The goal of any trace mineral program is to provide adequate but non-toxic amounts of each trace mineral required by the animal. Even marginal deficiencies of certain trace minerals can reduce feed intake and milk production, impair reproduction and reduce the immune response, leading to increased disease susceptibility.
Trace mineral deficiencies can occur due to a lack of a mineral in the diet or a low bioavailability of a mineral in the diet. For example, a TMR containing only 5 ppm of copper is considered deficient for dairy cattle.
If one supplements the diet with 10 ppm of copper from copper oxide, the TMR will contain 15 ppm of copper, which is above the NRC requirement. However, the diet will still be inadequate in copper because copper has been supplemented in a form that is essentially unavailable for cattle to absorb and utilize.
While it is important to provide adequate quantities of trace minerals to meet the animal’s requirement, it is equally important not to oversupplement trace minerals. Clinical signs of trace mineral toxicities can usually be detected in cattle. However, subclinical signs of certain trace mineral toxicities are difficult to detect but may impair animal performance and health in a similar manner to marginal deficiencies.
Trace mineral requirements
In the last Nutrient Requirements of Dairy Cattle, published in 2001, estimated requirements for cobalt, copper, iodine, iron, manganese, selenium and zinc were given.
Trace mineral requirements listed in the Dairy NRC are for total dietary concentrations (those coming from feedstuffs plus supplemental), not supplemental concentrations. Therefore, trace minerals from feedstuffs should be considered when formulating trace mineral supplements.
The trace mineral chromium was discussed in the 2001 Dairy NRC, but information was not sufficient at that time to estimate chromium requirements of cattle. Chromium functions by enhancing the action of insulin.
A number of studies have indicated positive responses in dry matter intake and milk production to chromium supplementation of dairy cow diets. Responses to chromium supplementation in dairy cows appear to be greatest under conditions that decrease insulin sensitivity (late gestation and early lactation, heat stress).
Since 2006, chromium propionate has been permitted by the FDA as a source of supplemental chromium in cattle diets at levels up to 0.5 mg chromium per kg diet dry matter.
Factors affecting trace mineral requirements
Trace mineral requirements are affected by age and physiological state (gestation, lactation, etc.) of the animal and bioavailability of the mineral from the diet.
Bioavailability is affected by the chemical form of the mineral supplemented to the diet as well as the chemical form of the mineral naturally present in feedstuffs and by antagonists present in the diet. An antagonist is a mineral or other compound that reduces the absorption or utilization of another mineral.
Sulfur, molybdenum and iron are important antagonists that can occur naturally at high levels in feedstuffs. Sulfur, at levels (0.2 to 0.5 percent) typically found in dairy diets, can reduce copper absorption. In recent years, the concentration of sulfur in many diets has increased due to increased use of byproducts such as distillers grains with solubles and corn gluten feed, that are high in sulfur.
Sulfur in the form of sulfide is believed to reduce copper absorption due to the formation of insoluble copper sulfide in the rumen. In the rumen environment, sulfide is produced from the reduction of inorganic sulfur sources and degradation of sulfur-containing amino acids.
The effects of dietary sulfur on copper metabolism is much more pronounced when dietary molybdenum is also high (greater than 2 to 3 ppm). Molybdenum can react with sulfide in the rumen to form thiomolybdates. Thiomolybdates bind copper very strongly and prevent it from being absorbed.
High dietary iron, when present in a bioavailable form, is a potent copper and manganese antagonist. It is not uncommon for dairy diets to exceed iron requirements by fivefold or greater. Hays and silages are highly variable but can exceed 1,000 ppm of iron. Many byproduct feeds are also high in iron, especially soy hulls.
Little is known regarding bioavailability of iron naturally present in feedstuffs. High dietary iron may not cause adverse effects on copper and manganese bioavailability 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 occur. 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 is due to soil contamination. In most soils, iron is extremely high.
Iron in soils is generally of low solubility and probably very poorly absorbed when ingested by cattle. Research has indicated that acid conditions occurring during the fermentation of silage or haylage greatly increases the bioavailability of iron from soil contamination. Legumes may also represent an important source of bioavailable iron in cattle diets.
Much of the iron in soybeans has been shown to be present in the form of ferritin, and this form of iron has been found to be very bioavailable in humans. If ferritin is a major form of iron in other legumes, such as alfalfa and clovers, this may represent a highly bioavailable form of iron in dairy diets.
