These weeds are not only an irritant to horses, they can cause them injury, warns Dr Mac.
The common blackjack (Bidens pilosa) is well-known to horse owners in South Africa. There can be few of us who have not spent ages picking them off our clothes after walking through the veld to catch horses in the early winter. Legend has it that, like the khakibos, this weed was brought into South Africa in the feed of horses imported from South America during the Boer War. However, the plant is listed as being indigenous to tropical Africa, so may have already been in South Africa at the time.
It grows easily on disturbed land or after fires, and in areas like manure heaps or horse paddocks, where the nitrogen level in the soil is high. The common blackjack is an annual plant belonging to the Asteraceae family. In the young plant, the leaves are flat and a dark purple-green. The white, daisy-like, yellow-centred flowers with their distinctively long, thin stems appear in late summer to autumn. They mature into star-like fruiting bodies. The typical ‘blackjacks’ that attach themselves to passing animals and people radiate outwards as 1cm-long seeds crowned by two or three sharp awns.
After fruiting, the plant dries out completely – even the roots become brittle. This is a mechanism to promote ‘zoochory’ – the dispersal of seeds through animal or human movement. If the seeds catch onto a passing animal, the whole plant is easily pulled out of the ground and dragged along. Stems break off and deposit seeds over the whole grazing area. Although Bidens pilosa is generally considered a weed, the leaves can be boiled and eaten.
Blackjacks that become entangled in the forelock of a horse can be a great irritant, and the animal will toss its head if you try to remove them. The spines can injure the eyes, so it’s better to clip the forelocks short. Blackjacks can also get caught up in the long hair behind the fetlocks and pasterns, causing chronic irritation and lameness. After cutting out the tangles, clip the hair and treat the skin with antibiotic creams.
In severe cases, you can use a poultice with Epsom salts to draw out the barbs before treating the area with Betadine.
It’s very important to check numnahs and horse blankets regularly for blackjacks, as they can be worked deep into the skin during riding, causing small infected wounds. These can become so painful that a horse will rear or buck while being saddled or mounted.
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