Is "Aslike Clover" poisonous to horses?

I heard this type of clover is poisonous to horses - but it makes up quite a bit of my horses' grazing pasture for years. What kind of poisoning is it? I've read alot of horsebooks that say clover is NOT poisonous - it just has bacteria that reacts with the horse's saliva and makes it turn green and foamy, but is not dangerous. Insight or recommended websites would be helpful.