Trace mineral metabolism in ruminant

Introduction

Trace minerals have long been identified as essential components in the diets of domestic livestock species. Included in the category of essential trace minerals (or microminerals) are chromium, cobalt, copper, iodine, iron, manganese, molybdenum, nickel, selenium and zinc (NRC, 1996). With the essentiality of each of the trace minerals becoming clear, for ruminants it has become necessary to identify the amount of each element that would be necessary for the maintenance of animal health and productivity. Although these values have evolved over time and continue to evolve, the current standards as set by the National Research Council
(NRC, 1996) for cattle are as follows: 0.1 mg of cobalt/kg DM, 10 mg of copper/kg DM, 0.5 mg of iodine/kg DM, 50 mg of iron/kg DM, 20 mg of manganese/kg DM, 0.1 mg selenium/kg DM, and 30 mg of zinc/kg DM. Values for molybdenum, nickel and chromium are not reported due to a lack of data to support a specific numerical value. It is also important to note that for gestating or lactating beef cattle, the manganese requirement doubles from 20 mg/kg DM of manganese to 40 mg of manganese/kg DM, while all other mineral requirements remain the same (NRC, 1996). Trace minerals exist in cells and tissues of the animal body in a variety of chemical combinations, and in characteristic concentrations, depending on the trace mineral consumed and the tissue in which it is metabolized (McDowell, 1992; Underwood and Suttle, 1999). Concentrations of trace minerals must be maintained within narrow limits if the functional and structural integrity of the tissue is to be maintained and the growth, health, and productivityof the animal are to remain unimpaired (McDowell, 1989; 1992; Underwood and Suttle, 1999). Ingestion of diets that are deficient, imbalanced, or excessively high in trace minerals may induce changes in the form or concentration of trace minerals in the body tissues and fluids, so that they fall below or rise above  the  tolerable  limits.  In  such  cases, biochemical lesions can develop, physiological functions may be adversely affected and structural disorders may arise, in ways which vary with the trace mineral, the degree and duration of the dietary deficiency or toxicity, and the age, sex, or species of animal involved (McDowell, 1989; 1992; Underwood and Suttle, 1999). Certain homeostatic mechanisms in the body can be activated which delay or minimize the onset of such diet-induced changes. Ultimate prevention of the aforementioned changes requires that the animal be supplied with a diet that is palatable and non-toxic and which contains the required minerals, vitamins and other nutrients, in adequate amounts, proper proportions, and available forms (Underwood, 1971; Underwood and Suttle, 1999).  Trace mineral deficiencies, toxicities, and imbalances require the animal to metabolically compensate for the nutrient deviation. In doing so, certain metabolic diseases can manifest and overall animal production can be depressed, thus decreasing overall animal performance and health. The intent of this review is to: 1) give a general description of trace mineral metabolism in ruminants, 2) discuss factors that affect trace mineral requirements in ruminants, and 3) give a brief description of the effects of trace minerals on the environment.

