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.
It is vitamin D3 that is absorbed from the sun and aids in the process of calcium being absorbed into the body. Vitamin D is a fat soluble essential vitamin. It cannot be produced in the body on its own. You can get vitamin D through supplements, foods and, of course, from the sun.