Mildew in hay a problem?

[garyws]

Started feeding the new hay and after a few days noticed it had a lot of mildew smoking from it when tossed around. I've never had this problem with hay, ever, but we've had a lot of rain the last month or so. The hay has been stored in a barn with lots of ventilation.
I've heard of moldy hay being bad for pregnant cows but never anything about mildew. Is mildew hay safe to feed cows? I've fed it to my bull and so far no apparent bad effects.