Botulism cause of horses' death

It has been found in tests run on feed, the 100 horses who died in Summerfield had eaten a grass product called haylage, which is cheaper to feed to your horses than regular hay. The horses died from botulism. Apparently, cattle are not prone to botulism from haylage, but horses are. Read the entire story here. I have never heard of haylage. You do learn something new every day.