The following is a review of plant, animal, and environmental factors affecting the availability of minerals in feed. These factors should be duly considered when planning a feeding program to achieve better animal performance.
Differences between legumes and grasses have been recorded in apparent mineral absorption. Application of magnesium fertiliser may increase or decrease absorption of magnesium from different grass species while absorption of magnesium from legumes does not respond to magnesium fertiliser application. There are also differences between species of grass in terms of cobalt absorption by ruminants. It was observed, for example, that the availability of cobalt differed between 2 varieties of the timothy grass Phleum pratense, despite the similar plant cobalt concentrations.
The phosphorus and potassium, but not the calcium contents, of crop and forage plants decline markedly with advancing maturity. The concentrations of magnesium, zinc, copper, manganese, cobalt, nickel, molybdenum, and iron also fall, but rarely to the same extent or degree as phosphorus and potassium. Whereas the concentration of silicon usually increases with plant maturity. The shedding of the seed is normally responsible for losses of many minerals so that the material remaining, e.g. the straw, is a poor source.
Certain minerals have antagonistic interactions by forming chemical bonds that inhibit absorption from the intestinal tract. Such interactions are crucial to producers under practical feeding conditions and have, thus, stimulated a great deal of investigation. For example, elevated dietary calcium can induce a deficiency of zinc, especially if there is a borderline deficiency of zinc resulting from the use of plant proteins in the diet. Also, there may be a chemical reaction between copper and sulphur involving the formation of the insoluble complex copper sulphide. Competition between minerals such as cobalt and iron for carriers in the intestinal wall has also been observed. At the cellular level, there can also be antagonistic processes resulting in the formation of insoluble complexes from copper, molybdenum, and sulphur, as well as competition among ions for the active centres in enzyme systems as occurs with magnesium and manganese. Many of such interaction problems may be alleviated through a proper balance of dietary minerals and an adequate supply of those minerals that are most affected.
In cases where distiller grains are frequently used as a source of undegradable protein, the mineral supplement should not contain phosphorus or sulphur as they are already present in high concentrations in the distiller grains. The supply of these minerals needlessly should, therefore be avoided because of cost and toxicity concerns. On the other hand, the distiller’s grain is deficient in calcium and copper and the addition of these elements to the diet is thus prompted in this case.
It was observed that the net absorption of zinc, for example, decreased from over 50% in very young calves to 12% in mature cows. The propensity of trace elements to form complexes with anionic ligands contributes greatly to this situation. Likewise, the availability of copper to milk-fed lambs decreased from 71% to 47% just before weaning, and to 11% 2 weeks after weaning. Sheep in which rumen function was well established had copper-availability levels of 4-8%. The reduction in copper availability as sheep age appears to be due to the production of sulphide by the rumen microorganisms which allows the formation of much less available compounds such as cupric thiomolybdate and copper sulphide.
The depletion of body mineral reserves caused by a period of dietary inadequacy also invokes homeostatic influences, which involve increased absorption and/or decreased endogenous excretion. The occurrence of mineral depletion such as calcium and phosphorus, and zinc during pregnancy and lactation resulted in the animals being able to absorb twice the amount absorbed by non-depleted animals.
In a study on sheep, utilisation of some minerals such as calcium and phosphorus was severely affected by parasite infestation due to endogenous loss and a reduction in the absorption of these minerals, which adversely affects bone formation and causes rickets and other diseases. There is also a reduction in copper absorption under parasitic infection. The infection by O. circumcincta can affect the metabolism of copper, especially because of the increased abomasum pH. The reduction of the abomasum acidity reduces the copper solubility and therefore the captivity of the mineral by the liver.
Heat-stressed animals present lower serum concentrations of phosphorus, sodium, potassium, and zinc, probably because of reduced voluntary intake during heat stress and hence the intake of these mineral elements. Therefore, increasing elemental concentrations in the diet is a strategy to offset the reduction in DMI to meet mineral element requirements. Of greatest importance is potassium (K) since more K is required to counteract losses through milk and sweat, thereby making the use of supplements necessary, especially with the increased use of concentrates for heat-stressed cows since most concentrated feeds are potassium-deficient. Potassium supply should be considered along with the supply of sodium (Na), because of the increased urinary losses of Na during heat stress, as a result of decreased levels of blood aldosterone. In one study, superior milk production and other biological functions of heat-stressed cows were obtained when supplementing the diet with 1.0 % K and 0.67 % Na.
Adequate amounts of iron should be supplied at high altitudes where atmospheric pressure is lower and there is less oxygen in each breath. In this case, iron would increase the oxygen-carrying capacity of blood and facilitates its utilisation by the cells, and would hence improve performance and adaptation to high altitude. It should be noted though that most conventional cereals and legumes used in feeding contain non-heme iron which is poorly absorbed and utilised by animals. Therefore, the diet of high-altitude animals should be duly manipulated to contain sources of heme-iron such as those of animal origin. Heme is a porphyrin ring combined with ferrous iron and protoporphyrin IX and is an essential prosthetic group in proteins that is necessary to perform diverse biological functions like haemoglobin and myoglobin.
References are available from the author upon request.