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Controlling weeds in the horse paddock


Teagasc Equine Specialist, Wendy Conlon explains how best to deal with certain noxious weeds, some of which can be deadly to horses.

Poor grass growth, bare patches, and poached fields are happy hunting ground for weeds. High levels of weeds in pastures not only reduce the pasture’s nutritional value, but restrict grazing areas and valuable grass growth. Low levels of weeds are of no consequence and their removal may not be cost effective. However, when weeds reach density levels of 10 to 20% of the total sward, they will impact on either grassland quality or productivity. Certain weeds can make hay and haylage unpalatable, while others are poisonous. Management practices such as drainage, fertility, grazing, topping and mowing are very important when it comes to controlling weeds. All of these encourage the grassland to be competitive and dense. Ragwort, thistle and dock are scheduled as noxious weeds under the Noxious Weeds Act 1936

Ragwort

(Senecio jacobaea) Ragwort is a highly poisonous plant causing serious cumulative damage to the liver. Early signs of ragwort poisoning include loss of appetite, weight loss, diarrhoea, depression, sensitivity to sunlight and mild jaundice. More severe signs include compulsive walking, circling, head-pressing (e.g. against a wall), apparent blindness and extreme depression. On good pasture horses avoid eating ragwort, but where there is over-stocking and grass is scarce ragwort may be unavoidably eaten. Poisons in ragwort are not destroyed by drying, and conserved forage containing ragwort is also a potential source of poisoning. The only way to safeguard against loss from ragwort poisoning is to eradicate the weed.

Pulling

Pulling by hand is recommended where infestation is not severe and labour is available. Pulling after heavy rainfall from soft ground gives best results when done in early summer before seed has set. Pulled plants must be removed and destroyed. As the rosette stage is not usually removed by pulling, repeat for two consecutive years to achieve satisfactory eradication. Wear gloves when pulling as the toxins are said to affect humans through the skin.

Cutting

Cutting before flowering prevents the weed from seeding and spreading. This is of limited value unless carried out over a number of years and accompanied by good grassland management.

Chemical Control

No single herbicide treatment will completely eliminate ragwort due to successive germination of the weed. Ragwort plants become more palatable after spraying and livestock *must be kept off treated fields until all plants are dead and removed. The removal of stock from pastures during peak grazing season is usually impractical and therefore spraying is generally carried out during the winter.

Spear thistle

(Cirsium vulgare) Spear thistles depend on their seed for regeneration so prevention of seeding is of crucial importance.

Physical Control

As spear thistle does not produce a spreading root system it is possible to control by hand hoeing individual plants and small patches, provided the growing point and top 20-40mm of the tap root are removed. This should be done before mid July when the plants are in late bud or early flower stage. Topping may help to reduce seed production but is, however, of limited value as spear thistles mature over an extended period and if soil moisture is adequate thistles are likely to recover and re-grow.

Chemical Control

For effective chemical control thistles must be growing actively. Spraying should be completed before the centre flowering stem develops (i.e. up to the end of June). 

Creeping Thistle

(Cirsium arvense) It thrives in fertile grassland. Winter poaching and overgrazing in spring encourages spread.

Physical control

Cutting is not an effective means of control for creeping thistle, as it can regenerate from its roots.

Chemical Control

Control at rosette stage when actively growing up to 200mm high or across.

Curled and Broad-leaved Dock 

(Rumex crispus and Rumex obtusifollius) Both dock species produce many seeds that can remain viable in the soil for decades. Spread may occur from fragments of taproot. Growth commences in early summer. They flower between June and October.

Physical Control

Pulling by hand is only effective in small infestations. Hand-pulling shoots before they have set seed can be effective when the soil is moist, but most need to be dug out, with care not to leave fragments behind. Continued topping before flowering will prevent seeding and wear down the taproot. Cutting or topping should not be carried out for at least two weeks after spraying to allow the herbicide fully penetrate the root.

Chemical Control

Spraying of docks should be done in warm weather, and if infestation is heavy, a second spraying may be required. Spraying should be carried out when the first flower stalk is emerging from May until the end of summer while docks are growing vigorously and not yet setting seed.

Chemical Control of Weeds

All herbicides have labels with data on weeds targeted, dose rates, dilution rates, and timing of spray. It is important to read and follow the printed information to get optimum results.

You might also like to read Soil and Fertiliser Advice for Equine Farms

Controlling weeds in the paddock (PDF) (First published in The Irish Field 2019)

Check out the Teagasc Equine page here