08/17/2026
Checking turnouts and pastures for hazards as well as toxic plants, shrubs, and trees is a preventative measure we can take toward keeping our equine healthy and safe!
๐ณ ๐๐จ๐ฑ๐๐ฅ๐๐๐ซ ๐๐ซ๐๐๐ฌ: ๐๐๐๐ฎ๐ญ๐ข๐๐ฎ๐ฅ ๐๐ฎ๐ญ ๐๐๐๐๐ฅ๐ฒ
This week we are switching things up a bit! Instead of highlighting a toxic plant, we are taking a closer look at a tree that may be growing right in or around your horseโs pasture.
Boxelder trees are common, widespread, and usually not something we think twice about. But female boxelder trees can produce seeds containing a toxin that can cause a rapidly progressive and often fatal muscle disease in horses.
๐ณ ๐๐ก๐๐ญ ๐๐ซ๐ ๐๐จ๐ฑ๐๐ฅ๐๐๐ซ ๐๐ซ๐๐๐ฌ?
Boxelder trees (Acer negundo), also known as an ash-leaved maple, are fast-growing but relatively short-lived trees, with an average lifespan of 60 to 75 years. These deciduous trees are medium-sized and can reach 35 to 70 feet tall.
They are often found growing along waterways, in floodplains, and in areas with moist soils, but they are highly adaptable and can grow in a wide variety of soils and habitats.
๐ ๐๐ก๐๐ง ๐๐จ ๐๐ก๐๐ฒ ๐๐จ๐ฌ๐ ๐๐ง ๐๐ฌ๐ฌ๐ฎ๐ ๐ญ๐จ ๐๐จ๐ซ๐ฌ๐๐ฌ?
There are male and female boxelder trees, and only female boxelder trees produce the winged seeds that are toxic to horses. These seeds are produced primarily in the fall and early winter.
This timing is important because pastures are often becoming sparse and overgrazed at this time of year. While horses do not typically seek out boxelder seeds, they may consume them when other forage is limited and the seeds are readily available.
This is one reason why fall pasture management is so important.
โ ๏ธ ๐๐ก๐ฒ ๐๐ฌ ๐๐ญ ๐๐จ ๐๐๐ง๐ ๐๐ซ๐จ๐ฎ๐ฌ?
These winged seeds contain hypoglycin A, a naturally occurring toxin that disrupts the body's ability to break down fats for energy.
When hypoglycin A is metabolized, it interferes with fatty acid metabolism in muscle cells, particularly affecting type 1, or aerobic, muscle fibers.
This is especially problematic because muscles rich in type 1 fibers include the postural and respiratory muscles. These muscles become unable to efficiently produce the energy they need to function, which is reflected in the clinical signs we observe.
๐จ ๐๐ก๐๐ญ ๐๐จ๐๐ฌ ๐๐จ๐ข๐ฌ๐จ๐ง๐ข๐ง๐ ๐๐จ๐จ๐ค ๐๐ข๐ค๐?
The condition in horses is often referred to as Seasonal Pasture Myopathy (SPM). It is a severe and rapidly progressing muscle disease.
Early signs can include:
โข Muscle weakness
โข Stiffness
โข Muscle trembling
โข Sweating
โข Dark or brown urine
Unfortunately, affected horses can deteriorate rapidly, sometimes within 24 to 72 hours. As the disease progresses, horses may develop rapid and heavy breathing as the respiratory muscles become affected, a high heart rate, signs of colic, weakness, and eventually recumbency.
Once severe clinical signs develop, the prognosis is poor.
๐ฅ ๐๐จ๐ฐ ๐๐ฌ ๐๐ญ ๐๐ซ๐๐๐ญ๐๐?
Treatment often requires intensive hospital care, including IV fluids, electrolytes, pain management, nutritional support, and close monitoring.
Treatment is costly and requires significant labor and resources. Unfortunately, even with treatment, approximately 75 to 90% of affected horses die or need to be euthanized within 72 hours of the onset of clinical signs.
