Realities of Mineral Nutrition Of Cattle: (Part B) Dietary Interactions of Minerals

Although it is well known that the absorption and utilization of a particular mineral can be significantly affected by the presence of other minerals in the ration, by far the greatest influence on availability is due to the level of intake of the mineral in question.  The classical example to illustrate this point is the absorption of calcium.  Although 98% of the calcium is contained in the bones and teeth of animals, the remaining 2% is essential for the transmission of nerve impulses, muscle contraction, blood clotting and numerous enzyme activities.  For this reason, the concentration of calcium in blood is maintained within a relatively narrow range.  This regulation is achieved through a system of hormones and target organs which release and take up calcium depending on the concentration of circulating hormones.  This is illustrated in Figure 1.

Fig 1.  Factors involved in maintaining blood calcium concentrations

           (Classical control by feedback loop)

In this way calcium availability decreases as the calcium intake increases.  When the animals requirement for calcium is exceeded, absorption ceases regardless of intake.  As an animal ages not only does calcium absorption from the  gut decline but the ability of the  skeletal bone cells to release calcium also diminishes (calcium absorption has been recorded at virtually 100% in a milk fed calf and falls to approximately 20% of intake in older animals).   For these reasons there is a higher incidence of uterine prolapse and milk fever in older cows.

Although calcium absorption is controlled by this hormone regulated feedback loop, the efficiency of absorption of other minerals is influenced directly by the dietary intake of the mineral in question.  This is sometimes expressed by the statement that “a deficiency of a mineral sensitizes its absorption”.  The minerals phosphorus, iodine, iron and especially zinc exhibit this phenomena.

There are however very clear interactions between different mineral elements which influence availabilities.  These interactions have been investigated for many years and a summary of known interactions is presented below in Table 1.

Table 1 – Minerals showing interactions with other minerals/nutrients which can influence availability.

Mineral	Interactive Nutrient
Calcium	Phosphorus, Zinc, Silica, Sodium
Phosphorus	Calcium, Zinc, Sodium
Magnesium	Calcium, Potassium, Sodium, Iron, Ammonia & Fatty Acids
Sulphur	Magnesium
Sodium	Calcium, Phosphorus
Zinc	Copper, Manganese, Calcium, Phosphorus, Cadmium, Lead
Copper	Molybdenum, Sulphur, Zinc, Iron, Calcium, Cadmium, Lead
Manganese	Calcium,Phosphorus, Zinc
Iron	Calcium, Phosphorus, Cadmium, Cobalt
Molybdenum	Sulphur, Copper
Selenium	Sulphur

However, the mineral interactions which have the most impact on the health and well being of animals are as follows:

(i)                  Copper, Molybdenum and Sulphur

(ii)                Copper and Zinc

(iii)               Magnesium, Potassium and Sodium

 

COPPER, MOLYBDENUM AND SULPHUR

The antagonism between copper, molybdenum and sulphur is without doubt the most widely researched example of adverse mineral interaction.  Copper deficiency has become recognized as one of the most important nutritional factors confronting livestock around the world.  In countries which have high rainfall, organic peat type soils which are poorly drained, molybdenum levels in pasture can be very high (20-100mg/kg DM).  The availability of dietary copper is reduced by this high molybdenum concentration especially when sulphur levels in forage are low.  In contrast, on very high sulphur intakes which generate a good deal of hydrogen sulphide in the rumen, molybdenum can actually help to prevent copper deficiency.   High levels of hydrogen sulphide produced in the rumen result in the formation of copper sulphide which is unavailable to the animal and is excreted in faeces.  Molybdenum in this situation helps to prevent the production of hydrogen sulphide and thus prevents the formation of insoluble copper sulphide.

Unfortunately to really make the situation even more complex, at low/normal sulphur levels in a herbage which is also low in copper, the addition of molybdenum can actually cause copper deficiency.  In this situation free from interfering high levels of rumen sulphide, the copper and molybdenum ions can complex together in the rumen to form copper molybdate which is excreted through the kidneys.

