Macro Mineral nutrient which, along with Calcium, is a major component of bone which holds 80% of total body Phosphorous, it is also required for production of the energy transfer molecules ADP and ATP plus production of phospholipid cell membranes, DNA, and phosphoproteins.
Phosphorous absorbtion from the gut averages ~35% and is depressed by high dietary calcium levels and formation of phytate compounds with phytic acid common in most plants. To a limited extent phytate phosphorous may be broken down by phytase enzyme in the hind gut where most dietary P is absorbed.
Forage feeds are not particularly rich in Phosphorous, but levels are higher in tropical than in temperate grasses. Seeds like beans are a good source as are grains and milling by products. Requirements are established for all classes of horse and are raised with increased exercise level, in young growing horses adding skeletal bone, and by lactation. Deficiency of Phosphorous will cause loss of bone strength and density called osteomalacia in adult horses and produce rickets like symptoms of long bone curvature and bowing in young stock. Serum P levels will be maintained as long as there are reserves available from the bones, sacrificing bone structure and strength to do so.
There is a critical relationship between dietary Calcium & Phosphorous which should be maintained in the ratio between 1.8- 2.2: 1.0 especially in young horses. When Phosphorous intake rises to equal or exceed that of Calcium, a nutritional fibrous osteodystrophy called Big Head can result. Care needs to be taken when feeding large amounts of grains and by products, particularly wheat bran which has an adverse Ca: P ratio of 1: 9 and can upset the overall dietary ratio quite easily. Compounded feeds normally have their Calcium: Phosphorous balance adjusted by the addition of inorganic sources such as Calcium Carbonate (lime) or Dicalcium Phosphate.