Absorption of zinc, copper and manganese

The general mechanisms of trace mineral absorption have been researched extensively in non ruminant species but, to date, are not clearly understood. As Bronner and Yost (1985) have suggested, however, trace mineral absorption probably involves both an active and saturable phase as well as a passive and unsaturable phase. Although the molecular mechanism of absorption has not been fully elucidated, the actual site of zinc and most likely copper and manganese absorption has been identified as the small intestine, primarily the duodenum (Davies, 1980).
Prior to introduction at the luminal side of the intestinal cell, it has been proposed that a binding ligand, most likely a protein, may be bound to the trace mineral (Cousins, 1985). This binding ligand may then act as a chaperone to introduce the trace mineral at the brush border or may remain intact with the metal and be absorbed as a cotransporter. The actual mechanism of transport across the brush border is not well understood, but it is likely that a membrane bound transporter of some sort exists. Data suggest that copper and zinc compete for the same transport mechanism as they enter the cell (Oestreicher and Cousins, 1985).
As Hempe and Cousins (1992) have illustrated, the trace mineral is probably bound to an intestinal binding protein after entrance into the cell. In the case of zinc, this intestinal binding protein has been specifically identified as Cysteine-rich Intestinal Protein (CRIP). The function of this binding protein is to act as both a protective mechanism for the cell by binding to free metal in the cytosol and as a specific carrier to chaperone the mineral across the cell to the basolateral membrane (Figure 1). If an intestinal binding protein fails to bind the trace mineral, it will most likely be bound to a non-specific binding protein or in the case of copper and zinc to metallothionein.  The  primary  function  of metallothionein is to maintain homeostasis of copper and zinc (Pattison and Cousins, 1986). As has been shown by Menard et al. (1981), as metallothionein levels increase, the absorption of zinc decreases; and as dietary zinc concentrations increase, an increase in metallothionein mRNA is also observed. This further supports the theory that trace mineral homeostasis is partially controlled via metallothionein and supports conclusions by Suttle et al. (1982) that zinc homeostasis is controlled at the level of absorption rather than excretion.
Once the bound trace mineral crosses through the cytosol and arrives at the basolateral membrane it is removed from the binding protein and transferred across the membrane via a poorly understood, but saturable, transport mechanism (Oestreicher and Cousins, 1984). The trace mineral is then bound to albumin as it enters circulation (Smith et al., 1979). The albumin remains bound to the trace element until it reaches the liver and the trace mineral is further metabolized before being released for transport to other body tissues (Cousins, 1985).

Figure 1. Proposed mechanism for zinc absorption (adapted from Hempe and Cousins, 1992).



Functions of trace minerals

The general functions of minerals can be broken down into four categories: 1) structural: minerals that play a role as components of tissues; 2) physiological: minerals that are involved in acid-base balance; 3) catalytic: minerals that are components of enzyme and hormone systems; and 4) regulatory: minerals that are involved in cell replication processes, (Underwood and Suttle, 1999). The specific functions of copper, zinc, manganese, and selenium, however, fall mainly into the catalytic and regulatory categories.

COPPER

Copper is second only to zinc in the number of enzymes that require it for appropriate function
(Underwood and Suttle, 1999). Copper is therefore essential to proper physiological function and is involved in an array of systems. These include iron metabolism, cellular respiration, cross-linking of connective tissue, central nervous system formation, reproduction and immunity as well as several other functions. (McDowell, 1992).
In order for hemoglobin synthesis to occur, iron must be converted to the ferric form before being incorporated into the hemoglobin molecule. This process is accomplished by ceruloplasmin, which is a copper-containing enzyme synthesized in the liver for this purpose (Saenko et al., 1994). Therefore, in a state of copper deficiency, hemoglobin synthesis is reduced (Hart et al., 1928). Copper is also an essential component in the enzyme cytochrome oxidase. This enzyme acts as the terminal oxidase in the electron transport chain and is essential to cellular respiration by converting oxygen to water (Spears, 1999). Cytochrome oxidase is also necessary for proper central nervous system function. Enzootic ataxia (swayback), which is associated with incomplete myelin formation, has been linked to an observed decrease in cytochrome oxidase activity in young lambs (Fell et al., 1965). Cross-linking of connective tissue is also facilitated by a copper containing enzyme, lysyl oxidase (Harris and O’Dell, 1974). In the absence of lysyl oxidase, dehydromerodesmosine cannot be converted to isodesmosine, which is an essential component in the cross-linking of elastin (Gallop et al., 1972).
The essentiality of copper for optimal reproductive performance has also been widely documented, although a specific copper-linked enzyme that is responsible has not been identified. It is likely that an array of copper-containing, or copper-activated, compounds are involved in the reproductive process making this identification even more difficult. Corah and Ives (1991) noted that clinical signs of copper deficiency associated with reproduction include decreased conception rate, overall infertility, anestrus and fetal resorption. Some of these problems may be associated with the function of a major intracellular enzyme, copper-zinc superoxide dismutase. This copper-containing enzyme functions as an antioxidant to protect cellular contents from oxidative stress. The same copper-zinc superoxide dismutase has also been implicated to have an important role in proper function of the immune system (Miller et al., 1979).

