Had a few hairy incidents when younger doing hiking competitions around Cheshire. We used to shout "I'm the f****r!" at them and wave our arms about; that seemed to work. Only had to ditch the rucksac and run once.

As I got older I got to thinking the profanity was unnecessary and it was just the shouting and positive body langauge that did the job, but I've camped fairly regularly only a working farm in the Welsh speaking part of Wales and they shout "Mother F****r" at the livestock when they're moving them about.