Can eating seeds and seedlings from maple trees be dangerous to my horses?

Hoof Nutrition Intelligence is a twice-a-month web segment that is designed to add to the education of footcare professionals when it comes to effectively feeding the hoof. The goal of this web-exclusive feature is to zero in on specific areas of hoof nutrition and avoid broad-based articles that simply look at the overall equine feeding situation.

Below you will find Part 2 of the latest question and answer installment that you can share with your footcare clients.

Q: Can eating seeds and seedlings from maple trees be dangerous to my horses?

By Kentucky Equine Research staff

A: Who would have thought the stately maple, with its festoon of fall color, could wreak havoc on horses across the world?

A muscle disorder in horses called atypical myopathy has been linked to a toxin contained in the seeds and seedlings of certain maple trees. While atypical myopathy is the term used in Europe, the disease is referred to as seasonal pasture myopathy in North America.
Maple trees shed distinctive winged fruits called samaras, each characterized by a paper-thin, nearly transparent tissue that envelops a seed. Children call them whirlybirds or whirligigs. Mature trees cast thousands of samaras each year. 

These seeds and seedlings contain hypoglycin A (HGA) and methylenecyclopropylglycine (MCPG) compounds that interrupt energy production once they are converted to metabolites after ingestion. Signs of intoxication include acute rhabdomyolysis involving weakness, recumbency, myoglobinuria, stiffness, depression, muscle tremors, sweating and congested mucous membranes.

A team of European researchers put together a short list of frequently asked questions and answers about the disease from two website questionnaires. Combined with a review of the scientific literature, data from over 3,000 cases of atypical myopathy in 14 countries was used to answer five frequently asked questions.

1 Which maple trees are toxic? Not all species are poisonous to horses and not all species have been assessed for toxicity. Specific to this disease, the sycamore maple (Acer pseudoplatanus) in Europe and the box elder (Acer negundo) in North America are predominant trees of interest.
Researchers found few horse owners could differentiate between tree species, even though an abundance of educational material is available on the internet. Wilted leaves and bark of the red maple (Acer rubrum) can also be toxic to horses, but have not been implicated in seasonal pasture myopathy.

2 How can the risk of atypical myopathy be reduced when horses are on pasture? Researchers suggested that all pastures be canvassed for toxic seeds and seedlings. Case studies have revealed areas of felled trees with dead leaves as well as piles of dried leaves that had fallen naturally from healthy trees are a risk factor. The wind can also carry samaras hundreds of meters from the parent tree, so pasture contamination may result from trees on adjacent properties.

Horses on pasture at all times seem to be at higher risk, probably because an absence of grass encourages horses to investigate other sources of nourishment. Because the toxin is water-soluble, it can pass from plants to water directly, so any area with free-standing water, such as ponds, should not be used as pasture, particularly if suspect trees are in the area and samaras are present.

3 How can the risk be mitigated for horses? Mainly through reduced exposure to toxic trees by managed grazing, through reputable feed-supply chains and safe water sources.

Nearly all cases of atypical myopathy occur when horses are on pasture. One study indicated that limiting grazing during high-risk periods might be prudent, such as in autumn or in spring following an autumn outbreak. 

Horses fed concentrates and hay year-round had a decreased risk of atypical myopathy, likely because owners did not depend on grass to meet the energy requirements of their horses. According to the researchers, atypical myopathy results from an energetic imbalance subsequent to poisoning.

Feed provides energy substrates, especially carbohydrates, that support the energetic metabolism and also vitamins and antioxidants known to increase the chance of survival.

Hay may contain samaras, so emphasis should be placed on purchasing hay from reputable growers. Forage products should not be harvested in areas where trees stand and shed samaras and leaves. When fed in the field, hay should not be fed from the ground if a sycamore tree is nearby, as horses may pick up toxic material.

4 What pastures are at risk? All pastures with neighboring maple trees should be judged unsafe. Atypical myopathy is considered to be an emerging disease, and multiple cases have been identified whereby no previous deaths have been recorded in an area.

5 When do cases occur in spring and autumn? While cases of atypical myopathy occur in both seasons, far more transpire in the autumn months. 

Spring cases have been recorded from March 1 to May 31, and autumn cases from Oct. 1 to Dec. 31. The seasonality of the disease has not been determined, but can’t be attributed to frost or deep freezing, as the toxin can withstand extreme cold. A flush of forage in the late spring and summer might keep horses from nibbling on unusual feedstuffs, such as maple seeds and seedlings.

Kentucky Equine Research is a nutrition consulting company located in Versailles, Ky.

Click here to read part 1 of the Dec. 15, 2020 installment of Hoof Nutrition Intelligence: Is there any new information you can share in regards to preventing or treating laminitis?  Click here to read more installments of Hoof Nutrition Intelligence.