This is why prevention and early recognition are so important.
๐ ๐๐ก๐๐ซ๐ ๐๐ซ๐ ๐๐ก๐๐ฒ ๐
๐จ๐ฎ๐ง๐?
Boxelder trees are native to the western hemisphere, ranging from Canada to Guatemala. In 1688, they were introduced to Europe as an ornamental park and garden tree.
Today, they can be found throughout much of the world due to human introduction and naturalization, and they are considered invasive in many regions.
Despite how widespread they are, documented cases of boxelder-associated toxicity have occurred in Eastern Canada, the Midwest, Texas, and the Southern USA. However, cases have likely occurred in other areas of North America and gone unnoticed.
A similar condition, called Atypical Myopathy, is reported in Europe, Australia, and New Zealand. It is caused by the same toxin, hypoglycin A, which is found in the seeds of the sycamore maple (Acer pseudoplatanus).
๐ณ ๐๐จ๐ฐ ๐๐จ ๐๐จ๐ฎ ๐๐๐ญ ๐๐ข๐ ๐จ๐ ๐๐ญ?
Prevention is key.
The best preventative method is to remove female boxelder trees from areas accessible to horses. However, if removing the tree is not possible, there are several management practices that can reduce the risk:
โข Fence off the tree and, importantly, the area where the seeds fall
โข Remove low-hanging branches when possible
โข Avoid overgrazing pastures, particularly in the fall
โข Provide hay when pasture forage becomes sparse
โข Regularly check pastures for fallen seeds during the risk period
Remember, fencing off only the trunk may not be enough. The toxic seeds can travel away from the tree and accumulate throughout the drop zone.
โ ๏ธ ๐๐จ๐ง๐๐ฅ๐ฎ๐ฌ๐ข๐จ๐ง
Boxelder trees may look like just another harmless tree growing along the edge of your pasture, but female trees can produce seeds containing a toxin capable of causing a rapidly progressive and often fatal disease in horses.
The best way to protect your horse is to prevent access to the seeds in the first place. Knowing what trees and plants are growing around your pastures, and understanding when they become a risk, is an important part of pasture management.
We have a male boxelder tree in our pasture, so thankfully we donโt have to worry about seed production from our tree.
๐ ๐๐ง๐ญ๐๐ซ๐๐ฌ๐ญ๐ข๐ง๐ ๐
๐๐๐ญ๐ฌ
A few things I learned while taking a deep dive into boxelder trees โ or, if youโre looking for a few fun facts to share at your next party as a conversation starter:
๐ณ ๐๐ก๐ฒ โ๐๐จ๐ฑ๐๐ฅ๐๐๐ซโ?
The name has a few potential origins. โBoxโ may refer to the treeโs ability to be tapped for low-quality syrup, its former use for making wooden crates, pallets, or boxes, or the resemblance of its pale wood to boxwood. โElderโ refers to the resemblance of its leaves to Elderberry.
๐ ๐๐ง๐ ๐ญ๐ก๐ ๐๐จ๐ฑ๐๐ฅ๐๐๐ซ ๐๐ฎ๐ ?
They were named for their strong association with boxelder trees. During the warmer months, they feed on the sap and seeds of boxelder trees, particularly female trees, and use them as a place to mate and lay eggs.
๐ฆ ๐๐จ๐ซ๐ฌ๐๐ฌ ๐๐ซ๐๐ง'๐ญ ๐ญ๐ก๐ ๐จ๐ง๐ฅ๐ฒ ๐๐ง๐ข๐ฆ๐๐ฅ๐ฌ ๐๐ญ ๐ซ๐ข๐ฌ๐ค!
Seasonal Pasture Myopathy has also been described in Pรจre David's deer, Bactrian camels, and gnus!
So, the next time you see a boxelder tree, you may look at it a little differently.
Do you have any experience with a boxelder tree?
Cheers,
Dr. DeBoer
