If an adult dog with a normal appetite suddenly loses interest in dog food, there could be several reasons.
1. He is ill. If there are other signs such as vomiting, diarrhea, listlessness, coughing or congestion, see your veterinarian.
2. You have changed his food. If the food change is for the better, i.e. healthier, it may take a while for him to accept the new food, even several months. "Junk food" pups sometimes need incentives and gradual change to get with the "program." Mix 25% of the new food with 75% of the old for a few days and gradually increase the amount of the new food.
A healthy adult dog isn't greatly affected if he refuses to for one or more days. So if he stages a hunger strike against the healthier food (especially if it is already mixed with the old), put down a tablespoon or two of the new mixture. Leave it down for 15-20 minutes and then take it up. Generally after a day or two of this (close your ears against the howls of protest and pleading big brown eyes!), he will give in and dive into the new food.
Picture this transition as similar to a child who is used to eating fast food and snack foods day in and out. Your dog's taste buds have been conditioned in the same manner by comparable pet foods, but if you love him, you will insist that he eats something that is more nutritious and life-sustaining.
3. Has the food gone bad? This is always a possibility, especially in the heat of the summer. Both canned dog food and dry dog food can turn rancid, and sometimes a batch of bad food is accidentally poorly processed. Usually, dogs seem to prefer their food a little on the "gamey" side, but there are limits!
Never leave a can of food out of the refrigerator all day. It is a good idea to transfer the remaining contents of a can of food into a glass container if it will not be consumed in one day. Although it has several advantages, naturally-preserved dry dog food has a much shorter shelf life than chemically-preserved dry dog food, so check the expiration date.
4. Is he snacking on dog treats in between meals? Cleaning ice cream bowls or plates? Is Grandma, the Softy, visiting? Is he raiding the garbage or visiting unsuspecting neighbors? Is Johnny slipping him half a sandwich? Just like kids, he may be ruining his appetite for dinner. If he doesn't appear underweight, that is probably the case. Try to exert your authority!
5. Bring food to room temperature or add a bit of hot water before serving. Cold food has less of an appealing aroma.
6. It is very difficult to switch a dog over from eating canned food or homemade food to eating dry food. This is because the wet food is so much more palatable. If it is absolutely necessary that you do so, ease the transition by adding some canned food, unsalted meat, poultry broth or bits of meat to the dry food.
7. Free-feeding sometimes produces finicky dogs. Since they are able to nibble all day long, when mealtime comes, why should they get excited? There are several reasons why free feeding is not a good idea. First, dogs prone to obesity will get fat. Second, dogs with a tendency to bloat or stomach torsion need to have controlled feedings. Third, with the gastric juices churning all day long, the dog's digestive system has no time to rest and medical problems often arise.
Changing a "free feeder" to a "scheduled feeder" may take some discipline on your part. Your dog may not eat his whole meal the first few times you feed him the new way because he is expecting to nibble on it all day long. However, when he sees there are no in-between-meal snacks, he will learn to take full advantage of mealtime. Until he has adjusted (usually within two weeks), you will have to bear with vocal protests and devastating "poor-me!" looks. Stick to it and you will soon get him on schedule.