ZINC

In addition to the functions mentioned for copper- zinc superoxide dismutase, zinc is also involved in an array of other systems as an enzyme component or activator. It therefore plays an indirect role in gene expression, growth, reproduction, immunity, vitamin A metabolism and many other processes (McDowell, 1992). Chesters (1997) indicated that zinc is involved as a component of a number of transcriptional regulators involved in the gene transcription process. These include zinc fingers in DNA binding proteins, RNA polymerase and DNA polymerase. Involvement in the basic transcription process may be the main role that zinc plays across all body systems, although it is not the only area of zinc influence.
Zinc has been shown to be essential for adequate growth and development. However, reduction in growth rate may partly be due to a decreased feed intake that has been observed in conjunction with zinc deficiency in rodent models (Mills and Chesters, 1969). Spears (1999) has suggested that poor growth may also be correlated with a reduction in protein synthesis due to impaired gene transcription processes under conditions of zinc deficiency. Reproduction has been identified as an area that is significantly affected by a state of zinc deficiency.
Apgar (1992) noted that rats fed a diet low in zinc exhibited several reproductive problems including fetal malformation, reduced birth weight, poor offspring viability and difficult parturition. Immunity is greatly affected by zinc status of the animal. Zinc supplementation has been associated with an increased antibody titer response and a decrease in respiratory disease in feedlot steers (George  et al., 1997).  This may be due to the function of copper-zinc superoxide dismutase as previously mentioned or to a variety of other processes that include zinc. These functions may include thymic hormone production and activity, lymphocyte function, natural killer function, neutrophil function and lymphokine function (Hambidge et al., 1986).
Zinc has also been shown to be involved in vitamin A metabolism. It has been reported that in cases of zinc deficiency the retinol binding protein responsible for vitamin A transport in the blood is reduced (Smith et al., 1973). This results in a decline in the amount of vitamin A that can be mobilized from the liver and moved to body tissues. Therefore, a zinc deficiency can lead to a vitamin A deficiency.

MANGANESE

Manganese is involved in many of the same processes already mentioned for zinc and copper, although the original research that identified manganese as an essential trace element was based on measurements of reproductive parameters (Orent and McCollum, 1931; Kemmerer et al., 1931). A study by Hidiroglou (1975) showed that manganese uptake was greater in the ovine Graafian follicle and corpus luteum when compared to other reproductive tissues. This author suggested that manganese may be essential for normal ovarian function. As Maas (1987) pointed out, manganese deficiency has been associated with the anestrus condition in cattle as well. Manganese has also been identified as a component of manganese superoxide dismutase, which functions in a similar manner to copper-zinc superoxide dismutase by reducing the risk of peroxidation damage to body tissues, particularly the heart (Malecki and Greger, 1996). Manganese has also been identified as an essential component in bone and cartilage formation and growth.  Leach (1971) noted that manganese is essential in the activation of glycotranferases that are partly responsible for mucopolysaccharide synthesis.  Without these important structural components of cartilage, skeletal defects often result.  Manganese is also involved in lipid and carbohydrate metabolism. Therefore, manganese deficiency can potentially lead to a decrease in overall animal growth (Prasad, 1984).
McDowell (1992) and Underwood and Suttle (1999) have also identified manganese as an essential component for brain function, structural integrity of cells, enzyme activity and most interestingly, blood clotting. Doisey (1973) showed that manganese-deficient chicks exhibit a reduction in the blood clotting response.  It has been hypothesized that this is due to the role of manganese in prothrombin formation, although the exact mechanism is not fully understood.

