I've never heard of horses having them till yesterday. I stopped by a friends barn real quick so she could grab her helmet. While there, walking around I noticed every horses stall had a blue cobalt block along with a red trace mineral block. I asked her how that was and she said that the barn owner feeds alfalfa hay which is known to be copper deficient, so the mineral block gives copper and the cobalt helps the horse keep the copper in their systems longer. I have never heard of this before. All I could picture in my head was a horse storing to much copper and becoming a copper horse shaped rock. (o_0) Does anyone else feed cobalt salt blocks to their horses?