Salt and Trace Minerals for Livestock, Poultry and Other Animals
BIOAVAILABILITY OF TRACE MINERAL SOURCES
Mineral bioavailability is defined as the proportion of the element consumed that is utilized for a biochemical or physiologic function (325). For a mineral to have high bioavailability it must be readily absorbed and easily assimilated by the body. Consequently, bioavailability is influenced by the chemical form of the mineral, the amount in the diet, the amount stored in the body, the concentration of other minerals in the diet, and the health age, and physiological state of the animal to which it is fed. To accurately assess bioavailability, more than one indicator of nutritional status should be monitored. Generally, the animals must be fed a deficient diet before or during the study to which graded levels of the mineral source in question are added (325). Although growth rate, milk production, egg production, etc. are economically important criteria to compare mineral sources, they may not reflect differences in bioavailability. Indicators such as metal-dependent enzyme activity, metallo-protein concentration, chemical balance and pathologic signs are more likely to reflect differences in bioavailability. Miles and Henry (338) have an excellent review of factors to consider when measuring trace mineral bioavailability.
Relative bioavailability will also vary between species. For example, phytate phosphorus is readily available to ruminants, but poorly utilized by mongastric animals. In contrast, copper is much more available to non-ruminants than to ruminants. Consequently, it is risky to predict relative bioavailability of different mineral sources without having measured it for the animal in question.
Knowing the approximate bioavailability of a mineral source is essential to being able to estimate the amount required in the diet. For example, Lawrence (303) used research data to estimate that a 500 kg horse needs 16 mg of bioavailable copper per day to replace endogenous loss. If the availability of the copper source was 70%, then 23 mg per day would meet the requirement. If the availability was 35%, then 46 mg of that cooper source are needed to meet the requirement. The amount of supplemental copper required in the diet is doubled due to differences in bioavailability. In addition, twice as much copper will be excreted into the environment with the low compared to the high bioavailable sources. Lawrence (303) summarized nine research studies measuring apparent copper digestion. The values ranged from a low of 9% to a high 48%. The amount of copper required and the amount excreted could be vastly different for the same horse fed these different diets.
When manure is applied at high rates or composted and then applied at lower rates, all of the trace mineral excreted are incorporated into the soil. Agronomists have begun to study the availability of the trace minerals in the soil and its impact on the concentrations in plants. Initial research suggests that the amount of copper and zinc in the soil will influence the amount taken up by some crops but not others (326). Consequently, feeding trace mineral sources with high bioavailability is good for the animal and good for the environment. Bioavailability data for the common trace mineral sources has been summarized for swine (327, 357), cattle (328, 337, 357) and poultry (338, 357).
Copyright: 2006