SELENIUM

Selenium was first identified in the 1930s as a toxic element to some plants and animals. However, selenium is now known to be required by laboratory animals, food animals, and humans (McDowell, 1992). Selenium is necessary for growth and fertility in animals and for the prevention of a variety of disease conditions. In 1973, Rotruck et al. reported that selenium functions as a component of glutathione peroxidase (GSH-Px), an enzyme that inactivates oxygen radicals such as hydrogen peroxide and prevents them from causing cellular damage. Since the discovery by Rotruck  et al. (1973), selenium has been shown to affect specific components of the immune system (Mulhern et al., 1985).  Earlier research by Reffett  et al. (1988) reported lower serum IgM (an antibody produced by B-cells) concentrations and anti-IBRV titers in selenium-deficient calves challenged with infectious bovine rhinotrachetis virus than when compared to selenium-adequate calves. Polymorphonuclear leukocyte function was reduced in goats (Azizi et al., 1984) and cattle (Gyang  et al., 1984) fed selenium-deficient diets compared with controls receiving selenium-adequate diets. Some studies have shown increased T-lymphocyte blastogenesis following in vitro stimulation with mitogen while others have not (Spears, 2000). Recently, bovine mammary endothelial cells growing in selenium- deficient cell culture media were found to exhibit enhanced neutrophil adherence when stimulated with cytokines (Maddox et al., 1999; Spears, 2000). These findings may indicate that selenium could affect neutrophil migration into tissues and subsequent inflammation.
The biggest challenge in interpreting trace mineral studies  with  ruminants  is  the  variation  in supplementation protocols that have been examined. Researchers have used a variety of mineral sources ranging from organic trace minerals (proteinates and amino acid complexes) to inorganic trace mineral forms.  In  addition  to  this,  trace  mineral concentrations in the supplements used have varied dramatically. The feeding period, as well as the feeding period relative to physiological status (i.e. date of parturition, growing, finishing, etc.), have also varied significantly. Furthermore, the interactions between trace minerals, animal production and disease resistance are extremely complex.

Factors affecting trace mineral requirements

Despite the apparent involvement of certain trace minerals in animal production and disease resistance, deficiencies of trace minerals have not always increased the susceptibility of domesticated livestock species to natural or experimentally-induced infections (Spears, 2000). There are many factors that could affect response to trace mineral supplementation such as the duration, form and concentration of supplementation, physiological status of an animal (i.e. pregnant vs non pregnant), the absence or presence of dietary antagonists, environmental factors and the influence of stress on trace mineral metabolism. For the purpose of this portion of the review, five areas deserve attention when discussing potential factors that may affect the trace mineral requirements of ruminants: breed, gestational status, stress, trace mineral antagonists, and age.

BREED

Although species differences in trace mineral metabolism have long been recognized, only recently have differences been noted between breeds within a species. Differences in trace mineral metabolism between breeds of dairy cattle have been reported. In an experiment by Du et al. (1996), Holstein (n=8) and Jersey (n=8) primiparous cows and Holstein (n=8) and Jersey (n=8) growing heifers were supplemented with either 5 or 80 mg of copper/kg dry matter for 60 days. At the end of the 60 day experiment, Jerseys had higher liver copper concentrations relative to Holsteins across both treatments. Furthermore, liver copper concen- trations increased more rapidly and were higher in the Jerseys supplemented with 80 mg of copper/kg DM compared to Holsteins supplemented with 80 mg of copper/kg DM by day 60 of the experiment. Overall serum ceruloplasmin oxidase activity (a copper-dependent enzyme involved in iron transport) was higher in Jerseys than Holsteins. Additionally, Jersey cows and heifers had higher liver iron and lower liver zinc concentrations than did Holstein cows and heifers at day 60 of the experiment. These data indicate that Jerseys and Holsteins metabolize copper, zinc, and iron differently.
Ward et al. (1995) conducted a metabolism study in which Angus (n=8) and Simmental (n=8) steers were placed in metabolism crates to monitor apparent absorption and retention of copper. At the end of the six-day metabolism experiment, plasma copper concentrations, apparent absorption and retention of copper were higher in Angus relative to Simmental steers. The authors indicate, from their data as well as from others, that Simmental cattle may have a higher copper requirement than Angus cattle and that these different requirements may be related to differences in copper absorption from the gastrointestinal tract between breeds. Furthermore, it has also been suggested that these breed differences in copper metabolism may not be due solely to differences in absorption, but also to the manner in which copper is utilized or metabolized post-absorption. Gooneratne  et al. (1994) reported that biliary copper concentrations are considerably higher in Simmental cattle than in Angus cattle. It is apparent that differences in copper metabolism exist between Simmental and Angus cattle both at the absorptive and post-absorptive level.
An extensive study comparing the mineral status of Angus, Braunvieh, Charolais, Gelbvieh, Hereford, Limousin, Red Poll, Pinzgauer and Simmental breeds consuming similar diets has also been conducted (Littledike et al., 1995). This work compared not only copper, but also zinc and iron status between all previously mentioned breeds of cattle. In adult cattle, it was shown that Limousin liver copper concentrations were higher than all other breeds, except for Angus. This same trend was not seen for zinc or iron, with very little breed differences observed except for lower liver zinc concentrations in Pinzgauer when compared to Limousin. Serum zinc and copper concentrations did not differ by breed.

