Minerals have both structural and regulatory functions. They are needed for bone and teeth formation, hormone actions, enzyme activation, and water balance. They are a structural part of hormones (iodine in thyroxine as an example) and hemoglobin (iron).
The minerals needed in largest amounts (macro minerals) by the dairy cow include: calcium, phosphorus, magnesium, sodium, chlorine, sulfur, and potassium. Sodium and chlorine usually are provided in the form of salt. Minerals required in small amounts (trace minerals) include iron, copper, manganese, zinc, iodine, cobalt, and selenium. Other minerals may be required in very minute amounts, including molybdenum and fluorine. Table 4 lists functions, deficiency symptoms, and feed sources of minerals. Feed tables A-9, A-10, and A-11 present the amounts of calcium and phosphorus in some common feeds. Table A-12 shows the composition of calcium and phosphorus supplements.Table A-13 gives major or macro mineral composition of some common feeds and mineral supplements. Tables A-14 and A-15 list recommended minimum and maximum mineral levels in diets for cows and heifers.
Sodium, potassium, chloride, and sulfur are strong ions and have a significant influence on acid-base regulation in the cow. When cations (minerals with a positive charge) exceed anions (minerals with a negative charge) in a solution the pH will be basic (above 7) and when anions exceed cations, the solution will be acidic (pH less than 7). The dietary cation-anion difference (DCAD) is the milliequivalent (meq) difference between the major cations and anions in a diet. The DCAD can be calculated when the percentages of Na, K, Cl and S (DM basis) in the diet are known:
DCAD, meq/100 grams of DM = [(%Na/.023) + (%K/.039)] - [(%Cl/.0355) + (%S/.016)]
Feeding a diet with a negative DCAD (anionic diet) in late pregnancy may reduce milk fever problems through alterations in calcium metabolism. Lactating cows need a high cationic diet to replenish the outflow of minerals in milk.
Many commercial mineral supplements are available. When a mineral supplement is chosen, mineral(s) needed to balance the diet should be considered. A supplement that properly meets the need should be selected. Cost per unit of the mineral needed should be the primary consideration. This can be determined by dividing the cost of the supplement (per hundred lb or per ton) by the number of pounds of element in the supplement. (Example: 15 percent phosphorus ÷ $15.00 per cwt = $1 per pound of phosphorus.)
Trace mineralized salt is a common source of salt and trace minerals. The adequacy of trace mineralized salt to meet requirements of an animal will depend on the potency of trace minerals in the salt and productivity level of the animal. Trace mineralized salt does not contain appreciable amounts of magnesium, sulfur, or potassium. In many diets, these minerals are present in adequate amounts and do not require supplementation. However, if they are needed, proper supplements will have to be provided. Magnesium oxide is a common source of magnesium. Sulfur can come from elemental sulfur, sodium sulfate, magnesium sulfate, or potassium sulfate. Potassium is available in potassium salts such as potassium chloride or potassium sulfate. Many commercially formulated feeds contain trace mineral information on the feed tag.
Mineral excesses should be avoided because of interaction with other minerals and possible toxicity. For example, even though copper is needed in small amounts, it can be toxic. Lead causes toxicity and can be a problem when stanchions and stalls are painted with a lead-base paint. Such paint should be avoided in painting barn equipment and other items to which cattle have chewing access.
Source: University of Minnesota Extension
