The 2005 NRC publication “Mineral Tolerance of Animals” changed my life and brought me to the point, years later, of blogging about water (weird, huh?). Like everyone else, I had half-baked assumptions (mostly incorrect) about whether minerals in water were or were not available to the animal consuming them. One sentence lead me to a much deeper exploration of the scientific literature on water chemistry: “Thus, knowing only the concentration of a mineral in water is an incomplete description as to the toxicity or availability of the mineral” (pg 470). The next paragraph goes on to explain that “an element can exist in water as a simple hydrated ion, as a molecule, as a complex with another ion or molecule, and as many other additional complexes (Stumm and Morgan, 1996)”.
In consideration of this concept, there are two laws that come into play: 1) that the resulting solution will be electrically neutral and 2) that the final solution will have the lowest possible energy state. To accomplish that, the ions in a solution will dissociate and re-associate in different ionic specie (combinations). If you add another ion to the solution, Na for instance, there would be a reshuffling of the ion specie until the solution was again electroneutral and in the lowest energy state.
For example, let’s look at some of the different ion specie of Cl in a solution. If you took a regular water sample that had 100 ppm of Cl (along with other typical minerals), that Cl would exist in several different combinations, or ion specie. Some of the possible combinations include: Cl-, FeCl+, FeCl++, ZnCl3-, MnCl+, etc. Each of these different ion specie will have a different solubility, hydration sphere and hence, availability to the animal.
There is a hydrogeochemical model, PHREEQC, published by the US Geologic Survey, that will calculate the concentrations of the various ion specie and their relative solubility in a given solution which satisfy the two laws mentioned above. For a dairy cow, the bioavailability of each of those ion specie will largely depend on its solubility (the Ksp, or solubility constant). So, unless we can calculate what all the various ion specie are, and determine their solubility, we can’t really make a good guess about their bioavailability.
And, those calculations are only in a solution with water and minerals. In cattle, this is further complicated by the presence of strong organic ions, like BHB or lactate, and many weak ions like amino acids, etc. These will influence the formation, concentration and solubility of the various ion specie.
So, all we can really say is that the minerals in water, or from the diet, once in the aqueous environment of the rumen, are electrochemically equivalent and so, will have the same bioavailability, because they will all be governed by the same electrochemical laws and scheme of ionic speciation.