portions. Alternatively, the sampling of regrowth from a recently shaved site (e.g. a liver biopsy site) can give a measure of mineral status to compare with contemporary diet, blood or tissue samples.
Advances in assay procedures for enzymes and hormones have greatly increased the range and sensitivity of the diagnostic techniques now available. Serum assays for vitamin B12 rather than cobalt, triiodothyronine (T3) rather than protein-bound iodine, ceruloplasmin rather than copper and GPX rather than selenium now provide alternative indicators of the dietary and body status of these elements. These newer assays brought with them fresh problems of standardization, which were only slowly recognized, and their use still does not invariably improve the accuracy of diagnosis. The diagnostic strength and limitations of these and similar estimations are considered later, in the chapters dealing with individual elements and their interrelations. Secondary changes in serum enzymes, arising from tissue changes or damage due to mineral deficiencies or toxicities, also have diagnostic value, but caution is still necessary.
Chronic exposure to mineral excesses leads to a sequence of biochemical changes which is, in some respects, a mirror image of events during depriva- tion (compare Fig. 3.3 with Fig. 3.1). Firstly, there is accretionat storage sites;
secondly, levels in transport pools may rise; thirdly, dysfunction may be manifested by the accumulation of abnormal metabolites or constituents in the blood, tissues or excreta; and, fourthly, clinical signs of disorder become visible. For example, marginal increases in plasma copper and a rise in serum aspartate aminotransferase (AST) precede the haemolytic crisis in chronic copper poisoning in sheep; the latter is indicative of hepatic dysfunction.
However, increases in AST also occur during the development of white muscle disease, caused by selenium deficiency, because the enzyme can leak from damaged muscle as well as liver. Assays of glutamate dehydrogenase (found only in liver) and creatine kinase (found only in muscle) are of greater value than AST in distinguishing the underlying site of dysfunction. The whole sequence in Fig. 3.3 can become telescoped during acute toxicity.
There will be similar marginal bands of uncertainty when assessing mineral excesses, as when deprivation is assessed.
Successful procedures for the prevention and control of all mineral deficiencies (and many mineral toxicities) have been developed. The procedure of choice varies greatly with different elements, climatic environ- Functional forms and indices
ments, conditions of husbandry and economic circumstances. The methods available fall into three categories: indirect methods that affect the mineral composition of pastures and feeds while they are growing and continuous or discontinuous direct methods, involving administration of minerals to the animals.
Correcting mineral imbalances in grazing stock by treatment of the soil often has serious economic limitations under extensive range conditions, because productivity from each unit area is often limited by inadequate or erratic rainfall or by low winter temperatures, while transport and application of the fertilizer or soil amendment are invariably costly. Furthermore, climatic effects may be dominant over soil effects in determining the mineral content of the herbage, as is pointed out in respect of phosphorus in Chapter 5. In more favourable environments, treatment of the soil is widely and successfully practised to improve both the yield and the mineral composition of herbage.
Treatment with copper- or phosphorus-containing fertilizers can increase plant yield and is therefore a logical first step when pasture growth has been poor. Soil treatments with cobalt and selenium do not affect plant yields but can be a practical means of ensuring adequate mineral intakes. By incorporat- ing small proportions of cobalt or selenium into fertilizers, such as super- phosphate, used to maximize herbage yields, the costs of application are minimized and all animals can secure more mineral from the treated herbage.
Accretion Excess Dysfunction Disorder
Duration of exposure
4 Clinical toxicosis 1 Stores
2 Transport pool 3 Abnormal constituents in blood, excreta
Marginal toxic supply 100
0
Nutrient pool
Fig. 3.3. Sequence of pathophysiological events during chronic exposure to excessive amounts of mineral: sequences become telescoped during acute exposure; rate of transition is also affected by dietary attributes (e.g. absorbability, Fig. 1.2), animal attributes (mineral may be safely disposed of in products like milk and eggs) and factors such as disease (e.g. liver toxins or pathogens).
Indirect methods
Problems arise if deficiency is caused by poor availability rather than poor mineral content of the soil. For example, cobalt-containing fertilizers are ineffective on calcareous or highly alkaline soils, because of the low avail- ability of cobalt to plants in such conditions. Soils of low available phospho- rus concentration are likely to trap added fertilizer phosphorus in unavailable forms. The application of sulphur or gypsum to seleniferous soils already high in sulphate or which contain a high proportion of their selenium in organic combination will not significantly reduce selenium uptake by plants, although it can be effective in other circumstances. Treatment with copper of pastures very high in molybdenum may not raise the copper content of the herbage high enough to counteract a severe conditioned copper deficiency.
The residual effects of all soil treatments and therefore the frequency with which they must be applied can vary widely from one location to another.
Animals that are being hand-fed can best be supplemented by mixing the required minerals with the food offered. When adequate trough time or space is allowed, variation in individual consumption is not pronounced and the amounts included should not exceed average mineral requirements. The precise amounts and proportions of supplementary minerals needed will depend on the nature and degree of the deficiency and the form and intensity of production. Where individual animal productivity is high, husbandry conditions are intensive and farm labour costs generally high, it is usual to purchase mixed feeds compounded to contain the required minerals in adequate quantities and with a universally sufficient margin of safety;
however, they often contain minerals that are not locally required as supple- ments. Where the conditions of husbandry are less intensive and farm labour is more plentiful and less costly, the same results can be achieved by home mixing and only those minerals known to be needed are added. Furthermore, home-grown roughage, rather than expensive concentrates, can be used as the carrier for the mineral supplement. In sparse grazing conditions, where little or no regular hand-feeding is practised, treatment of the water-supply may be practicable.
