I've seen this question quite a bit. Can the cost of caring for your barn cats be written off for your business? The answer is... it depends.
Rat and mice control is a serious issue on a farm or ranch. Rodents can destroy crops, feed, wiring, and other structural components on your farm. Rodent control is absolutely a necessity to keep the farm in working order and animals healthy. Just like you can write off rat poison or mouse traps you can write off the costs to maintain a barn cat whose purpose is pest control.
A lot of us understand the benefits of having barn cats versus using bait or other types of rodent control. It is also reasonable that the barn cat(s) must be fixed so as not to populate beyond a beneficial number and should be vaccinated so they do not spread diseases to other animals or people on the farm.
Supplemental food, flea and tick control, deporting, etc are all part of maintaining a barn cat that can adequately hunt rodents and do its job on the farm. With this is mind the costs associated with these activities are expenses that can be written off. But remember we're talking for barn cats being kept for pest control, not for pets.
Keep in mind that the cost of rodent control would be much larger without the barn cats. Caring for the cats can be a more cost efficient and safer method of rodent control.