GESTATIONAL STATUS

Although few reports have been published examining the effects of gestational status on trace mineral metabolism in cattle, several experiments have been conducted using lab animals and humans that indicate trace mineral metabolism is altered during pregnancy. Research has indicated that zinc concentrations increase in bovine conception products (placenta, placental fluids, and fetus) as the fetus grows (Hansard et al., 1968). Studies using rats have shown that the overall maternal body stores of copper and zinc increase during pregnancy and then decrease during lactation. Mean zinc total body stores at the start of pregnancy were recorded at 5260 mg of zinc versus 5810 mg of zinc at day 15 of pregnancy. By day 14 of lactation, maternal body stores of zinc had decreased to 5640 mg of zinc, which was still considerably higher than at the onset of pregnancy (Williams et al., 1977). These same trends were observed with copper. In a recent experiment by Vierboom  et al. (2002), pregnant cows and sheep absorbed and retained zinc to a greater degree that non-pregnant cows and sheep. These data indicate that certain physiological and/ or metabolic parameters are altered in pregnant cows and ewes consuming an alfalfa-based diet that enhance the apparent absorption and retention of certain trace minerals.
The trend for serum zinc concentrations to be lower in pregnant women has also been observed by Swanson and King (1982). Pregnant women in this study had lower serum zinc concentrations relative to non-pregnant women. The researchers noted, however, there was a greater apparent zinc retention in pregnant women (1.9 mg/day) versus non-pregnant women (0.9 mg/day). Furthermore, findings by Turnland et al. (1983) suggest that even though the need for additional trace elements may exist during pregnancy, the body may facilitate absorption and retention without increased intake being necessary.  Turnland  et al. (1983) also reported that pregnant women absorbed more copper from a plant protein-based diet than did non-pregnant women (40.7% versus   33.8%, respectively). This same trend was also observed when an animal protein-based diet was consumed, however, the differences were not statistically significant.
The aforementioned data indicate that copper and zinc metabolism is altered in pregnant vs  non- pregnant animals. Further research is required to determine the metabolic mechanisms that enable pregnant animals to alter copper and zinc metabolism as well as the animal’s specific metabolic requirement for both maintenance and fetal development. In addition, research is needed to determine the effects of gestational status on the metabolism of other trace minerals, as well as if breed differences exist relative to trace mineral metabolism and gestational status.

STRESS

As mentioned earlier, trace minerals such as copper and zinc are involved in immune response. Deficiencies and or imbalances of these elements can alter the activity of certain enzymes and function of specific organs, thus impairing specific metabolic pathways as well as overall immune function. Stress and its relationship to the occurrence of disease has long been recognized. Stress is the nonspecific response of the body to any demand made upon it (Selye, 1973). Stressors relative to animal production include a variety of circumstances such as infection, environmental factors, parturition, lactation, weaning, transport, and handling. Stress induced by parturition, lactation, weaning, and transport has been shown to decrease the ability of the animal to respond immunologically to antigens that they encounter. Furthermore, research has indicated that stress can alter the metabolism of trace minerals. Stress in the form of mastitis and ketosis has been shown to alter zinc metabolism in dairy cattle. Orr et al. (1990) reported an increase in urinary copper and zinc excretion in cattle inoculated with IBRV. Furthermore, Nockels et al. (1993) reported that copper and zinc retention was decreased in steers injected with ACTH (a stressor) in conjunction with feed and water restriction. These studies, in conjunction with several others, indicate that stress in the form of an infection (IBRV), a metabolic disorder (ketosis), or deprivation of feed and water can increase copper and zinc depletion from the animal.

