The minerals are usually classified into macro elements (Ca, P, K, NaCl, and Mg) and trace elements (Cu, Co, Se, Mn, I, Zn, Fe Mo and Cr) depending upon the quantities (NRC, 2001). The “trace elements” are those elements existing in natural and perturbed environments in small amounts, with excess bioavailability having a toxic effect on the living organism (Wada, 2004). Trace elements are essential dietary components for life and necessary for numerous metal-dependent enzyme and protein activities (Kramer et al., 2007).Trace mineral requirements are affected by a number of factors like age, stage (lactating vs non-lactating) and level of production, breed, and bioavailability of the mineral from the diet. Iron (1.0-2.0 ppm) is most abundantly found in serum followed by zinc (0.8-1.2 ppm) (Radostits et al., 2007; Andrieu, 2008) and copper (0.57-1.0). Along with these cobalt (1-3 µg/dl), iodine (2.4-14 µg/100ml), manganese (18-19 µg/dl) and selenium (50-220 ng/L) are required in least amounts (Radostits et al., 2007; Andrieu, 2008).Trace minerals are required by cattle because of their roles inthe production of hormones, enzyme activity, synthesis of tissues, energy production andcollagen formation (Paterson and Engle, 2005). Even short periods of insufficient alimentary supply can promote significant physiological changes (Brugger et al., 2014). Its improper level may affect embryonic development, post-partum recovery activities and over all fertility of animal may be impaired along with reduction in quality and quantity of milk production. In male animals it may change spermatogenesis and reduce libido. So, in order to maintain adequate trace mineral status in dairy animals, balanced intake and absorption are necessary.

Chemical Structure of Trace Mineral Supplements

Conventionally, Mn, Cu and Zn supplements have been fed as inorganic salts, for example magnesium sulfate, cupric sulfate and zinc sulfate. Trace mineral in these salts is associated with sulfate in a dry form but dissociates from the sulfate when hydrated in the rumen. A current trend is to feed “organic” forms of Zn, Cu, and/or Mn in place of inorganic salts. In organic Zn, Cu and Mn the mineral is bound to an organic (i.e. carbon-containing) molecule, typically an amino acid or protein and further these supplements are classified as complexes, chelates or proteinates based upon their chemical structure. The minerals and organic molecules are associated in complexes, but not necessarily by covalent bonds (Spears, 1996). Covalent bonds exist between the minerals and organic molecules for both chelates and proteinates (Spears, 1996). For Zn, an example of a complex, chelate, and proteinate is Zn-methionine (ZN-met), Zn-methionine hydroxy-analogue (Zn-MHA), and Zn-proteinate, respectively. Selenites and selenates provide inorganic Se in the form of sodium salts while organic Se is provided by Se-yeast, a product made from growing yeast in media supplemented with Se (Weiss, 2005). Yeast incorporates Se into a variety of compounds, with the predominant compound being selenomethionine (Schrauzer, 2000). In Se-met, Se is covalently bound to the amino acid and takes the place of Sulfur in the molecular structure.

Bioavailability of Trace Mineral in Ruminants

Bioavailability determines efficacy of trace minerals. Several factors directly or indirectly affect the concentrations of minerals in plants and hence the amounts available for animals that depend on plants for feeds. Soil condition has been reported to influence the mineral compositions of feed ingredients (Kavanek and Janicek, 1969). In animals, supplemental trace minerals are supplied through inorganic sources. Although some new organic sources have been introduced in recent years, like sulphates and oxides, are still most widely used in animal feeds. For Se, most studies indicate that the bioavailability of selenium from selenite and selenate is similar in ruminants (Podollet al., 1992; Ortman and Pehrson, 1999). Organic selenium in selenized yeast results in much larger increases in blood and milk selenium concentrations than selenite (Podoll et al., 1992; Knowles et al., 1999). Absorption of selenium is much lower in ruminants than in non-ruminants. Absorption of orally administered ⁷⁵Se was only 34% in sheep compared with 85% in swine (Wright and Bell, 1966). The relative bioavailability of copper from tribasic cupric chloride (Cu₂OH₃Cl) was 121% (based on plasma copper) to 196% (based on liver copper) that of cupric sulfate when supplemented to cattle diets high in molybdenum and sulfur (Spears et al., 1997). Tribasic copper chloride and cupric sulfate were similar in bioavailability when evaluated in copper-deficient steers fed diets that were low in molybdenum (Spears et al., 1997). Copper absorption in ruminants is low (<1.0–10.0%) relative to values reported in non-ruminants (Underwood and Suttle, 1999). Manganese is poorly absorbed (1% or less) from ruminant diets (Hidiroglow, 1979; Van bruwaene et al., 1984). Manganese from two feed-grade manganese oxide sources tested in lambs was 70% and 53% as bioavailable as manganese from reagent-grade manganese sulfate (Henry et al., 1992). Relative bioavailability of manganese from manganese methionine was 120% of that present in the sulfate form (Henry et al., 1992). The percentage of dietary zinc that is absorbed decreases as dietary zinc increases in ruminants (Miller, 1970). Zinc requirements of ruminants appear to be affected by dietary factors based on the variable animal responses that were observed after zinc supplementation (Underwood, 1977). Studies indicate that addition of 250–1,200 mg of iron (from ferrous carbonate)/kg of diet greatly reduces copper status in cattle (Bremner et al., 1987; Phillippo et al., 1987) and sheep (Prabowo et al., 1988). High dietary iron did not affect copper status in young pre-ruminant calves, which suggests that a functional rumen is needed for iron to interfere with copper metabolism (Bremner et al., 1987).

