BS - Myth.
The west was just as populated with sheepherders. It was part of ranch work, and any ranch hand that wanted work, worked with sheep when the job required it.
This myth of cowboy is just a glorified ranch hand.
As for Brokeback Mountain, well that POS homo movie just further explodes the myth of cowboy vs. sheepherder.
That with one really quick search. More can be found if you want to look.