Over here it is normaly well rotted cow manure or non organic fertilisers. Sometimes you will get a farmer using calcified seaweed.
My ponies have been on adlib haylege in the field for the last 13 years, we get it from the same supplier year after year and it is always still good after 10 days if there is any left.
We recently had to send a bale back when it was too wet and too sweet smelling. He took it back and fed it to his cows!