Introduction

The importance of calcium and vitamin D3

The eggshell is a highly specialized mineralized structure, which provides protection against physical damage and penetration by micro-organisms. The egg shell consists of the inner and outer shell membranes, the true shell and the cuticle. The crystalline layer of the shell, which is responsible for its mechanical strength, consists of more than 90% calcium in the form of calcium carbonate. Calcium is absorbed from the feed in the intestine. Provided that sufficient calcium (3.8�4.2%) is present in the feed, the process of calcium uptake, deposition and excretion is regulated by vitamin D₃ and its metabolites.

Vitamin D₃ is absorbed from the intestine in association with fats and requires the presence of bile salts for absorption. It is transported via the portal circulation to the liver, where it is accumulated. The first transformation occurs in the liver, where vitamin D₃ is hydroxylated to become 25-hydroxyvitamin D₃(25-OH D₃). This vitamin D₃ metabolite is then transported to the kidney where it is converted to the most active hormonal compound 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂ D₃). The production of 1,25-(OH)₂ D₃ is tightly regulated by parathyroid hormone (PTH) in response to serum calcium. If plasma calcium is low, PTH secretion is induced, which stimulates the hydroxylation of 25-OH D₃ to 1,25-(OH)₂ D₃. This compound will increase calcium absorption in the intestine, mobilize calcium from the bones and reduce calcium excretion via the kidney. If plasma calcium is high, first PTH secretion and then 1,25-(OH)₂ D₃ production are suppressed, which result in a reduction of calcium absorption in the gut as well as calcium resorption from the bones and an increase in calcium excretion. Therefore it is of utmost importance for an optimum egg shell quality to optimize calcium supply and secure sufficient vitamin D₃ activity available to the laying hen.

Despite adequate fortification of layer feeds with vitamin D₃, clinical signs of vitamin D₃ deficiency such as rickets or cagelayer- fatigue can frequently be observed in laying hens kept under commercial conditions. Such disorders indicate insufficient utilization of the dietary vitamin D₃, which can be counteracted by a special feed product such as ROVIMIX^� Hy�D^�.

Chemically, Hy�D^� is 25-OH D₃, representing the first metabolite in the cascade of vitamin D mobilization. In numerous studies, Hy�D^� has been demonstrated to support the homeostatic function of vitamin D₃, which is important to provide sufficient minerals for incorporation into the bone matrix as well as for optimizing the stability of the eggshell. Hy�D^� also helps to maximize bone mass before the onset of lay and thus prevents layers from a fatal demineralization of the bones, resulting in osteoporosis. Therefore, Hy�D^� is an effective and more flexible source of vitamin D₃ activity for optimizing vitamin nutrition and ultimately maximizing profitability of modern layer production.