Buttercups are a common sight in fields across the country at the moment, with dandelions and docks also common problem weeds.
How do grassland weeds affect pastures?
High levels of weeds in grass swards not only reduce pasture’s nutritional value, but restrict grazing areas and restrict valuable grass growth, particularly in new reseeds. Farmers are also obliged to control certain weeds by the Noxious Weeds Act and as part of cross-compliance obligations.
At what level of weed infestation should I take action?
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 reach density levels of 10%-20% of the total sward, they will impact on either grassland quality or productivity.
It is quiet difficult to accurately assess the damage that weeds do, except where animal death or severe sickness is the result of plant poisoning (due to ragwort etc).
Research carried out in the UK suggests that for every 1% of ground cover by docks, you can assume that pasture growth is reduced by 1%.
Certain weeds (such as thistles) discourage grazing by the nature of their prickly habit and can make hay and silage unpalatable.
What grassland management practices can be applied to reduce weed effects?
Management practices such as drainage, fertility, grazing, topping and mowing are very important when it comes to controlling weeds. All of these encourage the grass to be competitive and dense.
Although there is an abundance of weed seeds waiting to germinate, very few get the chance, as the grass stops the light reaching the buried weed seeds.
How do I control docks?
The amount of light reaching the soil surface is the main factor required for a dock seed to germinate.
Open swards or swards after cutting facilitate this. Also, in silage fields, potassium may be in over-supply for the needs of the grass, thus favouring the dock.
Teagasc recommends that soil potassium levels should be maintained at index 3 (101-150 mg/l).
This strategy will reduce the competitiveness of docks in your grassland.
Best control of docks is achieved in good growing conditions, when docks are actively growing and nutrients are actively being transported to new leaves 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 or top or graze, and allow re-growth of the docks before applying chemicals.
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.
Allow adequate time between spraying and cutting silage for the herbicide to work.
Herbicides based on aminopyralid, dicamba, triclopyr, fluroxypyr, etc, will give season-long control of a wide range of common grassland weeds.
In swards containing clover, Eagle or Prospect may be applied.
These products do not harm clover but Prospect may have some effect on the constituent grasses in the sward.
These are best applied in good growth conditions and will give season-long control.
Recent Teagasc trials showed that longer term (up to four years) control of docks can be achieved by applying a suitable herbicide (such as Starane2 at 1.5 l/ha) onto small docks shortly after re-seeding.
Docks that emerge in the following years rarely establish, due to competition from the grass.
How do I control dandelions?
Dandelions can also reduce the overall value of a pasture, if allowed to establish.
Once established, dandelions develop a large tap-root, but will not propagate from their root system.
Dandelions present a challenge, as they flower once they get a warm week in March, usually before most people think of spraying them.
Once flowered, it is very hard to get good control of the root system. In small amounts, MCPA or 2, 4-D will keep them at bay, but where long-term control is required, the aminopyralid/Fluroxypyr/dicamba based sprays are best, if applied in the summer or early autumn.
How do I control buttercups?
Control of this weed is quiet difficult, mainly because most buttercup plants emerge from seed in the autumn or late winter, and corrective action is normally not taken until after the flower appears. At this stage, the new seeds have already been produced, and control measures are often too late.
Maintaining a dense, leafy grass sward will smother out emerging buttercups. Avoid any overgrazing or poaching of grazing ground.
A heavy infestation of buttercups will require chemical control.
The best time to spray buttercups is in early spring (February to April), before it flowers and the plant is small and actively growing.
Spraying after flowering may require greater amounts of herbicides and will not prevent seed formation.
Effective control can be achieved with herbicide combinations containing MCPA, 2,4-D.
Seedling white clover and red clover in pasture will be killed by these herbicide products.
For example, MCPA will check white clover and kill red clover.
Spraying at the correct time can achieve up to 80 to 100% control of buttercups.
Consult the herbicide label for guidelines on grazing restrictions, precautions, and other limitations.
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