How to reduce speed of character when collide with specific game object

Actually using the A* pathfinding in my project, I set it up. It’s working great.

My problem actually is I’m trying to reduce the speed of the character when collide with some specific object.
For example, I created some barrel object. They are not ignore by the pathfinding because there are not on a different layer. That’s correct because I don’t want my AI avoiding my barrel. But I want my AI reduce their speed when collide with them. My barrel are physic object so they are pushed.

How I can make my AI reducing their speed when collide with my barrel?
Is there a way to do it, or can someone help me to do it please?

Thanks in advance.

Could you use collision detection or set up a trigger?

http://unity3d.com/learn/tutorials/modules/beginner/physics/on-collision-enter
http://unity3d.com/learn/tutorials/modules/beginner/physics/colliders-as-triggers