Monensin is a feed additive used in cattle and poultry feeds to promote growth and although it is effective for its purpose in these species, it is highly toxic to horses.  Most cases of poisoning occur when a feed mill makes a mistake and monensin is added to horse feed instead of cattle feed.  However, toxicity can also occur when a horse breaks into a feed room and eats cattle or poultry feed containing it. Also, if a producer does not realize it is toxic and has a horse in the same pasture as cattle that are eating feed containing monensin, horses may be exposed. 

Erica Larson indicated at The Horse.com that monensin primarily affects the muscle cells of horses and can cause a loss of appetite, rapid or irregular heart rate, exercise intolerance, sweating, colic and even death.  Some horses that consume monensin eventually recover and return to their previous level of performance and it all depends on the amount consumed.  A concern is that since the heart is a muscle, it has not been shown how monensin affects equine hearts that survived so Dr. Clementine Gy from the University of Montreal did a study on affected horses.  She looked at 76 horses exposed to monensin that were in the acute phase of exposure in the last 2 weeks, horses that were exposed 15-45 days prior, and horses 4-10 months after exposure.  With a stress test, ultrasound and electrocardiogram, she found heart abnormalities in all horses recently exposed, 65% of those exposed 15-45 days ago, and 44% had heart abnormalities even at 4-10 months.  At 16 months after exposure, only 50% of the horses had returned to their previous function.  So, if your horse is exposed to monensin, call your vet immediately and provide rest for your horse.