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.
Vitamin D is responsible for proper absorption of calcium, so you need lots of this vitamin every day for optimal bone strength. Vitamin D-3 is the form found in many supplements and most foods that are rich in vitamin D, and it's the kind your body makes when you get sun exposure. Although D-3 is beneficial, too much can lead to toxic effects.