One rattlesnake can cause a stampede. In certain parts of the country there are places where large concentrations of rattlesnakes are present. Plenty enough to cause a large number of cattle to stampede/scatter -- making the cowboy's job much more difficult and deserving of praise if well done.

It is of course true that a cow usually wins a confrontation with a snake by (mostly accidently) stepping on it.

But you forget two important factors: 1) Cows have ears 2) Rattlesnakes are named after their rattles for a reason.

If you ever surprise a rattlesnake you'll find that their rattle is (assuming the snake is old enough to have a decent size) very very loud and quite good at making mamals with ears run the other way.