Skeletal Depletion in Hens Laying on a Low-Calcium Diet

Abstract

It is generally accepted that the skeletons of birds and mammals act as a reservoir of minerals, in particular calcium and phosphorus, which can be drawn upon when dietary sources are inadequate for requirements. In birds these bone reserves are especially important during the egg-laying period. In the laying hen the rate of calcium deposition in the shell is often greater than the rate of absorption from the intestine, even when the calcium level of the diet conforms to accepted standards for egg production (Tyler, 1940). The immediate source of the additional mineral is thought to be the medullary bone contained in the marrow cavities of the long bones of the legs (Kyes & Potter, 1934; Bloom, Bloom & McLean, 1941 ; Bloom & Domm, 1941) and in other parts of the skeleton (Taylor & Moore, 1953). This highly specialized bone undergoes rapid changes during the egg-laying cycle, periods of intense bone formation alternating with periods of equally intense bone destruction (Bloom et al. 1941) and in birds that are in calcium balance the mineral reserves lost during the period of shell calcification are restored when active shell formation is not taking place.. On a low-calcium diet the minerals for the calcification of the newly formed medullary bone are derived from structural bone (Benoit & Clavert, 1945), which becomes progressively depleted, and presumably a similar state of affairs exists in birds that are in negative calcium balance, even though their ration supplies theoretically adequate amounts of calcium (Morgan, Mitchell, Ringrose & Lease, 1942).