It depends on your Marksmanship skill, the quality of the bow, and the quality of the arrow.
I would guess two shots should down a cow with all at 50.
I strongly recommend you try using a sling first though. It is easier to target and faster to use. I have 50 marksmanship, a Q 100 bow and Q 60 plus arrows and I prefer to use a sling.
Good luck and good gaming to you.
edit - It is usually easier to get higher Q bone arrows and I think a bone arrow is generally better (in game, not in Real life

).