did they use trip wires?

I've been watching the first episodes and began to wonder with all the horses going down. I found this on thehorse .com and that made me wonder even more.

"That worked until 1966, when the U.S. Supreme Court ruled that a branch of MPAA charged with regulating movie content was practicing censorship and the regulatory power of MPAA was broken. With MPAA's regulatory power went the AHA's role as an overseer for movie animals.

There was something of a hiatus in the humane treatment of animals on production sets from 1966 until 1980. Often, AHA safety representatives weren't even allowed on sets. Animals were frequently overworked and kept in unsafe environments, and the trip wires were once again used."