You may see nary a whisker nor a tail, but you know that mice and/or rats are in your stable when you spot their gnawed holes. These least welcome of barn visitors can be nixed if you build in or add rodent barriers to your stable and eliminate the easily available feed that attracts them. When building or remodeling, place a layer of metal mesh or "rat wire" between the frame of the feed room and the wallboard to physically stop rodents from chewing entry holes.
If rodents have already gotten the jump on you and chewed portals for themselves, now is the perfect time to act. For the most part, mice and rats move their operations outside during the warmer months, so the barriers you set up now will ruin their likely homecoming in the fall.
First and most important, clean up the feed room to eliminate the motivation rodents have for chewing their way through walls. Store grain in sealed, unchewable containers, and stack unopened feed bags off the floor on pallets in a rodent-proof bin or screened area. Sweep the feed-storage area as a matter of routine, and clean up spills immediately.
The second phase of your offensive is to search out and seal up all existing rodent holes, which are most likely to be at structural intersections -- where walls meet floors, ceilings and other walls. Rats also like to work in secrecy, so the holes are likely to be hidden behind something, such as a tack trunk or refrigerator. Seal off holes by nailing rat wire over and around them, by stuffing them with steel wool (which rodents won't pull out because it hurts their mouths) or by plugging them with concrete mixed with crushed glass. But unless you've already removed the food source within, the rodents will simply chew on through in another location.