Ok-so I've been riding for 40 years, and teaching and training for 20. I have always based a horses' jump on his athletic abilities, scopiness, etc. I've seen the solid, 2'6 packers that can stretch to 3' but are better suited to the 2'6 job.
Bioactive vitamin D or calcitriol is a steroid hormone that has long been known for its important role in regulating body levels of calcium and phosphorus, and in mineralization of bone.
Although vitamin A's chief function involves the eyes and vision, it also plays important roles in the development of bones, including the teeth. In addition, vitamin A aids immune function and helps provide a barrier against infections of the skin, lungs and mouth.