Functions of Trace Minerals

Trace mineral functions are categorized into four broad categories (Underwood and Suttle, 1999).

Structural Function

Structural function refers to minerals forming structural components of body organs and tissue, for example, the contribution of zinc to molecular and membrane stability.

Physiological Function

Physiological function occurs when minerals in body fluids and tissues act as electrolytes to maintain osmotic pressure, acid base balance and membrane permeability.

Catalytic Function

Catalytic function is probably the largest category for trace minerals as it refers to catalytic role of metalloenzymes in enzyme and hormone systems. Trace elements serve as structural components of metalloenzymes. Upon removal of the trace element or lack of adequate trace mineral levels the enzyme activity is lost. There are numerous metalloenzymes that are required for a wide range of metabolic activities such as energy production, protein digestion, cell replication, antioxidant activity and wound healing.

Regulatory Function

Regulatory function is exemplified by the role of zinc to influence transcription and iodine serving as a constituent of thyroxine, a hormone associated with thyroid function and energy metabolism.

Effect of Trace Minerals on Productive Performance of Animals

The role of trace minerals in animal production is an area of strong interest for farmers, producers, feed manufactures, veterinarians and scientists. Micro minerals are very essential part of animal’s ration which is required only in little amount and excess feeding of some of these may show toxicity symptoms. Deficiency of trace minerals in the diet alone can reduce animal production by 20-30%. Therefore, supplementation of trace elements in animal diets has long been practiced in order to ensure their rapid growth, boost reproductive performance and improve immune response (Overton and Yasui, 2014).

Table 1: Trace minerals requirement for maintenance of dairy cattle

Source: ICAR (2013)

Table 2: Trace minerals requirement during gestation period of dairy cattle

Source: ICAR (2013)

Recent data indicate that micronutrient management will enhance the production of good quality milk. The keratin lining of the teat canal has been described as a physical and chemical barrier for protection of the mammary gland. Keratin lining may physically trap bacteria and prevent migration into the mammary gland. Because the mammary gland is a skin gland, it is highly likely that zinc will have a positive role in its protection.

Table 3: Trace minerals requirement for per kg milk production of dairy cattle

Source: ICAR (2013)

Zinc deficiency in ruminants has been postulated to weaken skin and other stratified epithelia as well as reducing the magnitude of basal metabolic rate following infectious challenge (Harmon, 1998). Skin integrity of the teat has been shown to be especially linked with mastitis prevention. Kellogg (1990) reported that chelated zinc decreased somatic cell counts (SCC) by 22-50% in eight trials, depending on the dose of zinc used and increased milk production. Selenium supplementation increased SCC resistance to intra-mammary infusion of Escherichia coli. Erskine et al. (1987) demonstrated lower blood selenium concentrations in cows with high SCC compared with cows with low SCC.

Effect of Trace Mineral on Animal Reproduction

According to Smith and Chase (2010), the interaction between mineral and reproduction in farm animals have been documented. These reports generally suggest that adequate mineral intake improves production efficiency and reproduction performance parameters in farm animals (Almeida et al., 2007; Griffiths et al., 2007). Excess mineral intake results in loss of body weight and condition, and may delay puberty, reduce ovulation, lower conception rates, interferes with normal ovarian cyclicity by decreasing gonadotropin secretion and increases infertility (Boland et al., 2001; Wright, 2012).

