Trace Mineral Nutrition-What is Important and Where do Organic Trace Minerals
Fit in?
J. W. Spears
Department of Animal Science
North Carolina State University
Corresponding author: Jerry_Spears@ncsu.edu
Introduction
One or more trace minerals are required for the normal functioning of essentially
all biochemical processes in the body. Deficiencies of essential trace minerals,
depending on the severity, can result in clinical or subclinical deficiency signs. Dietary
requirements for many trace minerals are affected by their bioavailability, and antagonist
that reduce bioavailability.
The action of trace minerals are dose dependent and even essential trace minerals
can produce toxic effects when consumed at high concentrations (Figure 1). Toxic
effects of trace minerals can be subtle with no clinical signs. For example, Olson et al.
(1999) reported that supplementing beef cows with zinc, copper, manganese, and cobalt
at concentrations at least twice the NRC requirement, following calving, reduced
pregnancy rates in beef cows. On the other extreme, high levels of certain trace minerals,
such as copper and selenium, can result in death.
Figure 1. Relationship between trace mineral intake and animal responses
Trace Mineral Functions and Bioavailability
Cobalt
The only known function of cobalt relates to its role as a component of vitamin
B12. Ruminal microorganisms are able to synthesize vitamin B12 from dietary cobalt.
The vitamin B12-dependent enzyme methylmalonyl CoA mutase is involved in the
metabolism of propionate to succinate in the liver. A second vitamin B12-dependent
enzyme (methionine synthase) is important in the recycling of methionine following
transfer of its methyl group and in the maintenance of tissue folate concentrations. A
lack of dietary cobalt for vitamin B12 synthesis by rumen microorganisms can also alter
ruminal fermentation. Low dietary cobalt in high concentrate finishing diets reduces
molar proportion of ruminal propionate (Tiffany et al., 2003; Tiffany and Spears, 2005).
Studies in growing and finishing cattle suggest that dietary cobalt requirements
are higher than the 0.10 mg/kg diet recommended by the NRC (Schwarz et al., 2000;
Tiffany et al., 2003). Depending on the criteria used, estimates of cobalt requirements
have ranged from 0.12 to 0.26 mg/kg diet. A recent study in dairy cows indicated that
cobalt supplementation of prepartum and postpartum diets containing 0.15 to 0.19 mg
Co/kg did not affect milk production, intake or serum vitamin B12 concentrations
(Kincaid and Socha, 2007). In addition to cobalt, other dietary factors may affect vitamin
B12 production by ruminal microorganisms. Research in finishing cattle suggests that
ruminal synthesis of vitamin B12 is lower in cattle fed barley-based diets compared to
corn-based diets (Tiffany and Spears, 2005).
Copper
Copper is a component of a number of enzymes including lysyl oxidase,
superoxide dismutase, tyrosinase, ceruloplasmin and cytochrome oxidase. These
enzymes are important in the structural integrity of collagen and elastin, detoxification of
superoxide radicals, pigmentation, iron transport, and energy metabolism.
Sulfur and molybdenum are potent copper antagonists and can greatly increase
copper requirements. Sulfide is produced in the rumen via reduction of sulfate and
degradation of sulfur amino acids. Sulfide and molybdate interact in the rumen
environment, when dietary molybdenum and sulfur are high, to form thiomolybdates
(Spears, 2003). Tri and tetrathiomolybdates can form stable insoluble complexes with
copper that are not absorbed. Certain thiomolybdates that are not bound to copper can be
absorbed and cause excretion of stored copper from the liver or removal of copper from
metalloenzymes (Suttle, 1991). Independent of dietary molybdenum, sulfur reduces
copper absorption probably through the formation of insoluble copper sulfide in the gut.
High dietary iron, when provided in an available form also reduces copper status in cattle
(Mullis et al., 2003).
Iron
Iron plays a vital role in oxygen transport in the blood as a component of
hemoglobin and in oxygen storage and transport in muscle as a component of myoglobin.
A number of cytochromes and iron-sulfur proteins involved in the electron transport
chain also contain iron as an integral component. In addition several enzymes either
contain iron or are activated by iron.
Most practical diets are more than adequate in iron, and iron deficiency is unlikely
in cattle unless parasite infestations or diseases exist that cause chronic blood loss.
Factors affecting bioavailability of iron in ruminants have received little attention in
ruminants because of the abundance of iron in ruminant diets.
Manganese
Manganese is an integral component on the enzymes, arginase, superoxide
dismutase found in the mitochondria, and pyruvate carboxylase. In addition, a number of
enzymes can be activated by manganese. Of the enzymes that can be activated by
manganese only the glycosyltransferases are known to specifically require manganese.
