Antler velvet (layer covering the antler) contains proteins and fats. The hard structure of the antler is different. Although sometimes incorrectly referred to as horns, deer antlers are cast and regrown annually. Horns, on the other hand, grow continually much like hooves. Another difference between horns and antlers is that horns, like hooves, are composed of keratin, whereas antlers are composed of bone. The actual composition of antlers depends upon their stage of growth. Growing antlers are 80 percent protein and 20 percent ash. Hardened antlers are roughly 63 percent ash, 22 percent calcium, 11 percent phosphorus and 4 percent organic matter. Antlers are most dense on young deer and tend to become more porous as the animals grow older.