Copper

Copper is one of the important mineral for reproduction point of view as such its deficiency is reported to be responsible for early embryonic death and resorption of the embryo (Miller et al., 1988), increased chances of retained placenta and necrosis of placenta (O’Dell, 1990) and low fertility associated with delayed or depressed estrus (Howell and Hall, 1970). In addition to this, proper copper supplementation is must for quality semen production (Puls, 1994). Copper treatment is reported to improve conception rate as the copper treated cow require 1 service and the untreated cow require 1.15 services per conception (Hunter, 1977).

Cobalt (Co)

Cobalt plays important role in the synthesis Vitamin B12 (Miller and Tillapaugh, 1967). Levels of Vitamin B12 are high in milk and colostrum which is required for the conversion of propionate into glucose and folic acid metabolism. Cobalt deficiency leads to reduce fertility and poor conditioning of the developing fetus. In dairy animal deficiency leads to prolonged uterine involution, irregular estrous cycle, lower conception rates and early calf mortality (Puls, 1994; Kumar, 2003). Deficiency of cobalt will in turn lead to Vitamin B12 deficiency. Manganese, zinc and iodine may reduce cobalt deficiency (Patterson et al., 2003).

Selenium

In pregnant animal marginal deficiency of selenium leads to abortion, birth of weak calves that are unable to stand. Research indicates that selenium supplementation reduces the incidence of retained placentas, cystic ovaries, mastitis and metritis (Patterson et al., 2003). Being having direct link to the uterine involution selenium is important dietary mineral (Arthington, 2005). Among dairy animals, where subclinical selenium deficiency is there, reproductive performance may get retarded with delayed ovulation, increased services per conception and high incidence of mastitis (Goff, 2005). Selenium helps in enhancing the reproductive efficiency by increasing the activity of glutathione peroxidase in blood and tissues. Selenium easily crosses placenta whether fed as inorganic or organic food. It has been reported that selenium supplementation leads to improved conception rate at first service (McClure et al., 1986). Selenium injections prior to parturition helps in reducing the incidence of retained placenta in deficient animals.

Manganese (Mn)

Manganese is important in cholesterol synthesis (Keen and Zidenberg-Cheer, 1990) which in turn is necessary for the synthesis of steroids like progesterone, estrogen and testosterone. Decrease concentration of these steroids in circulation following manganese deficiency may lead to related reproductive abnormality. Deficiency cause poor fertility problem in male and female. It is responsible for silent estrus and anoestrus (Corrah, 1996) or irregular estrus (Brown and Casillas, 1986) and decrease conception rate, birth of deformed claves and abortions in females and absences of libido and improper or failure of spermatogenesis in males (Sathish Kumar, 2003). Postpartum anestrus in dairy cows has proven to be reduced following manganese supplementation (Krolak, 1968) so thus the number of services required per conception increased (Rojas, 1965).

Zinc

Zinc act as cofactor and coenzyme of many enzymes and various reproductive hormones. Zinc plays an essential role in the maintenance and repair of uterine lining after calving, helps in early involution. Abnormal levels of zinc is associated with decreased conception rate, abnormal estrous and abortion. Zinc as coenzyme, is involved in the formation of prostaglandins form arachidonic acid suggesting its profound effect on reproductive cycles and maintenance of pregnancy (Kumar et al., 2011). Zinc also increases the plasma beta carotene level that has been directly correlated to higher conception rate and embryonic development (Staats et al., 1988). Delayed puberty and low conception rates, failure of implantation and reduction of the litter size are also found in association with the zinc deficiency in feed. The recommended dietary requirement of zinc for dairy cattle lies between 18-73 ppm (Patterson et al., 2003) depending upon the stage of the lifecycle and dry matter intake, whereas according to the feeding standards the requirement is 40 ppm (NRC, 2001).

Iron

Iron is essential for the synthesis of hemoglobin and myoglobin and various other enzymes that help in formation of ATP through electron transport chain. It helps in transport of oxygen to tissues, maintenance of various oxidative enzyme systems (Khillare et al., 2007). Deficiency is rare in adult animals due to its abundance in feed stuffs. But in cases where deficiencies are there, reproductive health is deteriorated due anemia, reduced appetite and poor body condition. Chances are there that deficient animal will become a repeat breeder and will require increased number of services per conception and may abort occasionally (Kumar et al., 2011).

