The bio-availability is the factor which converts physiological requirements to feeding standards, and is a product of availability and absorption efficiency, but contains in many cases also a security margin for the uncertainties that might be. There can for instance be uncertainties in the determination of the availability of minerals, a variation in the amount of minerals in the feeds and in the validity of the mineral needs.

The bio-availability is a factor between 0.00 and 1.00.

The feed standards are determined by use of an assumed bio-availability, for instance 0.55 concerning phosphorus - see the calculation example below.

The availability of a given mineral source can be defined as the fraction of the mineral which is dissolved into mineral-ions and is available for absorption in the digestive tract. The availability is a factor between 0.00 and 1.00. Many circumstances influence the availability of minerals:

• Feed type: Minerals in concentrated feeds are generally more available than those in roughage feeds.
• Feeds digestibility: The availability is much dependent on the digestibility of the cell walls as the minerals are incorporated in the cells.
• Feed treatment: Heat or formaldehyde treatment has a strong negative influence on the mineral availabillity.
• Feeding plan: A high level of digestible protein in the ration increases the amount of NH₃ in the rumen. The increased level of NH₃ can restrict the ability of the rumen wall to transport Mg, and provoke a coagulation of magnesia-ammoniumphosphate in the rumen.
• Used mineral salts in the mineral supplements: It is for instance widely known that white Norwegian MgO has a much higher availability than brown Chinese Mg(I). Oxides are normally with a lower availability than phosphates, and chelated minerals with a higher availability.
• Time dimension: The availability is normally correlated with the time the mineral salt has been in the rumen. The availability is normally evaluated on basis of 16 hours stay in the rumen, based on average time for feed to pass the rumen.
• Interaction: The figure below shows how the different minerals influence the availability of other minerals. The influence is indicated by the direction of the arrows. An excess amount of calcium will have a negative influence on the availability of magnesia, and vice versa. Balancing of the ration is therefore not only a question of ensuring the mineral norms as a minimum are fulfilled; it is also important to avoid excess amounts of individual minerals, or, with other words, to assure the correct ratio between different minerals in the ration. It is for instance seen from the figure below, that phosphorus has a great impact on the availability of many other minerals.

The availability is normally measured by the nylon bag technique, whereby it is possible to measure the availability over time.

The absorption efficiency can be defined as the probability that an available mineral-ion in the digestive tract by physiological processes is transferred into the blood vessels. The absorption efficiency is a factor between 0.00 and 1.00. The absorption efficiency in the intestines is an important component in the homeostatic regulation of ruminants mineral concentration in the blood. The absorption efficiency is found by comparison of mineral availability with results of dose-response or balance-trials.