Calcium is essential for bone formation, muscle contraction, proper functioning of the nervous system and egg shell formation.Phosphorus is also essential for bone formation, metabolism of carbohydrates and fat, and calcium transport in egg formation.
The diet provided to keep a bird healthy must contain adequate calcium and phosphorus and they must be present in the proper ratio. The desired calcium to phosphorus ratio is 1.5 : 1. Additionally, vitamin D3 is essential for the proper utilization of calcium and phosphorus.
Seed diets, in addition to lacking many required nutrients, contain too little calcium and too much phosphorus.
An endocrine gland known as the parathyroid gland is important in maintaining calcium and phosphorous homeostasis (balance). The parathyroid glands are located at the thoracic inlet, just below the thyroid glands.
When plasma calcium levels decrease, the parathyroid glands secrete a hormone known as parathormone (PTH). PTH elevates blood calcium by causing bone resorption, increases absorption of dietary calcium in the intestine and increases renal excretion of phosphorus.
Low blood calcium may occur with insuffient dietary calcium (seed diet), increased demands made by egg laying, and in renal disease. Young growing birds have a greater demand for calcium than mature non-egg layers.
Inadequate calcium in the diet or an improper calcium to phosphorus ratio leads to a condition known as secondary nutritional hyperparathyroidism in which parathryroid homone secretion increases and the parathyroid glands enlarge. Excessive production of PTH leads to progressive demineralization of bone. Fibrous tissue replaces the reabsorbed bone, resulting in bowing and bending of bones and “pathologic” fractures.
Breeding hens will lay thin shelled eggs and develop weak bones with folding fractures. Their babies will also have weak, poorly mineralized bones that develop folding fractures as well.