I've also seen horses who died of hardware disease. Not from eating the metal but from getting cut up from being kept in junkyards and having infections that took over from lack of treatment. THAT place should have had the cops called on it!
When we bought our farm it had been a pig farm for 35 years. The children worked in restaurants and brought garbage home to feed the pigs. The place was filled with inedible garbage. There buckets filled with bent up eating utensils, piles of chipped plates, and stacks of cups and serving pieces - and that was just the stuff that might be used. The rest had been thrown in the pig pens and ground into the muck.
We spent YEARS cleaning the place up and hauled tons of garbage off the place. Even so when we built our house thirty years after buying the farm we still found junk at the house site. In addition to more eating utensils, we found an entire metal bed frame with springs, lots of broken crockery, and a few more buckets of junk.
Old plastic lasts forever - we still find condiment packets that are in one piece with the printing still legible.
Our work was worth it. After all that we never had a single horse show up with nails in their feet or with injuries from old metal.