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For constipation you an add some canned pumpkin (not the pie filling) to the daily diet as this acts as a roughage. (2 tablespoons) You can also sprinkle bran over the food or you can give metamucil.
metamucil information and dosage: http://www.petplace.com/drug-library/psyllium-metamucil-etc/page1.aspx
If the dog eats any non food items such as raw hides, bones, sticks, rocks, etc. then it is not a good idea to force evacuation of the stool as you may be dealing with the start of a blockage of the intestines. symptoms of a blockage are sudden lack of appetite, vomiting, diarrhea or off color /wrong consistency stool and lethargy.
Causes for constipation http://www.petplace.com/dogs/constipation-in-dogs/page1.aspx