Iodine

Iodine due to its action on thyroid gland affects the reproduction. Iodine is regarded as essential for the developing fetus and maintaining the basal metabolic rate. Iodine through its effect on thyroid gland helps in secretion of gonadotropin by stimulating the anterior pituitary gland, thereby affects the estrous cycle (Khillare et al., 2007). Deficiency of iodine affects the fertility and increases the abortion rate (Hetzel, 1990), the incidence of retained placenta and post-partum uterine infections, respectively (Hemken, 1960). Conception rate and ovarian activity is reduced with the impaired thyroid functions. Thus, iodine affects the reproduction in many ways and a recommended dose of 15-20 mg of iodine each day is necessary for a cow to have good reproduction status. Excess of iodine also have deleterious effect on reproductive health by inducing premature births of weak calves, abortions and lowers the immunity status of animal (Kumar et al., 2011). Subclinical iodine deficiency is characterized by increased stillbirths, suppressed estrous, increased chances of retained placentas and prolonged gestation periods (Hess et al., 2008). Normal plasma level of inorganic iodine in cows should be maintained between 100-300 ng/ml.

Chromium

Chromium is essential for carbohydrate metabolism (Tuormaa, 2000). It is present in nuclear protein in higher amount thus has a role in gametogenesis and for healthy fetal growth. It is also an integral part of the pregnancy specific protein that is secreted by uterine endometrium which helps in preventing the early embryonic mortality (Kumar et al., 2011). It is having a crucial role in maturation of follicle thus maintaining the estrous cycle and also in LH release which triggers the ovulation. Deficiency of chromium will lead to irregular estrous cycle, delayed ovulation, early embryonic mortality and retarded fetal growth (Tuormaa, 2000). In lactating animals, it may predispose the animal to ketosis and decreased milk production.

Molybdenum

Molybdenum deficiency in animals delays the onset of puberty, decreases conception rate and causes anestrus (Kumar, 2003). Molybdenum and copper are interlinked with each other as deficiency of one occurs in the presence of toxic levels of other. Therefore, a proper balance in feeding the copper and molybdenum must be followed to avoid the reproductive problems (Randhawa and Randhawa, 1994).

Deficiency of Trace Minerals

Trace mineral deficiencies in livestock are often divided into two distinct categories:

1. Primary: A deficiency resulting from the consumption of an essential trace mineral at levels inadequate to support the physiological functions associated with that element.
2. Secondary: A deficiency resulting from the consumption of an element which antagonizes the pre- or post-absorption of an essential trace mineral rendering the element incapable of supporting the physiological functions associated with that element.

Reproductive performance of cattle may be compromised if zinc, copper, or manganese status is in the marginal to deficient range. Common copper deficiency symptoms in cattle include delayed or suppressed estrus, decreased conception, infertility and embryo death (Phillippo et al., 1987; Corah and Ives, 1991). Inadequate zinc levels have been associated with decreased fertility, abnormal estrus, abortion, and altered myometrial contractibility with prolonged labor (Maas, 1987; Duffy et al., 1977). Manganese deficiency in cows results in suppression of conception rates, delayed estrus in post-partum females and young prepuberal heifers, infertility, abortion, immature ovaries and dystocia (Brown and Casillas, 1986; Maas, 1987; Corah and Ives, 1991). Dairy producers can benefit from year-round complexed trace mineral supplementation due to additional effects such as enhanced milk production and reduced somatic cell counts. Improving reproductive performance of dairy cows by achieving confirmed conception rates early in the breeding period could have economic returns to the producer. Subclinical or marginal deficiencies may be a larger problem than acute mineral deficiency, because specific clinical symptoms are not evident to allow the producer to recognize the deficiency; however, animals continue to grow and reproduce but at a reduced rate. As animal trace mineral status declines immunity and enzyme functions are compromised first, followed by a reduction in maximum growth and fertility, and finally normal growth and fertility decrease prior to evidence of clinical deficiency.

