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Here are several web sites that list not only the Japanese Maple, but other trees, schrubs and plants that are poisonous to horses.

http://www.horses-and-horse-information.com/articles/0800perils.shtml

             Pasture perils
Following is a list of plants and trees that are potentially toxic to horses. While this list is not all-inclusive, it includes trees and plants common in ...
www.horses-and-horse-information.com/articles/0800...


Following is a list of plants and trees that are potentially toxic to horses. While this list is not all-inclusive, it includes trees and plants common in the eastern United States.
Red maple: In the summer and fall, red maple leaves (wilted or dry) and bark are toxic to horses. The onset of signs, including weakness, depression, increased heart and respiratory rates, dark red urine and icterus, or yellow colored mucus membranes, occurs one to three days following ingestion of leaves or bark. Do not allow your horses any access to fallen red maple leaves or branches.
Cherry, apricot, peach and plum trees: While these trees are rarely a problem, wilted leaves may contain significant amounts of cyanide. The cyanide potential is greatest three to four days after branches have been cut or blown down. Dry leaves are not hazardous to horses. Horses will show signs within an hour of eating wilted leaves. Clinical signs include severe respiratory distress, even sudden death. Owners need to be aware of any tree trimmings or storm damage and remove branches from the pasture.
Japanese yew: This is a common evergreen ornamental shrub around houses. Owners will often feed trimmings to their horses. Approximately one-quarter pound is sufficient to kill an adult horse within 15 minutes.
Clinical signs will include lack of coordination, respiratory distress and sudden death. The toxin of this shrub causes the heart to stop.
Mountain laurel and rhododendron: Horses will generally avoid these trees unless pastures are severely depleted. Approximately four to six pounds will be toxic to horses. Clinical signs usually occur within six hours of eating the leaves, and include severe colic and heart problems.
Black walnut: Trees in the pasture are not associated with any problems in horses. The problem arises when bedding contains even a small amount of black walnut shavings. The toxin is present in the heartwood and is most likely ingested. Clinical signs begin eight to 48 hours after new bedding. These include depression, edema and laminitis (founder). Check with your bedding supplier that black walnut has not been milled at the factory. Any dark wood shavings should be suspect.
White snakeroot: This plant prefers wooded pastures where it grows in the shade close to a water source. It grows up to five feet tall with serrated leaves and white flowers in clusters. Poisonings generally occur in the fall, and horses must consume several pounds over several days. Clinical signs include muscle tremors, uncoordination and stiffness.
Jimson weed: Jimson is an annual that grows between two and five feet tall. It has irregular 'toothed' leaves with large white or purple trumpet flowers with pitted, kidney-shaped black seeds. All parts of the plant are toxic, but the seeds are especially bad. Horses that consume Jimson weed will generally show clinical signs - increased heart rate, decreased gastrointestinal motility and hyperexcitability with muscle twitches - within 24 hours.
Tall fescue: A fungus growing within most tall fescue in the Southeastern U.S. causes reproductive problems for mares and their foals. The toxic principle in this fungus is called 'ergovaline,' and very small amounts in grass or hay will cause problems in mares. These problems may not become evident until the last months of pregnancy when the mares fail to have mammary gland development and milk production. Mares will often go a month past their due date, and the foals will appear large and spindly.
Foals will quickly develop other problems due to the mare's lack of milk production. Pregnant mares should be kept off all fescue pasture and hay for one to two months prior to foaling.

Equine Health
Poisonous Plants and Trees; That Can Harm Your Horse; Portions of this below are from an article in the; March, 2001 issue of EQUUS magazine: By Joanne ...
www.naturalhorsetraining.com/PoisonousPlants.html [Found on Google, Yahoo! Search]

Poisonous Plants 2
The only thing in her paddock that she was addicted to was a horse chestnut tree - no one ever lists this as poisonous but it occured the following year ...
www.newrider.com/Library/Misc_Tips/poisonous_plant...


Meadowsweet Acre Herbs for Horses
1 Manufacturer of all natural and totally holistic equine herbal supplements.This website is ... There are at least 200 plants/trees or scrubs that are toxic to horses at some level if ...
www.meadowherbs.com/toxic.htm

I hope this helps. If you need futher information, please feel free to contact me.

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