TRACE  MINERAL  ANTAGONISTS

Many element-element interactions have been documented (for an in depth review see Puls, 1994). These include zinc-iron, copper-iron, copper-sulfur, copper-molybdenum, and copper-molybdenum- sulfur interactions and interactions between elements and other dietary components. Peres et al. (2001) used perfused jejunal loops of normal rats to characterize the effects of the iron:zinc ratio in the diet on mineral absorption. When the iron:zinc ratio in the diet was held below 2:1, no detrimental effects on absorption were observed. However, once concentrations were increased to a ratio between 2:1 and 5:1, zinc absorption was decreased. Similar effects have also been seen for copper absorption, with depressed copper uptake in the presence of excess iron (Phillippo et al., 1987). The best known mineral interaction that can cause a reduction in copper absorption and utilization is the copper-molybdenum-sulfur interaction. However, even molybdenum or sulfur alone can have antagonistic effects on copper absorption. Suttle (1974) reported that plasma copper concentrations were reduced in sheep with increasing concentrations of dietary sulfur from either an organic (methionine) or inorganic (sodium sulfate) form. In another experiment, Suttle (1975) demonstrated that hypocupraemic ewes fed copper at a rate of 6 mg copper/kg of diet DM with additional sulfur or molybdenum, exhibited slower repletion rates than sheep fed no molybdenum or sulfur. However, when both molybdenum and sulfur were fed together, copper absorption and retention was drastically reduced. Current research would support these findings and suggest that in addition to  independent  copper-sulfur  and  copper- molybdenum interactions, there is a three way copper-sulfur-molybdenum interaction that renders these elements unavailable for absorption and/or metabolism due to the formation of thiomolybdates (Suttle, 1991).
Ward (1978) also investigated the independent effect of molybdenum on copper absorption and concluded that elevated molybdenum intake reduces copper availability and can lead to a physiological copper deficiency. This was attributed to a copper- molybdenum complex which forms in the rumen and cannot be broken down and absorbed. Based on this and previous experiments, it appears that the ratio of the antagonistic elements seems to be more important than the actual amounts. Miltimore and Mason (1971) reported that if copper: molybdenum ratios fall below 2:1, copper deficiency can be produced. Therefore, feeding additional copper has been recommended in areas where a molybdenum interaction is suspected. Huisingh et al. (1973) further concluded, in their attempt to produce a working model of the effects of sulfur and molybdenum on copper absorption, that both sulfur (in the form of sulfate or sulfur-containing amino acids) and molybdenum reduce copper absorption due to the formation of insoluble complexes. They also noted that sulfur and molybdenum interact independently and suggested that they may share a common transport mechanism. Interactions between and among minerals are not the only possible inhibitors of mineral absorption. Other dietary components can also inhibit or enhance the amount of mineral that is absorbed. Protein, as might be expected from the discussion involving sulfur-containing amino acids, is an example of a dietary component that can affect mineral metabolism. Snedeker and Greger (1983) reported that high protein diets significantly increase apparent zinc retention. In contrast, diets high in sulfur- containing amino acids have been shown to decrease copper absorption, most likely due to the formation of insoluble copper-sulfur and potentially copper-sulfur-molybdenum complexes (Robbins and Baker, 1980).
O’Dell (1984) noted the potential for carbohydrate source to affect copper absorption. This was attributed to phytate as well as oxalate concen- trations in the diet. Fiber can also act as a mineral trap due to its relatively negative charge, which serves to bind the positively charged divalent metal cations rendering them unavailable for absorption (van der Aar et al., 1983).

AGE

Age has also been shown to alter trace mineral needs.  Trace mineral requirements have been reported to vary with age of dairy cattle (NRC, 2001). Wegner  et al. (1972) reported that dairy cattle in their second to fifth lactations had higher serum zinc concentrations than either first lactation or bred heifers, 131 mg/100 ml, 85 mg/100 ml and 93 mg/100 ml respectively. This change in mineral needs over time is most obvious in young, growing animals.