The most widely used method of supplementation is the provision of mineral mixtures in loose (‘lick’) or block form. Common salt (NaCl) is a vital ingredient of such supplements, because it is so palatable and attractive to most animals. Individuals are relied on to consume sufficient for their needs where supplies are freely available and to seek it out actively where it is less accessible. The siting of dispensers can be arranged to attract stock to parts of a grazing area that they might otherwise undergraze. Because of its palat- ability, salt is a valuable ‘carrier’ of other minerals. So long as these mineral mixtures contain 30–40% salt, they are commonly consumed in amounts sufficient to meet the needs of livestock for other minerals (McDowell et al., 1993). However, individual variation in consumption from licks or blocks can Continuous direct methods
Free-access mineral mixtures
be marked (Bowman and Sowell, 1997; see p. 450). In one study, 19% of the ewe flock commonly consumed nothing, while others consumed 0.4–1.4 kg day21 (Ducker et al., 1981). In a study of salt supplementation (presented in protected, trailer-drawn blocks) of a beef suckler herd grazing Californian rangeland, mean daily consumption per head (cow ± calf) ranged from 0 to 129 about a mean of 27 g day21 over consecutive 7-day periods (Fig. 3.4;
Morris et al., 1980); provision is therefore ‘semicontinuous’, rather than
‘continuous’. Seasonal variation in mineral consumption can also occur, but intakes are more uniform from granulated than from block sources of minerals (Rocks et al., 1982). Provision of several, widely scattered sources can minimize competition and hence individual variation in uptake of minerals. This form of treatment cannot be relied on where the water- supplies are saline and may be unsatisfactory with an element like cobalt, which is required by the animal regularly.
The oral dosing, drenching or injection of animals with mineral solutions, suspensions or pastes has the advantage that all animals receive known amounts of the required mineral at known intervals. This type of treatment is unsatisfactory where labour costs are high and animals have to be driven long distances and handled frequently and specifically for treatment. With minerals such as copper, which are readily stored in the liver to provide reserves against periods of inadequate intake, large doses given several months apart can be satisfactory. Oral dosing with selenium salts works well in selenium-deficient areas, particularly where it can conveniently be combined with oral dosing of anthelmintics. In contrast, cobalt deficiency cannot always be prevented fully if the oral doses of cobalt salts are weeks
Fig. 3.4. Variation in average salt intake by a group of beef suckler cows given free access to salt from covered trailers on Californian rangeland: observations began with calving in November (data from Morris et al., 1980).
Discontinuous direct methods
apart. With iron, iodine and copper, some of the disadvantages of oral dosing can be overcome by the use of injectable organic complexes of the minerals.
Such complexes are more expensive, but, when injected subcutaneously or intramuscularly, they are translocated slowly to other tissues and provide protection against a dietary deficiency of the injected element for lengthy periods. For instance, a single intramuscular injection of iron-dextran, supplying 100 mg Fe, at 2–4 days of age can control piglet anaemia (Chapter 13).
The efficacy and costs of administering minerals to individual animals can be improved by using large doses in relatively inert, slowly mobilized forms. The administration of heavy pellets (Millar and Meads, 1988), glass boluses (Millar et al., 1988) or particles of the mineral (e.g. copper oxide: Chapter 11), which are retained in the gastrointestinal tract, all act as slow-release sources. The problem of cobalt deficiency can be solved by use of heavy cobalt pellets, as described in Chapter 10, which lodge in the reticulorumen and yield a steady supply of cobalt for many months, unless they are regurgitated or become coated.
The choice of a mineral supplement is determined by: (i) cost per unit of the element or elements required; (ii) the chemical form in which the element is combined; (iii) its physical form, especially its fineness of division; and (iv) its freedom from harmful impurities, particularly fluorine. With calcium and phosphorus supplements, these factors can greatly influence the choice of material used, but, with the trace elements, because of the small quantities required and the relatively low costs involved, such considerations are generally of minor significance and the choice of supplement widens.
Molybdate-containing salt-licks have been highly effective in preventing chronic copper poisoning in sheep and cattle in certain areas. There is a remarkable range in the composition of proprietary free-access mineral mixtures, which is influenced by factors such as cost of ingredients and colour, as well as nutritional need, and care is needed in choosing appro- priate mixtures (e.g. Suttle, 1983). The mineral supplements commonly employed are given in detail in Appendix Table 2 on p. 600 and the optimum methods of treatment discussed in more detail in the following chapters.
The provision of extra minerals beyond the animal’s needs is economically wasteful, confers no additional benefit on the animal and can be harmful.
Excesses of phosphorus and magnesium can cause death from urinary calculi (Hay and Suttle, 1993), while an excess of copper soon causes toxicity in Slow-release methods
Mineral sources