Landmark Inventions

This year, I'm going to do a project for National History Day. The topic is "Innovative Inventions", or something like that. I want to do something with horses. So, what invention do you think changed the American horse industry forever? Any ideas are loved.

The tractor greatly reduced the number of draft horses in the U.S. And relegated those left to the Amish or the hobbyist. The same could be said for the passenger car and saddle horses. As far as the actual use of horses I would say that blevins buckles on western sttirup leathers and girths with tongue buckles are almost universally used today but were rare only 50 or 60 years ago.