Research is continuing in this area. Biological availability is usually expressed on a comparative basis. One source is selected as a standard and its availability assigned a value of 100. All other sources are then ranked in availability according to the standard source. Availability can be highly variable in natural feedstuffs and additives with the former dropping in concentration later in the season.
Inorganic sources tend to be more available over organic sources.
The quantity of minerals required will vary depending on the age, weight, health, species and type and level of production of the animal. Young animals absorb minerals such as Ca more efficiently than older animals, but they have higher mineral requirements. High rates of gain or milk production and poor health or parasitic burden will increase mineral requirements. Sheep have different requirements than beef cattle, with copper being the prime example. As Cu toxicity in sheep is swift and deadly, it is crucial that only minerals designated for sheep be used.