Glycosyltransferases are involved in the synthesis of proteoglycans, and thus are required
for normal formation of skeletal cartilage.
Recent studies with growing and finishing cattle (Legleiter et al., 2005), and
growing heifers (Hansen et al., 2006a) suggests that manganese requirements do not
exceed 16 mg Mn/kg diet. However, this level of manganese was found not to be
sufficient for normal fetal development in gestating heifers (Hansen et al., 2006b). When
heifers were continued on the control diet, containing 16 mg Mn/kg, during gestation
signs of manganese deficiency were observed in some of their calves at birth. Deficiency
signs observed included dwarfism and superior brachygnathism. None of these signs
were noted in calves born to dams supplemented with 50 mg Mn/kg.
Factors that may reduce manganese bioavailability, and thus increase dietary
manganese requirements are not clearly defined. High dietary iron may reduce
manganese bioavailability in certain instances. Possible effects of iron on manganese
metabolism will be discussed in more details in the next section of this paper. Limited
evidence suggests that high dietary calcium and phosphorus may reduce manganese
bioavailability (Hidiroglou, 1979).
Selenium
Selenium functions in the antioxidant system as a component of a family of
glutathione peroxidase enzymes. These enzymes prevent cellular damage by destroying
hydrogen peroxide and lipid hydroperoxides. Selenium also is involved in the
deiodination of thyroxine (T4) to the more metabolically active triiodothyronine (T3) in
tissues. The immune system is adversely affected by selenium deficiency, and it is well
documented that selenium deficiency increases the incidence of mastitis and retained
placenta in dairy cows.
Sulfur and selenium have similar chemical properties and increasing dietary sulfur
reduces the absorption of selenium (Ivancic and Weiss, 2001). There is also evidence
that selenium is less bioavailable in legume hay than in grass hay or concentrates (Spears,
2003). Selenomethionine, which is the major form of selenium found naturally in
feedstuffs and in selenized yeast, is more bioavailable in cattle than sodium selenite
(Pehrson et al., 1989).
Zinc
Zinc is an essential component of over 70 enzymes found in mammalian tissues.
Enzymes that require zinc are involved in protein, nucleic acid, carbohydrate, and lipid
metabolism. Zinc is also important for normal development and functioning of the
immune system, in cell membrane stability, and gene expression.
Responses of cattle to zinc supplementation of practical diets have been highly
variable, suggesting that dietary factors affect zinc bioavailability. However, dietary
factors that may affect zinc bioavailability in ruminants are not well defined. Some
studies would suggest that high dietary calcium reduces zinc status in cattle (Spears,
2003).
Possible Consequences of High Dietary Trace Mineral Concentrations
It is obviously important to supply adequate quantities of essential trace minerals
to meet nutritional requirements in order to maximize productivity and health in cattle.
However, excessive supplementation of trace minerals or use of feedstuffs naturally high
in certain trace minerals can create mineral imbalances or produce other toxic effects that
negatively affect animal performance. Supplementing zinc and copper in high
concentrations relative to requirements may also lead to environmental concerns due to
run-off and land application of waste containing high levels of these metals. Copper and
zinc imports and exports were recently measured in six dairies in Idaho over a 1-year
period (Hristov et al., 2007). Imports far exceeded exports resulting in whole-farm
surpluses for both minerals. They concluded that reducing dietary concentrations of
copper and zinc was the most efficient way of reducing excretion and whole-farm
surpluses of these minerals.
Most trace minerals are generally supplemented in dairy diets at levels equal to or
above NRC recommendations. With iodine, cobalt, and selenium this is probably a good
practice because of their low requirement, the expense of analyzing feedstuffs for these
minerals, and the minimal interactions expected between these minerals and other
minerals. However, numerous interactions can occur between iron, copper, zinc, and
manganese. The balance between dietary concentrations of these minerals, especially
bioavailable concentrations, is an important factor affecting mineral utilization and thus
animal requirements. With these trace minerals we should consider their concentrations
in major dietary ingredients. Variation in the mineral content of feedstuffs presents a
problem when formulating mineral supplements. Forages vary considerably in mineral
concentrations. However, much less variation in mineral content is usually found in
cereal grains and oilseed meals.
