I have some beautiful Passion vines on my property. They surround my horse pastures and so far the horses have not touched the vines or fruit. I wonder if they are poisonous for horses, cows etc?
Don't know what types of passiflora you have but you may wish to read the following article.
Here is a link that might be useful: Horse Herbs
Passionflowers contain cyanogenic glycosides, which can be toxic to most animals. You might google cyanogenic glycosides and horses to more information regarding toxicity. My understanding is that they are more toxic to ruminants than animals with simple stomachs. Might also ask a veterinarian for his or her opinion.
Asking a veterinarian or other animal health professional is always sound advice and I agree with it.
As far as toxicity, I can only comment on the effect in humans and not in horses, but Passiflora incarnata has a long and relatively safe herbal use history. As a medicinal herb, passionflower is generally used for its calming effects. I do not know of other passiflora species being used herbally and so would doubt their safety for grazers.