I am supposing your fruitless plum is Purple Pony Cherry Plum (Prunus cerasifera). At any rate, I am sure it is a member of the genus Prunus which includes peaches, plums, apricots, cherries and chokecherries. P. cerasifera is not listed specifically in any poisonous plants database, but many other members of the genus are listed and all have the same warning about the ingestion of leaves, twigs or seeds of fruit. These parts of the plants contain cyanogenic glycoside or cyanogens that are highly toxic and may be fatal if eaten. Cyanogenic glycosides are changed into free cyanide either in plant material that has been damaged or in the gastrointestinal tracts of animals. It is most severe in ruminants (cattle, sheep, goats, deer) but is also very dangerous for animals with single stomachs (e.g., dogs, cats, horses, pigs).
See the following poisonous plants websites for information about the Prunus spp.:
If there is a Prunus spp. tree in the field where the horse is to be pastured, I believe I would remove the tree or move the horse to a different pasture. This is especially so if it is a small pasture where the horse is more likely to come in contact with the tree or if other vegetation is sparse so that the horse would consider eating leaves off the tree.
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