Environmental Issues and Ration Formulation Strategy

Currently producers are faced with many challenging issues related to sustainable agriculture. These environmental issues make deleterious impact on livestock production practices. In the near future, regulations may possibly limit the level of trace minerals fed in order to reduce the amount found in animal wastes. When producers are confronted with these types of restriction, form of trace minerals fed may become more critical in relation to bioavailability to the animal. The question then becomes, can lower levels of more bioavailable organic minerals give the same response as higher levels of inorganic minerals? In the swine industry it is a common practice to include copper sulfate at elevated levels (200-250 ppm) to enhance growth in the nursery. In a trial evaluating levels of copper sulfate and copper lysine, pigs fed 100 ppm copper lysine gained more weight and consumed more feed than those fed 250 ppm copper sulfate (Coffey et al., 1994). Feeding lower levels will also reduce the total amount of mineral excreted in the feces. In a study evaluating copper metabolism in growing calves, retention was improved when copper complex was the sole source of supplemental copper or blended with sulfate source and compared to copper sulfate alone.

Conclusion

A well-coordinated nutrition, health care, and management program is required to maximize efficiency and productivity. Trace elements are required for numerous metabolic functions in livestock, and optimal production and performance require adequate intake of balanced trace minerals. As trace mineral status of the animal declines from adequate to marginal, immunity and enzyme function are compromised followed by the loss of performance and reproduction. In order to avoid the chances of reproductive failure and other reproductive disorders we have to supplement adequate quantities of mineral required by the animal but Still trace mineral nutrition continues to be an area of interest for research and production applications.