TRACE MINERAL SOURCES

Trace minerals are available from several different sources  –  typical  feedstuffs,  water,  and commercially available supplemental mineral packages.  However, as Underwood and Suttle (1999) indicated, the amount of mineral in a feedstuff or supplement is not as important as the availability of a mineral for absorption and utilization in biological systems.
Currently, trace minerals are available in both organic and inorganic forms. Trace minerals defined as inorganic are those that are typically bound to sulfates, carbonates, chlorides, or oxides, while those defined as organic are bound to amino acids or protein complexes. The general premise behind increased bioavailability of organic trace minerals is that organic trace minerals are protected from many of the interactions (as previously mentioned) that can potentially make them unavailable for absorption (Hemken  et al.,  1996). It has been theorized by some researchers that organic trace minerals remain intact in the gastrointestinal tract, through the sight of absorption, and perhaps beyond absorption.
A number of studies have been conducted using in vitro and  in vivo techniques to determine the relative bioavailability of trace mineral sources. These experiments typically use an inorganic mineral as a benchmark (100%) and compare other mineral sources to it. Results have been variable, however  under  certain  circumstances  (as summarized below), organic mineral sources have been shown to be more bioavailable than inorganic sources.
Ward et al. (1996) reported that organic copper was more bioavailable when high levels of molybdenum were present in the diet. These researchers concluded that the use of organic copper may be advantageous when antagonists are present. Furthermore, Du et al. (1996) concluded that organic sources of copper are better utilized than certain inorganic sources of copper in rats. By adding high levels of iron to the diet, these researchers were also able to conclude that organic copper did not interfere with iron absorption to the same extent seen with inorganic copper sources. Limited data suggest that manganese from organic sources is as available if not more available than from inorganic sources.  A study using crossbred wether lambs compared organic and two forms of inorganic manganese. Results clearly show that the organic form was more available than the two inorganic forms of manganese (Henry et al., 1992). These conclusions were based on multiple linear regression comparisons of bone, kidney, and liver manganese concentrations on total dietary manganese concentrations from the varying sources.

Effect of organic mineral supplementation on reproductive performance

Numerous trials have been conducted to identify the impact of supplementing trace minerals on reproductive performance in beef cattle. The challenge in interpreting these data is that researchers have used various methods of supplementation, different sources of trace minerals, a variety of different cattle types, and a variety of reproductive response variables. Therefore, caution must be taken when interpreting/comparing results between experiments.
One of the earlier studies involving cattle was performed by Manspeaker et al. (1987). In their study, bred Holstein heifers were supplemented with either a control diet or the control diet plus an organic trace mineral supplement. The organic supplement supplied additional iron, manganese, copper, and zinc in  addition  to  potassium  and  magnesium. Supplementation began 30 days prepartum and continued through parturition, rebreeding, and the confirmation of pregnancy by rectal palpation. Follicular dynamics were monitored via rectal ultrasonography to determine the effects of supplementation on ovarian function. Results of this study  indicated  a  tendency  for  organic- supplemented females to exhibit more ovarian activity than controls, as well as a decrease in embryonic mortality. Conception occurred earlier in the supplemented animals, which may have been attributed to improved endometrial regeneration of the uterus. This was verified by the measurement of the amount of periglandular fibrosis present, with the organic treatment group exhibiting lower periglandular fibrosis. Manspeaker  et al. (1987) therefore concluded that feeding excess trace elements in an organic form improved reproductive performance in first-calf Holstein heifers while not causing toxicity problems.
Researchers in Ireland (O’Donaghue et al., 1995) have also examined the effects of supplementing organic trace minerals to dairy cattle. When organic copper and zinc were added to diets fortified with selenium, Friesian cattle tended to have a higher first service conception rates and lower somatic cell counts. Similar results were reported in an experiment utilizing organic copper, zinc and manganese in conjunction with selenium, with somatic cell counts decreasing in supplemented groups compared to controls. However, this same study reported no positive effects on reproduction (Boland et al., 1996).
Stanton et al. (2000) conducted research using Angus cows (n = 300) that examined the efficacy of supplementing inorganic minerals at either a low or high level versus supplementing organic minerals at a high level. The composition of the mineral supplements fed in free-choice mineral feeders were as follows: 501, 1086, 1086 mg copper/kg DM; 2160, 3113, 3113 mg zinc/kg DM; 1225, 1764, 1767 mg manganese/kg DM; and 11, 110, 110 mg cobalt/ kg DM (inorganic low, inorganic high, and organic high respectively). Although overall pregnancy rate at the end of the breeding season was not affected by  treatment,  conception  rate  to  artificial insemination was increased in the organic high treatment versus the inorganic low and inorganic high  treatments  (75%   vs   61%  and  56%, respectively). Higher rates of gain were also observed for calves born to organic supplemented cows, although cow weight and body condition score did not differ by treatment. Calves in the inorganic high treatment also had lower weaning weights compared to calves in either organic high or inorganic low treatments. Other studies have shown trace minerals are required for optimal reproductive performance. Gengelbach (1990) reported a tendency for cows supplemented with organic and inorganic trace minerals to have higher conception rates than cows receiving the control supplement. However, conception rates were similar for cows supplemented with organic and inorganic trace minerals.
As indicated previously, it is challenging to interpret data from different experiments because researchers have used various methods of supplementation, different sources of trace minerals, a variety of breeds, ages, and a variety of reproductive variables. Moreover, breed of cattle, antagonists present in the diet, as well as physiological status of the animal must be taken into consideration when comparing the results from different experiments.

