Reports from the world of equine research.
Ragwort: a growing problem.
© Copyright Equine Science Update 2002-2006
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New evidence points to the ingestion of ragwort in pasture as an important cause of ragwort poisoning, contrary to the widely held view that the plant`s bitter taste prevents horses eating it. Research conducted by Ragwort-UK has identified that continuous low level poisoning occurs through two routes. Both are caused by seedling ragwort plants in good dense grass.
"Once ragwort germinates it throws up a single small leaf, carried on a long rib to reach the sunlight. It is easy for this single leaf to be bitten off with a mouthful of grass, clover etc and be eaten without its taste being noticed" Derek Smith of Ragwort-UK explains. " The ragwort crown will soon send up another leaf and so the problem continues. We have witnessed horses grazing single seedling leaves in this way, and the stock involved was diagnosed by blood tests to have liver damage typical for ragwort poisoning."
The second route is probably even more dangerous, Smith suggests, because greater quantities of ragwort can be eaten. As the ragwort seedling establishes its crown and extensive root system, it throws up three or four long leaves into the grass above it. The leaf is bitten off with a mouthful of grass. By now there is enough ragwort for the horse to taste it. So it spits it out. One or two days later, the leaf is dead and has lost its bitter taste.
"Everyone knows that dead ragwort is poisonous though it has lost its bitter taste, but your stock do not and the dead leaf is eagerly eaten to play its small but cumulative part in the destruction of your horses liver. In field trials we have witnessed single leaves being bitten off and spat out in this way and have recorded many seedling plants with evidence that several leaves had been removed. These dangers are magnified as grass growth slows in October and stock graze closer or start grazing long or rough patches."
Ragwort poisoning is often called the 'iceberg disease' because the damage is being done bit by bit with virtually no outward signs. Each little ragwort leaf destroys a few more liver cells until in many cases when symptoms become apparent, it is just too late and the horse must be destroyed to save it from a piteous death.
There is also growing concern about the human health risks of ragwort. Derek Smith predicts that the next two years will see an escalation in equine and human liver damage as ragwort further invades both food chains.
At the heart of the ragwort problem are the huge reserves of ragwort seed now present in the ground. In many areas densities of many thousands of seeds per square metre are commonplace. The seeds are very long lived (7 years to 20 years often reported). Seeds can only geminate if they have been in the ground for a certain amount of time and then only if they are exposed to sunlight. Even when further seed production is eventually stopped, the seed store now present will remain viable for many years. Earthworms, ants and hoof damage will continue to trigger germination virtually all year round.
The key to bringing ragwort back under control is to prevent further seeding from adding to these reserves. Today's piecemeal spraying and pulling programmes are leaving sufficient plants to seed such that the problem is still escalating by about 50% each year.
Researchers at Ragwort-UK have been developing a method of biological control of ragwort using Cinnabar moth caterpillars, which feed exclusively on ragwort.
"Our trials show that Cinnabar caterpillars are capable of achieving sufficient densities to completely prevent seeding." says Smith." One of our trial plots, a 1 hectare natural meadow, was completely clear of flowering ragwort this year after previously being infested with about 5 plants / square metre for the previous three years. This control was achieved by Cinnabar alone.
The major advantage of Cinnabar, apart from being self replicating, Smith points out, is that they do not stop at fences or boundaries. Pulling and spraying must respect the law of trespass, and as a consequence plants 'just over the fence' are left to seed and compound the problem. Obviously the law does not apply to the Cinnabar and when present in sufficient numbers they will completely consume all flowering ragwort whichever side of the fence it is growing on.
However, Smith warns that while Cinnabar caterpillars can play an important part in bringing ragwort under control by halting seed production, there still remains the substantial problem of seed reserves. "Our models predict that even if all ragwort seeding could be halted immediately, it would take at least another 3 - 6 years before seed germination ceases to be a significant issue for equines."
"You cannot remove the seed and you cannot prevent it from germinating. Your only defence is to inspect your pastures regularly and remove the seedlings the moment they show. One trial paddock controlled by Ragwort-UK yielded over 200 seedlings per acre every week for over 2 years with only a slight reduction through the winter months."
"Walk every inch of your paddock now " he advises. "Seedling ragwort is not easy to spot until you 'get your eye in'. They might only have a single leaf or be less than an inch across, but they all have to come out to safeguard your horses."
"If there are too many seedlings to lift you may have to consider herbicidal sprays. Despite the damage sprays cause to the land and the length of time you must keep stock off the grass until the seedlings have rotted (typically 3 - 6 weeks), spraying with a 2.4D ester based herbicide is often the only practical way of bringing seedling numbers down to the level that they can then be controlled by regular weekly inspection and lifting."
"Seedling ragwort is an insidious killer. It is growing 12 months of the year and until the seedlings establish a crown and rosette structure, they are very hard to spot and remove. For the past five years, ragwort seed production has increased inexorably. Ragwort seed blows for many miles guaranteeing that every piece of land gets its share of the burden. Doubtless you did not create the problem, but equally doubtless, you are the recipient of it. If you do not aggressively and persistently remove ragwort from your grazing, your horses will be the ones that pay the final price with their lives."
for more details see: www.ragwort-uk.com