soils
- Remove sources of weed seed: Identifying where weed seed is coming from, such as bought-in silage containing large amounts of dock, or patches of docks and thistles around slurry stores and muck heaps is the first stage to eliminating these sources. Applying slurry contaminated with dock and thistle seed provides the ideal conditions for weed establishment
METHODS OF CONTROL
Weeds can be controlled by cultural methods, such as cutting and/or grazing in limited circumstances.
Mr Dilks says: “Repeated cutting can be effective against weeds such as creeping thistle and dandelion but may prove less successful when controlling docks. Topping can also be useful if weeds have grown beyond the optimum growth stage for chemical control to allow spraying of re-growth.”
The Plant Protection Products (Sustainable Use) Regulations 2012 govern the use of all pesticide products. The regulations set down strict requirements regarding training and qualifications needed prior to the application of plant protection chemicals.
All individuals using any form of plant protection product should ensure they are suitably qualified to do so and must always adhere to the product stewardship information on the product label.
Mr Dilks offers recommendations on chemical control:
What to spray: Consult a suitably qualified agronomist regarding the recommended selective herbicide to use on the problematic weeds and the correct method of application.
When to spray: When weeds are growing actively and taking up nutrients but well before flowering. If docks have black spots or are purple it indicates the plant has ‘shut down’ so will not translocate the chemical to the roots if sprayed.
If weed growth has progressed beyond the ideal growth stage, consider topping the weeds before allowing them to re-grow and spraying.
Do not spray during drought conditions as the chemical will not be taken up in sufficient quantities by the plant leaves
Do not spray during windy conditions or when rainfall is imminent.
Points to remember:
- Using the maximum water volume rate when applying herbicides to grassland weeds will help to maximise coverage and penetration
- Refer to the label for withdrawal periods for grazed livestock
- Always use recommended products when spraying using a knapsack
- If spraying ragwort, ensure the plant has disappeared entirely before returning livestock to the field
- If weeds are already flowering, it is too late to spray
- Take extra care when spraying near watercourses; product stewardship advice will set out the minimum buffer zones necessary
Where weeds have become abundant within a grass sward, reseeding may be the only option, according to Mr Dilks…
- Always spray off the old ley with glyphosate
- If practical, include a break crop such as a brassica/cereal crop or a fallow period between grass leys to reduce problems caused by leatherjackets and frit flies
- After reseeding into a suitable seed bed, roll to create good seed to soil contact and to deter slugs
- Monitor for slugs and other pests
- The grass is well established once it is possible to pull the leaves off without pulling out the roots. At this stage, graze with sheep or young cattle to encourage tillering; this will create a dense sward and enable the grass to compete effectively with perennial and annual weed species
Where grassland is included in an agri-environment scheme, restrictions apply to methods and timing of weed control:
- Read the option prescription carefully to understand what control methods are allowable and when
- Check dates of closed seasons for operations for individual options and schemes
- Spot spraying and the use of weed wipers is sometimes allowed for the control of pernicious weeds in grassland in agri-environment schemes
- Keep records of management undertaken, dates and weather conditions in case of scheme inspections
- In certain circumstances, derogations may be issued to allow control of problematical weed infestations – contact the scheme administrator for more information
The following weed species are defined as ‘injurious’. They should be treated as a priority for weed control.
- Broadleaved dock (Rumex obtusifolius)
- Curled dock (Rumex crispus)
- Spear thistle (Cirsium vulgare)
- Creeping thistle (Cirsium arvense)
- Common ragwort (Senecio jacobaea)