What is broom tail in a horse?

In the late 19th and early 20th centuries, wild horses were increasing in numbers and competing with cattle for available grazing on the open range. To remedy the situation, cowboys would capture and castrate the wild stallions. Part of the process involved trimming the tip of the tail so that it resembled a broom, rather than a shaggy pony tail. That way, it could be determined at a distance that a particular horse had already been captured.