What is the mason of the decline of the number of the wild horses ?

At the turn of the last century, more than 2-million wild horses roamed free across public lands in the American west. But decades of poaching and culling decimated the herds, and by 1971, when they were granted federal protection through the "Wild Free Roaming Horse and Burro Act," there were only around 20,000 left. Today a limited number of mustangs still roam public lands in ten Western states, their numbers regulated by annual roundups by the Bureau of Land Management.  Late last year, without public hearings or debate, a provision was slipped into the federal spending bill that allows the BLM to sell thousands of these captured wild horses for slaughter. That’s prompted horse lovers to try to save them.