Herd the Yakows into their pen
It is also related to a trophy titled Yee Haw!, completing this mission will reward you with a bronze trophy.
After talking to the Farmer you'll have to move the Yakows into their pen. There's a total of five and they will move automatically if you walk up to them and keep following them to ensure they won't trail off. Punching or kicking won't have much effect though, once they are inside their pen they'll stay there.