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NOTES ON ZINC

Zinc is an element in Group 118 of the Periodic Table (atomic number 30) with an atomic weight of 65. It is a metal with a close affinity to copper. The two metals can be combined to form brass and there are many similarities among their compounds. It is estimated that 0.004% of the earth’s crust is composed of zinc which makes it the 25th most abundant element. Its metallic properties have been recognised for hundreds of years and it has been used very widely as a constituent of various alloys. More recently it has been used as a coating for ferrous metals to prevent rust (galvanising).
The essential nature of zinc for the growth and survival of living things was first recognised in the 19th century. The earliest recorded work was with moulds which could not survive without a trace of zinc in the substrate. This research has subsequently been extended to all other plant and animal species with similar results.
In the early 1920’s, it was shown that the growth of laboratory animals on a nearly zinc-free diet was retarded and there was poor hair development. However, it was not until the mid 1950’s that zinc deficiency was demonstrated in farm livestock.
Zinc is present in variable amounts in soils with an average of about 100 mg/kg although it can vary from almost zero to 200 mg/kg and sometimes more. It is derived from basic rock substrate and rock minerals such as olivine, hornblende, augite and biotite are good sources. It tends to be part of the clay matrix structure in the soil rather than part of the soil solution. As a result it is not noticeably affected by free drainage, but its uptake is more dependent on soil pH. Whole plants contain in general, 20 to 200 mg zinc per kg dry matter with some parts such as roots and stems containing a greater proportion than leaves. Potato tubers may contain 600 mg/kg DM or even more.
Animals contain about 30 mg zinc per kg body weight The highest concentrations are found in epidermal tissues such as skin, hair, feathers and wool with smaller amounts in bones, muscles, organs and in blood.

Zinc is an essential trace mineral for plants and animals. In plants it is concerned with the formation of chlorophyll and in the synthesis of tryptophan. Unlike iron, manganese and copper it cannot take part in redox processes, though it is required for a number of dehydrogenase enzymes. It has been suggested that zinc is indispensable as an acid catalyst in all biological space.
In animals, zinc fulfils a number of functions. It affects growth, development, reproduction, bone and blood formation and metabolism of nucleic acids, proteins and carbohydrates. In all these processes, zinc acts in conjunction with enzymes. Up to 6 atoms of zinc are present in the construction of the molecules of at least 12 enzymes including pancreatic carboxypeptidase. Zinc also activates a number of other enzymes such as intestinal dipeptidase, as a nonspecific cation. It forms complexes with nucleotides in various tissues but these are, less stable than its complexes with amino acids. It seems that it helps to maintain a definite configuration of RNA so that, through this association, it has an indirect effect on the biosynthesis of proteins and the transmission of genetic information. Zinc may also have a direct effect on reproduction by complexing with a specific ligand in the gonads. In 1983, over 200 zinc metalloenzymes in various species had been defined. Some of these enzymes also contain other metals or require them as cofactors such as alkaline phosphatase which requires magnesium and superoxide dismutases which contain copper or manganese.
Thus zinc is directly or indirectly involved with almost every metabolic function in animals. Every animal requires a daily supply because utilisation and excretion rapidly reduce the available stores. Supplies from, the daily feed intake are absorbed mainly in the upper part of the small intestine. Efficiency of absorption is relatively low, regardless of the form of zinc. It varies from as low as 7% in some adult monogastrics to 50% in young ruminants. A high calcium level in the presence of phytic acid inhibits or reduces zinc uptake and can lead to a secondary zinc deficiency. Generally, however, animals show an effective homeostatic mechanism which is maintained by varying the amount of zinc absorbed and its endogenous excretion with the faeces.
Zinc is stored mainly in the mucosal cells of the gastro-intestinal tract, liver, kidney and bone. Since the mucosal cells are continually being sloughed they probably function as part of the homeostatic mechanism preventing excess zinc absorption. Liver and kidney also accumulate excess zinc and prevent toxicity. These stores are of limited value since they are rapidly depleted at times of reduced intake. The zinc reserves in bone are the major stores and can be mobilised for metabolic use even when calcium intake is high.
If Insufficient is supplied
One of the first indications of zinc deficiency in monogastric species is the development of skin, hair or feather problems. Pigs develop parakeratosis which appears as skin scaliness; poultry show poor feathering and feather—fraying. Even ruminants, who are more efficient in removing phytate-bound zinc from their ingested feed, show skin, hair or wool problems. Sheep fed rations deficient in zinc develop a break in the wool and show abnormal changes in the horns.

Apart from these observable changes, zinc deficiency reduces growth rate, delays the healing of wounds, leads to poor testicular development and causes impairment of glucose tolerance. There are also other consequential changes. Feed intake is reduced, leading to overall reduction in performance. The immune system is also affected. This is most serious in the new-born animal whose dam has been fed a zinc-deficient diet during the last phases of pregnancy. This results in immunodeficiencies and the new-born animal is unable to build adequate defences against endemic diseases. At least two systems are involved - the thymic response to T-cell mutogens and the production of immunoglobulins.
Sometimes the results of marginal zinc deficiency do not appear for a long time. Work with sows showed that growth rates of piglets were not affected until the fourth and fifth parities.
High calcium levels in the feed exacerbate the effects of zinc deficiency. The ways in which calcium exerts this effect are subject to speculation. Calcium certainly increases the losses of excreted zinc and also appears to decrease absorption. But there is published information showing a metabolic interference between calcium and zinc.