Superimpose on this situation the ingestion of soil and its mineral content, (especially iron) it is easy to see the complexities of the interactions which have led to conflicting results under field conditions.

 

COPPER AND ZINC

Copper and zinc both compete for the same absorption sites in the gut and for this reason a mutual antagonism exists.  High levels of copper interfere with the absorption of zinc and vice versa.  Cattle grazing temperate pastures in the south of the country during the summer months can run the risk of facial eczema caused by fungal toxins.  Zinc given in the feed or water supply has been shown to be highly effective in preventing liver damage caused by these toxins.  However, prolonged treatment with high levels of zinc will also predispose the animal to copper deficiency.

Finishing diets for beef cattle usually contain commercial trace element mixtures.  Of the seven minerals (cobalt, copper, iron, iodine, manganese, selenium & zinc) considered essential for feedlot cattle, zinc concentrations usually show the greatest variations in such mixes.

In one American survey of feedlot consultants, Galyeen (1996) observed that zinc fortification ranged from a low of 24-30 mg/kg of feed to over 300mg/kg.  Many Nutritionists believe that high dietary zinc levels are necessary for improved immunity, hoof health and carcass quality.  The scientific evidence however, for beneficial effects of super nutritional levels of zinc above the levels recommended by the NRC (30-40 mg/kg feed) is scanty at best.

High levels of zinc in such situations may not only be antagonistic to copper availability but also impose an unwanted environmental burden by increasing zinc levels in manure.

Limestone additions to diets have also been implicated in a reduction in zinc availability to the beast.  This is certainly true in monogastic animals.  However, several good quality research studies have revealed very little influence of calcium carbonate on the availability of zinc in cattle.

As stated previously, zinc is one of the minerals which is absorbed more efficiently when dietary concentrations are reduced.  It is questionable whether high levels of zinc in feed are really necessary except in circumstances such as the prevention of facial eczema in cattle.

 

POTASSIUM, SODIUM AND MAGNESIUM

Although magnesium is a vital element necessary for the well being of animals there appears to be no hormonal regulatory system as in the case of calcium.  In spite of this, magnesium related disorders do not exist in simple stomach animals.  In cattle however, low magnesium disorders (hypomagnesaemia) are very common.  This is particularly true of cattle production systems based on high nitrogen grasses in temperate areas or in green oat finishing systems in central and southern Queensland.  Tropical grasses usually contain higher levels of magnesium than temperate grasses.  Magnesium is absorbed primarily through the rumen and omasum (book organ) in both cattle and sheep.  Absorption of magnesium is markedly decreased by high potassium intakes but enhanced by high sodium intakes.  Young grasses are low in sodium and due to intensive use of potash fertilizers usually contain high levels of potassium (3-5%).  For this reason cattle in the south are at greatest risk from hypomagnesaemia especially if the areas are subject to occasional low temperatures which promote a decrease in magnesium concentrations in leaf material. (Magnesium is withdrawn into the root of the plant during periods of cold stress).

High potassium intakes prior to calving predisposes the animal to disorders centered around poor calcium metabolism.  Potassium itself does not interfere significantly with calcium availability.  It is the mobilsation of calcium from bone which is reduced when potassium levels in diets for dry cows are excessive.

 

BIOCHEMICAL CHANGES IN THE COW IN RESPONSE TO AN INCREASED NEED FOR CALCIUM

On the day of calving the cow can produce 10 litres or more of colostrum containing in excess of 25g of calcium.

An insufficient rate of calcium release from body reserves coupled with high output of calcium from the body is the prime cause of milk fever.  However, as cows become more and more productive and therefore under greater metabolic stress, we are seeing an increase in the incidence of subclinical milk fever or "sad cows".   Dry matter intake in such animals is generally reduced and milk production is below target levels. 

