For the scene where a tree branch breaks through the window at Aaron's house, the reactions from the actors were real. the special effects team designed a specific rig so they could actually send a branch crashing through the glass.
The sequence where the herd walks through a minefield was shot by combining practical and visual effects. For the shots of exploding walkers, the practical effects team made foam walker dummies with blood bags on top and then blew them up in front of a green screen.