Excess zinc in the diet is more likely to lead to feed refusal than to toxic effects. Animals persuaded to consume excessively large amounts of zinc show a reduction in feed intake, reduced growth rate, arthritis, gastritis and haemorrhage in axillary space.
The concentrations of zinc necessary to produce these effects vary with species. Pigs are more sensitive than poultry and show symptoms when zinc concentration is above 1000 ppm whereas chicks can tolerate 1500 ppm and turkeys about 3000 ppm. Ruminants have a lower tolerance and should not receive more than 750 ppm.
Most of these toxicity studies have been done in isolation. In practice the interaction with other micronutrients might have an important bearing on toxicity. For example, ewes given 750 ppm of zinc throughout pregnancy showed inter-reactions with dietary copper. Combined with very low copper, high zinc resulted in a number of lamb deaths which did not occur on adequate dietary copper. Zinc supplements to broiler feeds>200 ppm significantly reduced tissue tocopherol. It is also probably worth noting that pregnant animals are less tolerant of high or low levels of zinc than non-pregnant.
Reference has already been made to the binding effect of phytic acid and the effects of high dietary calcium. This is a complex relationship and it appears that the excess calcium reduces the availability of phytate-bound zinc still further. In diets where phytate is present, the amount of zinc absorbed is decreased as the calcium level is increased. However in the absence of phytate, calcium levels had little effect on zinc absorption. But, as has been shown earlier, high calcium increases zinc excretion. Vitamin D also appears to be implicated in this complex relationship. At least one trial has shown that absorption and retention of zinc decreased as dietary vitamin D increased.
Pigs given diets high in copper appear to have an increased requirement for zinc. This was also the implication of the toxic study with ewes. The utilisation of iron can also be affected by excess zinc intake, leading to an induced anaemia.
Cobalt and cadmium have also been linked to zinc. Zinc deficiency symptoms have been alleviated by supplying additional cobalt, whereas increased zinc in the diet helped to overcome cadmium toxicity. Magnesium and/or nickel might also assist in overcoming zinc deficiency problems.

The efficiency of absorption of zinc from the ingested feed is comparatively low and varies from about 10 to 50 percent. It tends to be highest in very young and lowest in mature adult stock. This low uptake is taken into account in preparing requirements and allowances.
Most sources of inorganic zinc appear to be equally bioeffective. However the zinc in some crude rocks and ores is less effectively absorbed and may be relatively unavailable.
The addition of a chelating agent such as EDTA increases the availability of zinc, presumably because of increased absorption. Zinc/protein and zinc/amino acid complexes may show some slight improvement in zinc availability compared to inorganic sources providing there is no interference from phytic acid.

The amount of zinc in a supplement or feed can be determined within fine limits of accuracy by atomic absorption spectrophotometry. The feed product is first digested with nitric acid and hydrogen peroxide to remove organic matter and separate the zinc from its protein or phytic acid complexes. The residue is then made into a solution. A nitrous oxide/acetylene flame gives the best results in the atomic absorption apparatus.

Since zinc is stored in bones, liver and kidneys it would be possible to determine the zinc status by assaying the amounts in these tissues. Obviously this is very difficult, if not impossible, in a living animal.

A liver biopsy or a liver sample taken post mortem may contain 20-120 mg zinc per kg fresh weight. This corresponds to 300-1800 µmol/kg. The amount present in whole blood or serum is not a good Indicator of status. Zinc stores vary with species, age, sex as well as the amount in the diet.
In adult mammals, the amounts of zinc usually found in the different organs and other biological tissues (fresh material) are -:

Liver 40 to 80 mg/kg, Kidneys 13 to 8 mg/kg, Heart 14 to18 mg/kg, Bones 60 to120 mg/kg
Muscles 8 to12 mg/kg ,Blood 2 to– 4 mg/kg, Plasma 0.6 to 1.0 m/kg
Most of the zinc in blood is present in the erythrocytes, almost exclusively as a component of the enzyme carbonic anhydrase. The amount in the plasma is often the best indicator of zinc status. For most farm animals, plasma zinc within the range 0.6 to 0.4 mg/litre can be regarded as marginal and <0.4 deficient. This determination can only give an indication since there can be conditions reducing plasma zinc when other tissues are adequately supplied.

An animal’s zinc requirement is not absolute but varies due to the interaction of a number of factors. For example a high calcium diet leads to an increased zinc requirement. Similarly the amount of phytic acid influences the absorption of zinc and therefore the requirement.
Published zinc requirement tables must be considered in the 1ightof all these variables and generous allowances given. There is a wide degree of safety with the “safe zone” of application lying between 20-1000 mg/kg feed. Breeding animals need more zinc than growing or maturing stock because it is an essential component of developing tissues and the carryover effect is important to the life and health of the offspring.
The zinc contents of the feed ingredients can be deducted from the allowances if they are known. If supplements are formulated for unknown combinations of ingredients a deduction of, say, 30 mg/kg can he made arbitrarily from the allowances if required. Generally, however, because of the wide safety margin, the supplement can include the full allowance and thus ensure adequate coverage even under the worst combination of affecting factors.

Livestock conditions suggesting further needs
Dermatitis, poor hair development, feather-fraying or breaks in the wool can be caused by a variety of conditions of which zinc deficiency is only one. Nevertheless many such conditions are modified or resolved by increasing zinc supplementation in the feed.