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The eating of non food items is called pica and there are many speculations as to why a dog does this. Boredom, malabsorbtion issues, metabolic issues, diabetes, or iron deficiency. Unfortunately this can lead to an obstruction of the bowel which will eventually need surgery it can also lead to zinc poisoning. I owuld bring the dog in for a check up and get a complete blood screening. Here is more on this.
Zinc poisoning http://www.petplace.com/dogs/zinc-toxicity-in-dogs/page1.aspx