Supplementation of trace minerals above requirements is often practiced as a
safety margin, to prevent any likelihood of deficiencies. Providing a certain level of
safety is sound, but the question becomes at what magnitude relative to requirements
should safety levels be provided. If trace minerals are present in the diet in a bioavailable
form, homeostatic control mechanisms in the animal serve to prevent deficiencies. It has
long been recognized that homeostatic mechanisms alter absorption and/or excretion of
essential trace minerals in response to changes in the amount of the mineral consumed or
required, resulting in tissue concentrations of most minerals being maintained within a
fairly narrow range (Miller, 1975). Transporters have been characterized in recent years
for zinc, copper, and iron that are involved in cellular transport and assimilation of the
metals by intestine and other tissues. Many of these transporters are down regulated
when trace minerals are fed in excess of requirements and up regulated when the mineral
is fed at low or marginal levels relative to requirements. Changes in the efficiency of
mineral absorption or post absorptive excretion serves to protect against trace mineral
deficiencies as well as toxicities. However, homeostatic control mechanisms can be
overwhelmed when extremely high concentrations of minerals are fed resulting in
subclinical or clinical signs of toxicosis.
High Dietary Iron
As mentioned previously, iron deficiency is unlikely in cattle because of the high
concentrations of iron commonly found in feedstuffs. It is not uncommon for ruminant
diets to exceed iron requirements by 5-fold or greater. Some feedstuffs that are likely to
be high in iron are shown in Table 1. Hays and silages are highly variable in iron content
as indicated by their high standard deviations. Many by-product feeds that are fairly high
in iron vary less in their iron content. Use of phosphate supplements also increases iron
intake by cattle, as commercial dicalcium and defluorinated phosphates contain
approximately 10,000 mg Fe/kg. Cattle grazing pastures may be exposed to high iron
through forage and/or soil ingestion.
High dietary iron, when provided in a form such as ferrous sulfate, that is highly
bioavailable, has been associated with reduced performance, elevated liver and spleen
iron concentrations, and decreased copper status (Standish et al., 1969; Mullis et al.,
2003). Iron absorption is well regulated, but exposure to high dietary iron may
overwhelm homeostatic control mechanisms resulting in iron accumulation in tissues,
especially the liver and spleen. Iron that is not bound tightly to ligands in the body acts
as a pro-oxidant and causes oxidative damage to tissues. At normal tissue iron
concentrations the oxidant properties of iron are minimal due to iron being bound to
proteins. However, with elevated iron in the body there is a greater likelihood of free
iron existing in certain tissues and causing increased production of free radicals that may
lead to tissue damage. Based on field observations, Pitzer (1994) reported that high iron
in feed or water was associated with impaired reproductive performance and increased
incidence of uterine infections. He attributed these adverse effects to the pro-oxidant
effects of iron.
A number of studies have indicated that high dietary iron is a potent copper
antagonist. The addition of as little as 250 mg Fe/kg diet (from ferrous carbonate)
reduced liver copper concentrations in calves (Bremner et al., 1987). High iron may also
reduced absorption of manganese. It has been demonstrated in rodents that high dietary
iron down regulates divalent metal transporter (DMT1), the major transport protein
involved in transport of iron into intestinal cells (Miret et al., 2003). Although DMT1 is
regulated by iron, it also transports other metals including manganese and copper (Miret
et al., 2003). Therefore, high dietary iron may reduce absorption of manganese and
copper by competition for DMT1 as well as reducing the actual amount of transport
protein.
Little is known regarding bioavailability of iron naturally found in feedstuffs.
High dietary iron may not cause adverse effects on animal performance or health if the
iron present in the diet is of low bioavailability. However, if iron present in feeds is
fairly bioavailable, adverse effects of high iron are more likely to be seen. When high
iron concentrations are detected in forages it is unclear how much of the iron is naturally
present in the forage and how much of the iron is due to soil contamination. In most soil
types iron is extremely high. Iron in soil is generally of low solubility and probably very
poorly absorbed when ingested by cattle. Acid conditions occurring during the
fermentation of silage or haylage may result in soil iron being much more bioavailable
when consumed by cattle. When 1 or 5% soil from different soil types was added to corn
silage prior to ensiling the amount of iron in the silage that was water soluble increased
by 18 to 84 fold compared to control silage with no soil added (Hansen and Spears,
unpublished). When the same levels of soil were added to silage after fermentation water
soluble concentrations of iron only increased 1.6 to 2.2 fold relative to control silage.
This suggests that iron bioavailability from soil may be greatly increased by ensiling.
Much of the iron in soybeans has been shown to be present in the form of ferritin
(Theil, 2004). This is the same ferritin protein that stores iron in the liver of humans and
animals. Recently, ferritin from soybeans was found to be absorbed as well in humans as
ferrous sulfate (Lonnerdal et al., 2006). If ferritin is a major form of iron in other
legumes, this may represent a highly bioavailable form of iron in ruminants. Based on
research in chicks (Henry et al., 1992) and swine (Kornegay, 1972), iron from feed grade
phosphate sources is 48 to 67% as bioavailable as iron from ferrous sulfate.
