Cow I lost bloated on an alfalfa/grass mix of hay. (Lost her inside of 20 minutes from first noticing a problem, BTW.) Had one calf bloat on grass pasture. Have had some problems with cows on alfalfa hay fields.
Usually caused by legumes - I don't believe we have clover in my area but we do have alfalfa - and I've heard folks point to a lack of free choice feed as a culprit for bloat. (They gorge themselves when you put out feed, then are empty 'til next feeding, gorge themselves again, etc.) Not a problem if they're running on pasture but it is if you're feeding hay.