Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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Dr. Getty
Hi Heidi,
This could certainly be a factor since a copper deficiency can result in anemia. And, the chewing is an attempt to add this mineral to the diet.
Minerals interact with one another and too much of one, can cause a deficiency in another. Copper needs to be balanced with zinc and iron. So, since the horses stopped chewing as a result of copper supplementation, it appears that this mineral was out of balance. But, care must be taken when supplementing one mineral, since another deficiency can result.
All the best,
Dr. Getty 
Where are you from? Bayfield, CO