Trace minerals and the environment

Livestock production has drastically changed over the past 50 years. The implementation of new technologies and production techniques has enhanced the efficiency of production of meat and dairy products. The increase in the efficiency of production has enabled producers to produce more product (milk and meat) with fewer animals, while maintaining a low end product cost for the consumer. However, the environmental impact of animal production system by-products (feces, urine, and gases) is of major concern. Excessive nutrient accumulation in areas of intensive animal production is primarily due to production operations importing greater amounts of elemental nutrients relative to the amount of nutrients exported in food and animal products (Van Horn et al., 1996). Therefore, the new challenge facing the agriculture community is to minimize the environmental impact of livestock production systems from confined livestock feeding operations.
Environmental concerns can be divided into three categories: soil, water, and air (Jongbloed and Lenis,
1998). Nitrogen, phosphorus, copper, and zinc have been of most concern relative to their accumulation in soil and water from animal waste and methane because of implications to global warming. In addition, odors emanating from areas of intense livestock concentration have come under scrutiny from residents living near confinement livestock operations. The major goals for the future relative to animal agriculture and the environment will be to increase the utilization of nitrogen, phosphorus, and trace  elements  while  decreasing  methane production. Therefore, in order to minimize excretory losses of nutrients (nitrogen, phosphorus and trace elements), sources providing better utilization to the animal must be found. In order to do this, criteria of mineral adequacy and availability must be established using immunological, hormonal, and biochemical indices.

Summary

The interactions between trace minerals, animal production and disease resistance are extremely complex. Many factors can affect animal response to trace mineral supplementation such as the duration and concentration of trace mineral supplementation, physiological status of an animal (pregnant  vs open), the absence or presence of dietary antagonists, environmental factors, and the influence of stress on trace mineral metabolism. Breed differences in trace mineral metabolism have also been documented (Weiner  et al., 1978; Gooneratne, et al., 1994; Ward et al., 1995; Du et al., 1996; Mullis  et al., 1997). Furthermore, research has indicated that different breeds of cattle respond differently to the same immune challenge (Schultz et al., 1971; Blecha et al., 1984; Engle et al.,  1999).  This may, in part, be related to differences in trace mineral metabolism among different breeds of cattle. Moreover, future research is needed to determine the effects of trace minerals on animal production, disease resistance, and certain factors that affect trace mineral requirements of beef cattle.

References

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