References

1. Almeida, A. M., Schwalbach, L. M. J., Cardoso, L. A. and Greyling, J. P. C. (2007). Scrotal, testicular and semen characteristics of young Boer bucks fed winter veld hay: The effect of nutritional supplementation. Small Ruminant Research, 73, 216-
2. Andrieu, S. (2008). Is there a role for organic trace element supplements in transition cow health? The Veterinary Journal, 176, 77-
3. Arthington, J. D. (2005). Trace mineral nutrition and the immune response in cattle, In: Proceeding of 64th Annual Minnesota Nutrition Conferences, Minneapolis. 106.
4. Boland, M. P., Lonergan, P. and Callaghan, O. (2001). Effect of nutrition on endocrine parameters, ovarian physiology and oocytes and embryo development. Theriogenology, 55, 1323-
5. Bremner, I., Humphries, W. R., Phillippo, M., Walker, M. J. and Morrice, P. C. (1987). Iron-induced copper deficiency in calves: dose response relationships and interactions with molybdenum and sulphur. Animal Production Science, 45, 403-414.
6. Brown, M. A. and Casillas, E. R. (1986). Manganese and manganese-ATP interactions with bovine sperm adenylate cyclase. Archives of Biochemistry and Biophysics, 244, 719-
7. Brugger, D., Buffler, M. and Windisch, W. (2014). Development of an experimental model to assess the bioavailability of zinc in practical piglet diets. Archives of Animal Nutrition, 68, 73-92.
8. Coffey, R. D., Cromwell, G. L. and Monegue, H. J. (1994). Efficacy of a copper-lysine complex as a growth promotant for weanling pigs. Journal of Animal Science, 72, 2880.
9. Corah, L. R. and Ives, S. (1991). The effects of essential trace minerals on reproduction in beef cattle. Veterinary Clinicsof North America: Food Animal Practice, 7, 40-57.
10. Duffy, J. H., Bingley, J. B. and Cove, L. Y. (1977). The plasma zinc concentration of nonpregnant, pregnant and parturient Hereford cattle. Australian Veterinary Journal, 53, 519-522.
11. Erskine, R.J., Eberhart, R.J., Hutchinson, L. J. and Scholz, R.W. (1987). Blood selenium concentrations and glutathione peroxidase activities in dairy herds with high and low somatic cell counts. Journal of the American Veterinary Medical Association, 178, 704.
12. Goff, J. P. (2005). Major advances in our understanding of with nutritional influences on bovine health. Journal of Dairy Science, 89, 1272-1301.
13. Griffiths, L. M., Loeffler, S. H., Socha, M. T., Tomlinson, D. J. and Johnson, A. B. (2007). Effects of supplementing complexed zinc, manganese, copper and cobalt on lactation and reproductive performance of intensively grazed lactating dairy cattle on the South Island of New Zealand. Animal Feed Science and Technology, 137, 69-83.
14. Harmon, R.J. (1998). Trace minerals and dairy cattle: importance for udder health. In: Biotechnology in the Feed Industry. Proceeding of Alltech’s 14th Annual Symposiuum. pp. 485-495.
15. Hemken, R. W. (1960). Iodine. Journal of Dairy Science, 53, 1138-1143.
16. Henry, P. R., Ammerman, C. B. and Littell, R. C. (1992) Relative bioavailability of manganese from a manganese-methionine complex and inorganic sources for ruminants. Journal of Dairy Science, 75, 3473-3478.
17. Hess, B. W., Moss, G. E. and Rule, D. C. (2008). A decade of developments in the area of fat supplementation research with beef cattle and sheep. Journal of Animal Science, 86, 188-204.
18. Hetzel, B. S. (1990). Present Knowledge in Nutrition. (Ed. L brown). International Life science Institute Nutrition Foundation, Washington D C, 308-313.
19. Hidiroglow, M. (1979) Manganese in ruminant nutrition. Canadian Journal of Animal Science, 59, 217-236.
20. Howell, J. M. and Hall, G. A. (1970). Infertility associated with experimental copper deficiency in cattle, sheep, guinea pigs and rats. In: Mills, C.F. (ed.) Trace element metabolism in animals. E. and S. livingstone, Edinburgh, 106-109.
21. Hunter, A. P. (1977). Some nutritional factors affecting the fertility of dairy cattle. New Zealand Veterinary Journal, 25, 715-
22. ICAR (2013) Nutrient requirements of cattle and buffalo. In: Nutrient requirements of animals. Indian Council of Agricultural Research, New Delhi, India.
23. Kavanek, M. and Janicek, G. (1969). Effect of locality and variety on the content of some trace elements in oat grains. Vys. Skoly Chemtechnol Praze E, 24, 65.
24. Keen, C. L. and Zidenberg-Cheer, S. (1990). Manganese. In: Present knowledge in nutrition. M.L. Brown ed. International life Science Institute, Washington, D.C.
25. Kellogg, D.W. (1990). Zinc methionine affects performance of lactating cows. Feedstuffs, 62, 35.
26. Khillare, K. P. (2007). Trace Minerals and Reproduction in Animals. Intas Polivet, 8(2), 308-314.
27. Knowles, S. O., Grace, N. D., Wurms, K. and Lee, J. (1999) Significance of amount and form of dietary selenium on blood, milk and casein selenium concentrations in grazing cows. Journal of Dairy Science, 82, 429-437.
28. Kramer, U., Talke, I. N. and Hanikenne, M. (2007). Transition metal transport. FEBS Lett, 581, 2263-
29. Krolak, M. (1968). Polsk Arch Leter, 11, 293-
30. Kumar, S. (2003). Management of infertility due to mineral deficiency in dairy animals. In proceedings of ICAR summer school on “Advance diagnostic techniques and therapeutic approaches to metabolic and deficiency diseases.
