If you have barn cats you must feed them or they'll be too weak to hunt.
My two girls are also spayed, because feral toms will hang around and impregnate your female barn cats if you don't take care of that little problem. They'll spray, eat the food you put out for your own cats, and fight with other toms, neutered males and females not in heat. Just complete ugliness all around.
Mine get dry food in the morning and canned at night. They are both superb huntresses, and between them and the black rat snakes have kept the rodent population under control.