The effect of high and low levels of dietary inorganic phosphate on the pre-laying storage of calcium and phosphorus and on the composition of the medullary and cortical bone in pullets.

There is considerable evidence that 7-10 days before a pullet begins to lay a new system of secondary bone is laid down, under the combined influence of androgens and oestrogens, in the marrow cavities of all the bones in the skeleton in which haematopoietic tissue is present (Taylor & Moore, 1956). This medullary bone forms a highly labile reserve of minerals which are mobilized during the calcification of the egg-shell. During the immediate pre-laying period, greatly increased amounts of calcium and phosphorus are retained from the food for the calcification of the medullary bone. Common (1933, 1936, 1938) found that with a high-Ca ration (approximately 2.0% Ca) the Ca : P ratio of the retained minerals was often far higher than that of normal bone, but not when a low-Ca ration (approximatelyo-26 yo Ca) was given. The effect of a high-Ca ration during the pre-laying period was confirmed by Taylor & Moore (1954). While the medullary bone is developing the ovary and oviduct show phenomenal growth, the ovary increasing in fresh weight from 1.0 to at least 60.0 g and the oviduct from 2.0-3.0 g to approximately 70.0 g, and in laying pullets these tissues together contain approximately 0.35 g more P than in pullets of the same age and weight which have not entered into reproductive activity (Taylor & Moore, 1955). The Ca:P ratio of the mineral actually available from the food for the calcification of the medullary bone is therefore even higher than the balance figures would suggest, since some of the retained P is required for the developing ovary and oviduct. Nevertheless, the Ca:P ratio of the medullary bone of pullets maintained on a high-Ca ration is within the range for normal bone (Taylor & Moore, 1954) so it is clear that P must be available from elsewhere in the body to counterbalance the P deficiency in the absorbed minerals. The only sources of additional P for the calcification of the medullary bone are the cortical bone and soft tissues, and these sources have been studied in the present work. At the same time, the hypothesis that the rations on which retention of bone-forming minerals with a very high Ca:P ratio occur are deficient in available P, owing to the fact that a large proportion of the P contained in these rations is in the form of phytate, has been tested.