31. Kumar, S., Pandey, A. K., Razzaque, W. A. A. and Dwivedi, D. K. (2011). Importance of micro minerals in reproductive performance of livestock. Veterinary World, 4(5), 230-
32. Maas, J. (1987). Relationship between nutrition and reproduction in beef cattle. Veterinary Clinics of North America Food Animal Practice, 3, 633-646.
33. McClure, T. J., Eamens, G. J. and Healy, P. J. (1986). Improved fertility in dairy cows after treatment with selenium pellets. Australian Veterinary Journal, 63, 144-
34. Miller, J. K., Ramsey, N. and Madsen, F. C. (1988). The ruminant animal. D.C. Church. Ed. Pp. 342- 400. Prentice Hall, Englewood cliffs, N.J.
35. Miller, J. K. and Tillapaugh, K. (1967). Cornell Feed Service, 62, 11.
36. Miller, W. J. (1970). Zinc nutrition of cattle: a review. Journal of Dairy Science, 53, 1123-1135.
37. NRC (2001). Nutrient requirements of Dairy cattle: 7th edn. National Acedemic press. pp: 105-1146.
38. O’Dell L (1990). In: present knowledge in nutrition. M.L. Brown, Ed., International life Sciences Institute Foundation. Washington DC. pp. 261-267.
39. Ortman K. and Pehrson B. (1999) Effect of selenate as a feed supplement to dairy cows in comparison to selenite and selenium yeast. Journal of Animal Science, 77, 3365-3370.
40. Overton, T.R. and Yasui, T. (2014). Practical applications of trace minerals for dairy cattle. Journal of Animal Science, 92, 416-
41. Paterson, J.A. and Engle, T.E. (2005). Trace mineral nutrition in beef cattle. Nutrition Conf. Proc. Dep. of Anim. Sci. UT Ext. and Univ. Prof. Dev., Univ. of Tennessee, Knoxville.
42. Patterson, H. H., Adams, D. C., Klopfenstein, T. J., Clark, R. T. and Teichert, B. (2003). Supplementation to meet metabolizable protein requirements of primiparous beef heifers: II. Pregnancy and Economics. Journal of Animal Science, 81, 503-
43. Phillippo, M., Humphries, W. R. and Garthwaite, P. H. (1987). The effect of dietary molybdenum and iron on copper status and growth in cattle. Journal of Agriculture Science, 109, 315-320.
44. Podoll, K. L., Bernard, J. B., Ullrey, D. E., DeBar, S. R., Ku, P. K. and Magee, W. T. (1992). Dietary selenate versus selenite for cattle, sheep, and horses. Journal of Animal Science, 70, 1965-1970.
45. Prabowo, A., Spears, J. W. and Goode, L. (1988). Effects of dietary iron on performance and mineral utilization in lambs fed a forage-based diet. Journal of Animal Science, 66, 2028-2035.
46. Puls, R. (1994). Mineral level in animal health. Diagnotic data. 2nd ed. Sherpa International, Clearbrook, BC, Canada.
47. Radostits, O.M., Gay, C.C., Blood, D.C. and Hinchliff, F.W. (2007). Veterinary Medicine. A text book for the diseases of cattle, sheep, pigs, goats and horses, 10th Ed.  Bailliere Tindall, London,
48. Randhawa, S. S. and Randhawa, C. S. (1994). Trace element imbalances as a cause of infertility in farm animals. Recent advances in animal reproduction and Gynaecology. 106-109. Proceedings of the summer institute, held at PAU, Ludhiana on July 25 to Aug 13. pp. 103-121.
49. Rojas, M. A. (1965). Manganese deficiency disease in bovine. Ph. D. Thesis submitted to the Washington University, USA. Cited from Hidiroglou, M., 1979. Canadian Journal of Animal Science, 59: 217- 236.
50. Schrauzer, G. N. (2000). Selenomethionine: a review of its nutritional significance, metabolism and toxicity. Journal of Nutrition, 130, 1653-1656.
51. Smith, R. D. and Chase, L. E. (2010). Nutrition and reproduction, dairy integrated reproductive management.
52. Spears, J. W., Kegley E. B., Mullis L. A. and Wise, T. A. (1997) Bioavailability of copper from tri-basic copper chloride in cattle. Journal of Animal Science, 75, 265.
53. Spears, J.W. (1996). Organic trace minerals in ruminant nutrition. Animal Feed Science Technology Journal, 58, 151-163.
54. Staats, D. A., Lohr, D. P. and Colby, H. D. (1988). Effects of tocopherol depletion on the regional differences in adrenal microsomal lipid peroxidation and steroid metabolism. Endocrinology, 23, 975-
55. Tuormaa, T. E. (2000). Chromium Selenium Copper and other trace minerals in health and reproduction. Journal of orthomolecular medicine, 15, 145-
56. Underwood E. J. (1977) Trace Elements in Human and Animal Nutrition, 4th ed. Academic Press, New York.
57. Underwood, E. J. and Suttle, N. F. (1999). The Mineral Nutrition of Livestock, 3rd ed. CABI Publishing, Oxon, U.K.
58. van Bruwaene, R., Gerber G. B., Kirchmann R., Colard J. and van Kerkom, J. (1984). Metabolism of 51Cr, 54Mn, 59Fe and 60Co in lactating dairy cows. Health Physics. 46, 1069-1082.
59. Wada, O. (2004) “What are trace elements? Their deficiency and excess states”. Japan Medical Association Journal, 47(8), 351-358.
60. Weiss, W.P. (2005). Selenium sources for dairy cattle. Page 61-71. In: Proceeding of Tri-State Dairy Nutrition Conference, Ft. Wayne, IN.
61. Wright CL (2012). Mineral nutrition and its impact on reproduction. In Proceedings, Applied Reproductive Strategies in Beef Cattle December 3- 4, 2012, Sioux Falls, SD. pp: 213-222.
62. Wright, P. L. and Bell, M. C. (1966). Comparative metabolism of selenium and tellurium in sheep and swine. American Journal of Physiology, 211, 6-10.