Table 1. Iron concentrations in feedstuffs
Mean
SD
222
290
123
230
122
176
171
308
523
187
147
76
17
28
24
27
47
44
172
99
619
104
156
331
286
367
392
284
990
206
617
109
157
324
270
490
309
307
796
124
a
By-product feeds
Almond hulls
Beet pulp
Brewers grains
Canola meal
Corn gluten feed
Distillers grains
Cane molasses
Safflower meal
Soybean hulls
Wheat meal run
Other feedstuffsb
Alfalfa
Corn silage
Grass hay
Grass silage
Legume hay
Legume silage
Sorghum grain silage
Sorghum Sudan hay
Sorghum Sudan silage
Soybean meal
a
From DePeters et al., 2000.
b
From NRC (2001).
Organic Trace Minerals
Organic trace minerals are complexed or chelated to organic ligands (amino acids
or polysaccharides). In theory the chemical bonds formed between the metal and
ligand(s) should allow organic trace minerals to resist many of the interactions
encountered by inorganic trace mineral sources. Organic sources of trace minerals have
been found to be more bioavailable than inorganic sources in some studies (Ward et al.,
1996; Hansen et al., 2008).
Using organic trace minerals to replace at least a portion of supplemental
inorganic trace minerals should allow nutritionist to reduce the overall inclusion rate of
trace minerals and still provide a reasonable safety margin. Reducing the inclusion rate
in concentrated cattle operations also will reduce the environmental impact related to
excretion of trace minerals such as zinc and copper in waste.
Responses of dairy and beef cattle to supplementation with organic trace minerals
have been variable. A number of factors such as the level and bioavailability of trace
minerals in feedstuffs can affect whether dairy or beef cattle respond to organic trace
minerals supplementation. In this paper I will emphasize studies where positive
responses have been observed.
Replacing a portion of the inorganic trace minerals with chelated or complexed
trace minerals has improved reproduction in a number of dairy studies (Uchida et al.,
2001; Ballantine et al., 2002; Nocek and Patton, 2002). In one study, over 500 cows
received total mixed rations supplemented with either inorganic trace minerals or a
combination of inorganic and proteinate trace minerals (Nocek and Patton, 2002). Cows
were fed their respective mineral treatment from 60 days prepartum until 150 days
postpartum. Trace minerals were added in both treatments to provide 120% of the NRC
requirement. In the chelated treatment, 50% of the supplemental zinc, manganese, and
copper were supplied from proteinate chelates. Reproductive performance is shown in
Table 2. Days to first heat and days to first breeding were lower for cows fed chelated
trace minerals. Days open, for cows confirmed pregnant by 150 days in milk, were 7
days less for the chelated treatment group. A recent study (Nocek et al., 2006) indicated
that reproductive performance was greater in dairy cows supplemented with a
combination of inorganic and organic trace minerals compared to cows supplemented
exclusively with inorganic or complexed minerals. Milk production has also been
increased by using organic trace minerals in some studies (Ballantine et al., 2002; Nocek
and Patton, 2002: Nocek et al., 2006).
In beef cows adding trace mineral proteinates (Tiffany et al., 2001) or complexes
(Stanton et al., 2001) to free choice mineral supplements has improved reproduction,
especially AI pregnancy rates. Finishing steers supplemented with zinc proteinate tended
to gain faster and more efficiently than steers receiving zinc oxide (Spears and Kegley,
2002). Hot carcass weights and dressing percentages were also high in steers
supplemented with zinc proteinate. Malcolm-Callis et al (2000) reported no differences
in performance of finishing cattle supplemented with zinc sulfate, zinc amino acid
complex or a zinc polysaccharide complex. However, subcutaneous fat thickness was
higher in steers fed either organic zinc source compared to zinc sulfate.
Table 2. Effect of trace mineral chelates on reproductive performance of Holstein cowsa
Treatment
Item
All cows
n
Days to first heat
Days to first breeding
Cows confirmed pregnantb
n
Days to first heat
Days to first breeding
Days open
Control
Chelates
P value
261
50.3
66.8
261
45.8
62.8
.04
.01
189
47.4
66.3
90.3
183
42.5
59.2
83.6
.05
.01
.04
Services/conception
1.7
From Nocek and Patton, 2002.
b
Cows confirmed pregnant by 150 days in milk.
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