Dog constipation typically occurs as they get older, lack of exercise, and poor diet.  Constipation in dogs can also occur due to the ingestion of objects such as sticks, trash and other not easily digested objects.  Bones can also be difficult to digest.  Indigestible objects can lodge in the intestines causing blockage in your dog’s digestive tract thus preventing feces to expel. As with humans, water is absorbed from the stool if it remains in the intestines too long.  This causes the feces to become hard and difficult to pass.  As a result your dog may strain to poop and may withhold defecating for several days.  This only causes the stool to become more impacted.  If this goes on for too long, the bowels may become irreversibly stretched causing your dog to lose the ability to defecate.  This condition is known as “mega colon”. Male dogs that have not been neutered may also become constipated from the onset of prostate disease, as can dogs infested with worms.

Treatment

• Feed your dog high-quality dog food that has not been processed.  Also keep your dog well hydrated.  Dehydration can cause your dog to become constipation.
• Make sure your dog has plenty of exercise
• Add the fiber supplement, psyllium to the diet. Add 1/2 teaspoon to your pet’s food twice a day.
• Add roughage to your dog’s diet.  Add 1/2 teaspoon to 2 tablespoons of oat bran in your dog’s meal.  The amount depends on the size of the dog.  You can also mix a few table spoons of bran flakes cereal in the food.
• Add a few tablespoons of non-spiced canned pumpkin to the food.  Pumpkin is high in fiber.  This can be used for cat constipation also.
• If needed give your dog 1 table spoon of mineral or olive oil
• Try giving your dog an enema.  Use a regular enema bag filled with warm soap water.  This will clean out the lower intestine.  Use a laxative to clean out the upper intestine.
• Add 1/8 cup to a 1/2 cup of milk per to relieve constipation
• Add a little Aloe Vera juice well mixed in water or wet food. 
• Feed your dog wet dog food

If these remedies do not work within a few days, then you should check with your vet.   There are other conditions that can cause constipation including: bowel obstruction in the digestive tract, enlarged prostrate, hernias, kidney disease and a few other conditions.