Our knowledge of vitamin requirements of deer is also pretty sketchy. Vitamins are classified as either fat-soluble (vitamins A, D, K, and E) or water-soluble (C and B vitamin complexes). Fat-soluble vitamins are stored in the body, and can in some cases become toxic. Water-soluble vitamins are not stored and are needed by most animals on a daily basis. Fortunately for the deer, the microorganisms in the rumen (bacteria and protozoa) produce all the vitamin K and B complex the deer needs. Ruminants also have no need for vitamin C.
Vitamin A is converted from a compound in plants called carotene. Deficiencies of vitamin A have been reported in deer. Deficiencies can lead to blindness and poor reproduction. Unfortunately, we really don’t know much about the vitamin A requirements of deer.
There has been some work done on vitamin D. Circulating levels of vitamin D in the blood vary with the antler growth cycle in bucks. That makes sense because vitamin D is needed for calcium absorption and metabolism in all animals. We do not know the requirements for vitamin D in deer, and there have not been reports of vitamin D deficiency symptoms in deer.