Insufficient calcium mobilisation at this time also can result in increased difficulty in calving, increased retention of afterbirth, rumen stasis, displaced abomasum and increased mastitis. 

First lactation cows almost never develop milk fever.  They may experience some degree of hypocalcaemia during the first days of lactation but their intestine and bone adapt rapidly to the calcium demands of lactation.  As the cow ages however:

• The adaptation process slows
• There is decreased absorption of calcium from the intestine

As stated previously the increased resorption of bone calcium is under the control of an active Vitamin D metabolite - 1-25 dihydroxy vitamin D (1-25 (OH)₂ D₃) which is produced by the kidney in response to parathyroid hormone stimulation.  However it has been shown that levels of the active metabolite can actually be higher in the blood of some cows which suffer from milk fever.  This has led to the hypothesis that cows with this condition have a reduced sensitivity to 1-25(0H)₂ D₃ preventing them from increasing calcium availability.

It is now believed that slight metabolic acidosis is important to allow the hormones and metabolites to function efficiently in order to mobilise calcium from bone. For this reason the acid-base balance of dry pregnant cows has been investigated.

The concept of balancing rations for cations and anions is not new.  Dishington (1975) successfully prevented milk fever in 92% of cases when prepartum dairy cows were fed a ration containing negative Dietary Cation-Anion Balance (DCAB) and high calcium content.

The Dietary Cation-Anion balance refers to the proportion of specific ions in the diet.  This can be a very confusing concept.  It is generally considered that anions form acidic residues and cations form alkaline residues.  This is incorrect.  For example HPO₄^2- and NH₄⁺ both act as proton donors (alkaline buffers) even though one is a cation and the other an anion.

The important concept is that the dietary cation-anion balance does not determine the acidogenic or alkalogenic properties of the feed.  It does however affect the metabolic processes within the animal.

The most commonly used expression for DCAB is:

Meq [(Sodium + Potassium) - (Chloride + sulphur)]

                                 100g Diet DM

 

To calculate DCAB (Meq/100g DM)

 

[(% Sodium in diet + % Potassium in diet)] - [(% Chloride in diet + % Sulphur)]

                0.023                            0.039                           0.0355                  0.016

It has been found that while lactating cows require a positive DCAB, it is important to have diets for dry cows in the 3 weeks prior to calving with a negative DCAB or close to negative DCAB as possible.

 

Negative DCAB in rations for pre partum cows prevents a decline in blood calcium at the initiation of lactation by one of the following mechanisms.

• By increasing the rate of bone calcium mobilisation directly.
• Increasing the rate of bone mobilisation of calcium indirectly via increased excretion of calcium.  Early work suggested that the salts effected changes in the intestine which increased the digestibility of calcium. However more recent research has not confirmed this.

Excess anions in relation to cations can produce metabolic acidosis. Chronic acidosis increases urinary excretion of calcium. This clearance of calcium stimulates the release of parathyroid hormone and synthesis of 1-25 (OH)₂ D₃ which mobilises bone calcium.

More importantly however, it has been shown that an excessive basic environment within bone cells inhibits the activity of the osteoclasts (insensitive to the metabolites) and therefore the rate of bone mobilisation falls.

For this reason reducing the daily potassium intake before calving is very important.  Molasses and dunder both contain large amounts of potassium and are often not fed to dry cows before calving for this reason.  However, both supplements contain large amounts of chloride and are therefore DCAB neutral and do not present a problem for dry cows.

Magnesium and calcium are also implicated in transit tetany.  During transport some animals (perhaps the more excitable) in a herd can become staggery and may be tramped on in the truck resulting in bruising or even death.  Cattle leaving the warmer north of the state for saleyards and feedlots in the colder south sometimes suffer from this disorder.   Allowing animals access to a mineral rich supplement prior to shipping aids significantly in this regard.

Supplied by Dr. Robert Elliott.