Last year we published a blog to help raise awareness of Atypical Myopathy or ‘sycamore poisoning’ in horses; an often fatal disease caused by the ingestion of the Hypoglycin A toxin (HGA) found in sycamore seedlings, seeds and leaves. Optimal pasture management to reduce the risk of toxicity is still unclear but new research conducted by the Royal Veterinary College (RVC) and The Animal Health Trust has shown mowing and herbicidal treatment to be ineffective at reducing the concentration of HGA in seedlings.


    Sycamore seed

Images courtesy of Gunther Van Loon, Gent University.


Sycamore seedlings were collected from 6 locations and assigned to either a ‘mowing’ or ‘herbicide’ treatment group.

  • Mowed seedlings were cut at mid stem length with the surrounding grass and left on the pasture.
  • Seedlings treated with herbicide were sprayed according to the manufacturer’s instructions and left to wither.
  • Seedlings were tested for the HGA toxin before treatment, 48 hours after treatment, 1 week after treatment and 2 weeks after treatment.



  • HGA concentrations remained high.
  • Neither treatment had a significant effect on the concentration of HGA in seedlings
  • The level of HGA toxin in grass cuttings increased from undetectable to 14.4 ug/g 1 week after mowing.


What does this mean?

  • Neither mowing seedlings and allowing them to dry or treating them with herbicide seems to reduce the level of the HGA toxin.
  • Cross contamination (seedlings to grazing) can occur when seedlings and grass are mown together.
  • Horses should not be allowed to consume sycamore seedlings, even if they have been dried or treated with herbicide. This includes seedlings found in hay.
  • Seedlings should be collected and removed from pasture.
  • If you can’t remove all of the seedlings from your pasture,  fence off the affected area and provide additional hay/ haylage to help reduce the risk of consumption


Horse eating grass

Essential facts

  • Sycamore poisoning can affect any horse or pony regardless of age, breed or sex. Donkeys are also susceptible.
  • Call the vet immediately if your horse shows signs of sycamore poisoning – the survival rate is approximately 30-40% but is highly dependent on rapid diagnosis and treatment.
  • The amount of the HGA toxin required to cause toxicity is unknown and some horses seem more susceptible than others (and some do not seem to be affected at all). It is not currently possible to identify which horses may be at greatest risk.
  • The level of HGA varies between individual trees. It is also possible that the level of HGA varies throughout the season and may be affected by a number of factors such as climate and soil conditions.
  • ‘Helicopter’ seeds can travel approximately 180 metres!
  • Streams/ rivers may be contaminated with fallen seeds so tape off the area and provide an alternative water source.
  • Horses may graze amongst sycamore trees for a number of years without being affected. However this DOES NOT mean these fields are safe either for horses currently grazing the pasture or those who may graze it in the future.
  • Horses may be more inclined to consume seedlings when grazing is sparse for example, during the current heat wave.


Where can I find more advice and information?                                            

The RVC continues to carry out research into this devastating disease and provides very useful advice for owners on their website and Facebook page. They also offer testing of seedlings and seeds for HGA, details of which can be found on their website.


Please share this blog to raise awareness of sycamore poisoning and help other horse owners reduce the risk of toxicity