The cyanide compounds in the twigs and older, wilted leaves are toxic (and possibly lethal) to horses and cattle.
In the spring of 2001 hundreds of thoroughbred horse foals where mysteriously miscarried or stillborn. The problem was traced to Eastern Tent Caterpillars that had fed on the many black cherry trees in the Lexington, Kentucky horse farm region. The caterpillars concentrated the toxic cyanide compounds present in black cherry foliage. Their feces contaminated the famous bluegrass pastures and was ingested by the grazing mares.
A spokesman for the University of Kentucky Agriculture Department reports: "The unusual weather pattern could have caused the cyanide levels in the trees to be higher..." The university recommends that horse breeders restrict access to pastures when caterpillar populations are high."--http://www.floridata.com/ref/p/prun_ser.cfm
Most animals can consume small amounts of healthy leaves, bark and fruit safely. Toxicity typically occurs when hungry animals consume large amounts of fresh leaves or small amounts of damaged leaves (as little as 2 ounces)."--http://www.canr.msu.edu/horseadults/publications/Toxic%20Plant%20Bulletins/Trees.pdf
If she was eating the bark, I would bet she was eating leaves as well or there could be an issue if you have the caterpillars in your area. Either way, the black cherry tree is a possible cause of the issue and it should be removed from the pasture or fenced so that she can no longer reach it.
Licensed Veterinary Technician