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CONCLUSIONS of CLOVER TRIAL While it was previously thought that grass sickness was more common in pastures with a high clover content, recent studies indicate that it can also occur on pastures with no clover. However, at worst it may be a trigger for a bacteria such as Clostridium botulinum. |
EQUINE GRASS SICKNESS CLOVER TRIALDr.
Bruce McGorum, Owners whose horses have succumbed to grass sickness often ask if there is some way in which they can make a practical contribution towards research. A field trial is about to commence which will now provide them with an opportunity to do so. Owners of horse pastures where equine grass sickness (EGS) has occurred, who wish to assist in the study of this devastating disease, are being asked to participate in a field trial. The trial will test whether EGS can be prevented by spraying fields with specific herbicides which kill white clover (Trifolium repens).
For the trial, a field on which EGS has occurred will be sprayed with Duplosan KV from BASF, a herbicide that is effective against a range of broad leafed weeds including white clover. Participants must arrange and pay for the herbicide application (estimated at £15/hectare or £6/acre), since the Equine Grass Sickness Fund is unfortunately not in a position to subsidise the treatment. The Equine Grass Sickness Fund can provide information on where to purchase the herbicide (contact Joyce McIntosh Tel 0131-445-6257). The herbicide must be applied according to the manufacturers' recommendations - the suggestions below are for guidance only. Information on the optimal time to spray, which may vary from region to region, should be obtained from the local agrochemical suppliers. However, in many instances it may be suitable to spray permanent/established pastures at the end of April or the beginning of May, when clover is actively growing, but prior to the main risk season for EGS (usually from middle of May in Scotland). The spray should be applied when the clover is actively growing, and not shielded by grass. The spray should not be applied when there are adverse weather conditions, such as frost, drought or severe day/night temperature fluctuations, to optimise killing of the clover, and to prevent scorching/blackening of the grass. While a single application of herbicide is likely to be effective, two applications may be required if the clover is shaded by a heavy grass growth. A maximum of 2 applications per year is permitted. The treatment may need to be repeated annually, depending on the amount of clover that re-grows the following year. The sward should not be topped for at least a week before or after spraying The manufacturers recommend that after spraying, horses and other stock are not returned to the pasture for at least 2 weeks. If ragwort or other poisonous weeds (including clover) are present, the herbicide manufacturers state that stock should be withheld from the treated field until the weed foliage has died and become unpalatable, which will take at least 2-3 weeks. However, it is probably safer to pull (wearing gloves) and burn ragwort, since dying ragwort retains its toxicity and is palatable to horses. To permit assessment of the efficacy of the treatment, it will be necessary to:- 1. Register with the trial co-ordinator - please contact Dr Bruce McGorum, Royal (Dick) School) of Veterinary Studies, Easter Bush, Roslin, Midlothian EH25 9RG, Tel 0131-650-6253 email: brucem@vet.ed.ac.uk (alternatively this can be done through the Equine Grass Sickness Fund - contact Joyce McIntosh, EGSF, Moredun Foundation, Pentlands Science Park, Penicuik, EH25 0PZ Tel 0131-445-6257). Participation in the trial will be confidential. All data collected will be confidential and will be used only for the purposes of the study. 2. Only those fields that have had cases of EGS should be sprayed. The other fields on the premises should ideally not be treated as these will act as control fields for the study. To permit assessment of the efficacy of the treatment, it would be ideal if all the grazing horses on a premises were divided up such that an equal number of horses were grazing on the treated and on the control fields. However it is realised that this may not be possible because of practical problems, and participants may wish to contact Bruce McGorum to discuss this aspect. 3. A record of the spraying date and application details should be made. A questionnaire that can be downloaded from the web site follows this information. Alternatively a questionnaire will be sent by post if requested. The completed form should be sent to Bruce McGorum at the address given on the form. 4. A diary recording the identity of horses grazing the treated and control fields, and the dates each horse was on each field, should be made. A diary sheet that can be downloaded from the web site follows this information. Alternatively a diary sheet will be sent by post if requested. The completed form should be sent to Bruce McGorum at the address given on the form. 5. The co-ordinator wishes to be informed if any horses on treated or control pastures develop EGS or other potentially related diseases - ideally this is best done via the participant's regular veterinary surgeon. The co-ordinator may request permission to visit the field to study and sample the pasture. Where possible, any horse which dies on treated or control pastures should receive a post mortem to ascertain whether EGS was the cause of death. If practical, The Equine Grass Sickness Fund may be able to subsidise a post mortem examination if the carcass can be transported to Royal (Dick) School) of Veterinary Studies. It should be noted that, as this is a scientific trial, we cannot guarantee that eradication of white clover will prevent the occurrence of EGS on treated fields. Furthermore, the herbicide spray must be applied in accordance with the manufacturers' recommendations. The co-ordinator can take no responsibility for any detrimental effects resulting from herbicide treatment. Click here to download FORM TO RECORD DETAILS OF HERBICIDE TREATMENT Click here to download FORM TO RECORD WHEN AND WHERE HORSES WERE GRAZING
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EGS
was first linked with ingestion of white clover almost 100 years ago, when
the first reports of the disease coincided with the introduction of wild
white clover into the seed mixtures used to improve agricultural
productivity in Scotland. White clover now grows in abundance on most, if
not all, fields where EGS has occurred. While white clover is commonly
considered to be innocuous to grazing horses, it does contain chemicals
called cyanogenic glycosides that can release cyanide, a well known nerve
toxin. Interestingly, the levels of cyanide in white clover increase when
the plant is rapidly growing, as occurs during the spring flush, which
coincides with the peak seasonal occurrence of EGS. Furthermore the levels
of cyanide also increase when clover is exposed to adverse weather stresses,
such as drought, cold, frost and snow. This may explain why EGS often occurs
after periods of dry cold weather, especially when there are sharp overnight
frosts.
Recent
work at the Royal (Dick) School of Veterinary Studies, Edinburgh, has
rekindled interest in the possibility that white clover may cause EGS.
Firstly, white clover collected from fields during outbreaks of EGS was
found to have increased levels of cyanide. Secondly, analysis of blood and
urine collected from horses grazing on EGS fields indicated that these
horses had recently been exposed to increased amounts of cyanide. Thirdly,
horses with EGS had alterations in blood amino acids (the building blocks
for proteins) which resemble those of humans with cyanide poisoning.