Weed Control in Grassland
Type Media Article
By Ivan Kelly, Teagasc Adviser, Galway/Clare Regional Unit
Grassland weeds are set to escalate over the next few weeks. Rushes thrived in last year’s wet summer, as they out competed grass on land with low soil fertility. Poached fields and poor grass growth also allowed the docks and thistles to multiply. The seeds left in your pasture in 2017 from the above weeds will readily germinate and spread this year if action is not taken.
Growing more grass so it can actively compete with weeds such as rushes, is one way of preventing infestation. An acidic soil does not directly favor rushes, but it will significantly inhibit grass growth, so liming low ph. soils is essential. This should be followed with an application of phosphorus and Potassium where necessary to bring your soil nutrient level to the optimum index 3 level.
Drainage, including new drains and cleaning of the existing drainage infrastructure can be an effective weed prevention measure. Identify what is the underlying cause of the waterlogged soil by digging test pits before remedial action such as field drains, sub-soiling or mole drains are considered.
The soft rush is the most common of the many rush species. It is recognised by the dense tuft of brown flowers coming from the side of the stems and a continuous white centre (pith) when the stem is peeled back
Soft rush can be controlled with MCPA or 2,4-D, applied in June or July when growth conditions are good and weather is suitable. Strong rushes should be cut and removed, with pesticide applied to the regrowth. Weed wiping with a Glyphosate product such as Roundup or Gallup, using a tractor or quad, has the advantage of the product being applied primarily to the target plant. It uses about 1/3 of the amount of pesticide, (compared to boom spraying) and produces much lower losses to waters. For farmers with Low Input Permanent Pasture in GLAS, chemical control of rushes can only be carried out by weed wiping or spot spraying. MCPA is not licenced for use with either of these application methods.
Dock seed can remain viable for over 50 years in soil, have a large root system, and are very opportunistic in terms of where they germinate. Open swards or swards after cutting facilitate light reaching the soil surface which allows germination. Best control of docks will be achieved when docks are actively growing and nutrients are actively being transported to new foliage and roots. If seed stalks are seen 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 chemical. Soil potassium levels should be maintained at index 3, as oversupply of potash favors the higher needs of the dock over grass. Rotating silage ground can also be an effective dock control strategy. Use of herbicides based on dicamba, triclopyr and fluroxypyr (e.g. Dockstar Pro, Ban Dock) will give season-long control of docks plus a wide range of common grassland weeds. Where clover is of consequence, Eagle or Prospect may be applied. If a suitable herbicide is applied to small docks after reseeding, long term control is achieved.
Creeping thistle is the most widespread and troublesome of the thistle family. It mainly spreads by creeping roots which can be meters in length but also spreads by wind-blown feathery seeds (July & August). To make matters worse, it can grow new plants from small fragments of its roots thus appearing to explode when reseeding is carried out. They cause most damage by preventing animals grazing around them. 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.
Sustainable Use Directive
All farmers that apply pesticide must be registered as a Professional User and have completed the pesticide application module. The following precautions should be taken with all pesticides:
- Read and follow the product label
- Don’t spray if rain or strong wind is forecast in the next 48 hours
- Mark out a minimum 5-metre buffer zone from any river or lake
- Containers should be triple rinsed and washings put in the sprayer
- All sprayers that are more than 5 years old having a boom width of >3m, must have passed a Pesticide Application Equipment Test by a DAFM approved inspector.
When spraying, remember to keep a record of the products used, when and where they were applied, and the concentration at which they were used. You should also record how you demonstrated Integrated Pest Management (IPM) in the overall control of weeds on the farm. You may be required to furnish this information if you are selected for a cross compliance inspection.