Use the "Brute Force" skill to Tip Cows. Press Left Trigger>Press LB or RB to highlight Cow>Move radial wheel to skill>press A
There are a few cows in the livestock stable in the East Fields through the AG Center. The faster you kill the enemies there the more cows will be left alive to tip over. There are also cow in the "infected farm". Talk to Sue to get him to mark the farm on your map.
The cows can take damage from your parties shots so don't aim towards them.