We recommend to always use a wall stiffener when your stall surface's horizontal length is greater than 9’. Most stall fronts are an industry standard of 12’ wide – with a 4’ wide door. For this reason, we don’t often see or recommend to use a wall stiffener for a horse stall front.
That being said, we almost always recommend to add a wall stiffener to a horse stall partition. This will help add much needed rigidity to your partition.