Colorful fall foliage is beautiful to look at and provides that certain autumn ambience we appreciate in the season of pumpkin-spice flavored everything, but some types of fall leaves can be fatal if ingested by horses.

Outward signs of leaf toxicity include lethargy, lack of appetite, pale or yellow gums, increased respiratory and heart rate, dark brown or reddish urine and progressive weakness. While those signs sound equally straightforward and alarming, the diagnosis of leaf toxicity can be difficult to diagnosis because the symptoms often mimic other medical conditions. To complicate matters, most horse owners aren’t aware that their horses have may have eaten the leaves.

We’ll look at what types of trees pose the most risk, explain why certain fall leaves are toxic to horses and offer management tips to help prevent your horse from eating them in the first place.

Acer and Prunus—the Most Common Culprits  

Acer rubrum

Commonly known as red maple, soft maple or swamp maple, these trees are native to the Eastern U.S. and Canada and provide stunning backdrops in the fall with their showy red leaves. While on the tree and alive, red maple leaves are fine, but once they fall and start to wilt, they can cause a severe toxic reaction in horses in relatively small quantities. As little as 1.5 to 3 grams per kilogram (that’s about 0.05 to 0.11 ounces per 2.2 pounds) of body weight can cause hemolytic (red blood cell-destroying) disease.

Why they’re toxic: Wilting leaves are thought to contain gallic acid, which can lead to red blood cell breakdown and hemolytic anemia, in which the body’s immune system attacks and kills its own red blood cells. Anthony Knight, BVSc, MS, DACVIM, explains that toxins in the wilted leaves damage the hemoglobin in the horse’s red blood cells, leaving them unable to carry oxygen. The damaged blood cells may rupture, overwhelming the kidneys with waste products. If the horse loses red cells faster than they can be replaced, tissues will be starved of oxygen, causing vital organs to fail.

“The onset, severity and duration of red maple toxicosis depend on how many wilted leaves the horse ingests. But once signs of poisoning develop, odds for survival are poor,” Knight said. “Red maple poisoning is fatal in 60 to 70 percent of cases.”

While leaves that fall from the tree and wilt are poisonous year-round, it appears that those falling after mid-September contain the greatest concentration of gallic acid and remain toxic for several weeks.

Symptoms: Horses that consume red maple leaves typically become seriously ill within 18 hours and often die within a couple of days. Initially after ingestion, a horse generally shows severe depression and becomes weak due to reduced oxygen flow around the body. Heart rate and respiration increase in an attempt to circulate oxygen, red or dark urine is often observed and their mucus membranes (including the whites around the eyes) might appear slightly yellow. Ultimately, this inability to carry adequate oxygen via the blood leads to death.

Important note: Not all red maples are toxic, however other species—including sugar maples and silver maples—have been found to contain gallic acid and should be avoided. To be safe, try to avoid having maples in areas where horses could consume the leaves.

Prunus 

This family of trees has more than 200 species and includes the stone fruit trees: plums, apricots, peaches and cherries. While less toxic to horses than cattle, the fruit stones contain cyanide and the leaves are particularly toxic while wilting. Choke cherries and black cherries are considered the most dangerous of the Eastern wild cherries.

Why they’re toxic: When metabolized, cyanide inhibits the body’s ability to absorb oxygen. Because horses don’t metabolize the cyanide compound as efficiently as ruminant animals do, grazing healthy adult plants is unlikely to harm them, but circumstances that injure the plant—wilting, trampling or frost—can chemically liberate the cyanide within the leaves, rendering them dangerous to all species.

Symptoms: Clinical signs of cyanide poisoning include increased heart and respiration rates, with horses breathing heavily through flared nostrils. Cyanogenic glycosides cause bright red mucus membranes, dilated pupils, difficulty breathing and shock.

Potential treatment: It’s possible to treat horses found early in the disease course by using chemicals that bind the cyanide, allowing cellular oxygen transport to be unblocked.

Preventing Fall Leaf Poisoning

Implement the following strategies to protect you horse from leaf poisoning:

• Learn the leaves in your area. Red maples have three distinct lobes when they are immature (other maples have five lobes). From there, they grow out into a more substantial shape—like the one on the Canadian flag. The edges of the red maple look they have been trimmed with a serrated kitchen knife.
• Supplement your horse’s pasture grass with high-quality hay. As we get deeper into fall, the nutritional value of pasture grass starts to dramatically drop, and your horse might be more inclined to snack on forage he’s not usually interested in, including toxic leaves.
• Identify all red maple tree locations on your property, as well as those within reach of horses along your fence line. If you’re not savvy about tree species, you can seek help from a professional or your County Extension Office. Some find it helpful to physically label the trees or mark them on a map and take photos. From there, you can determine if it’s safest to have the trees removed or if careful monitoring of their fall leaves will suffice.
• Even if you don’t have any red maple trees on your property, inspect your pasture for any leaves or branches that might have blown over from another location and immediately remove. Following a storm, check your pasture’s far reaches to ensure no branches are down within reach of hungry horses.
• Consider fencing off any trees that are toxic to horses, even if they’ve never caused a problem before.
• Whenever you buy hay, inspect it carefully for leaves and weeds. If it’s from a trusted, reliable source, there most likely won’t be any issues, but if your horse is eating hay from an unknown source while away from home, always check it first.
• If you keep a feeder or hay rack in your pasture, clean it out often during the fall. Leaves will collect in the bottom where horses can find them.

To read more about plants that are poisonous to horses, click here.