The seeds of box elder trees, also called boxelder maple, can be fatal to horses that consume them. The seeds can cause a disease called seasonal pasture myopathy (SPM), which is triggered by the ingestion of Acer negundo. Most prevalent in the Midwestern United States, symptoms come on rapidly and the fatality rate for affected horses is about 70 percent. Though recognized for years, the cause of SPM was not pinpointed until 2013, reports The Horse.
The amount of toxin in the seeds can vary, so there is no hard-and-fast number of seeds a horse must ingest to become ill; it can vary from them less than 100 seeds to several thousand to make the horse sick. Signs of SPM include reluctance to walk, stiffness, muscle tremors, depression, sweating, dark urine, colic, weak muscles or collapse.
The seeds of the box elder are wing-shaped and can travel a great distance on breezes; though horses generally do not eat the seeds, if pasture is sparse, they may be inclined to sample some. The Horse suggests pasture maintenance, supplementing with hay, and not overstocking pastures can reduce the chances a horse may ingest too many seeds.
Read more at The Horse.
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