Supplemental sources of trace minerals
Inorganic trace mineral sources (primarily sulfate and oxide forms) are the cheapest sources of supplemental trace minerals and have been used in dairy cattle diets since the 1930s.
Supplementation of inorganic trace minerals has been effective in correcting as well as preventing trace mineral deficiencies in cattle. Sulfate sources of trace minerals are generally considered to have a higher bioavailability than oxide sources, and some of the oxide forms (especially copper oxide) have a very low bioavailability.
However, in the presence of certain antagonists, bioavailability of sulfate sources can be low. Various feed-grade sources of a particular metal (oxide, sulfate, etc.) can also differ in purity and other factors that can affect bioavailability of the mineral.
Organic trace mineral sources are complexed or chelated to organic molecules (amino acids or polysaccharides). Use of organic trace mineral sources to replace a portion of supplemental inorganic trace minerals has increased greatly in the past 20 years.
In theory, the covalent bonds formed between the metal and organic molecule should allow organic trace minerals to resist many of the interactions encountered by inorganic trace mineral sources. Organic trace minerals are considered more bioavailable than inorganic sources. The downside is the cost. Organic trace minerals are seven to 12 times more expensive than inorganic forms.
The newest category of trace minerals is hydroxy trace minerals. Basic copper chloride or copper hydroxychloride was introduced in 1995. Zinc and manganese hydroxychloride were introduced to the market in 2012.
In contrast to sulfates, where the metal is weakly bound to sulfate via ionic bonds, the metals in hydroxy trace minerals are covalently bonded to multiple hydroxy groups. Hydroxy trace minerals are relatively insoluble in water but become soluble under acidic conditions typical of those found in the abomasum of cattle.
The low solubility in water results in hydroxy trace minerals being non-hygroscopic and less reactive in feeds and premixes than sulfates, resulting in greater vitamin stability and less oxidation of fats.
Hydroxy trace minerals have also been shown to offer a greater bioavailability than sulfate forms. The higher bioavailability of hydroxy trace minerals may at least partially be explained by their ability to bypass the rumen, thus minimizing interactions that normally occur in the rumen.
As discussed earlier, many of the interactions among trace minerals in ruminants occur in the rumen environment. The cost of hydroxy trace minerals relative to inorganic sulfate forms varies between trace minerals but usually ranges from 100 to 300 percent the cost of sulfates.
Overfeeding trace minerals
Many producers believe that if a little is good, more must be better. This is not necessarily true for trace minerals. The range between an adequate amount and a level that adversely affects cattle health or performance varies among trace minerals. However, all trace minerals can produce toxic effects or trace mineral imbalances when fed at high enough concentrations.
Overfeeding of trace minerals can be due to high concentrations of a mineral occurring naturally in feedstuffs or oversupplementation of one or more trace minerals. Iron is a good example of a trace mineral that can sometimes be found at high concentrations in silage, haylage, hay and byproduct feeds used in dairy diets.
Copper is an example of a mineral where oversupplementation has led to toxicosis problems in dairy cows. If cattle are supplemented with copper well above their requirement, liver copper levels can increase to the point that toxicity occurs. This increase in liver copper usually occurs over a period of months and possibly years.
When copper concentrations in the liver reach a high level (generally 1,000 ppm or higher on a dry matter basis), copper can suddenly be released from the liver into the blood, and clinical signs of toxicosis become evident. Even in cows not showing clinical signs, elevated liver copper can cause subclinical signs (liver damage).
In the early 1990s, copper from copper oxide was shown to be of very low bioavailability in cattle. Prior to this time, copper oxide was a major source of copper used in cattle mineral supplements.
In the past 20 years, as nutritionists have switched to more bioavailable copper sources, and in some instances, higher supplemental copper levels, there has been a large increase in case reports of copper toxicosis in dairy cows. Jerseys appear to be more susceptible than Holsteins, but several cases of copper toxicosis have been reported in Holstein cows.
Summary
As their name implies, trace minerals are required in trace amounts. It is important to provide sufficient amounts of each trace mineral to meet the requirement of dairy cattle. Trace mineral requirements are affected by bioavailability of the mineral (supplemental source and that naturally present in feedstuffs) and antagonists present in the diet.
Once the animal’s requirement for a trace mineral is met, no additional improvement in performance or health can be expected from increasing the level of the mineral in the diet. Oversupplementation of trace minerals (copper in particular) can result in subclinical (liver damage) or even clinical signs of toxicosis. PD
Jerry W. Spears
Professor Emeritus
North Carolina State University
