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BIOAVAILABILITY OF MINERALS IN CATTLE IS AN IMPORTANT CONCERN

Dr. Steve Blezinger
Ph.D.

On several occasions we have discussed mineral nutrition in beef cattle including what their needs are as well as how these needs can be met. One of the more critical areas we have found in overall mineral nutrition is not so much in determining these needs or in delivering a suitable supplement but in determining how well the various minerals are actually absorbed.

No nutrient is absorbed and utilized to the full extent that it is fed. This may be due to the overall digestibility of the plant material being fed from which the nutrients are to be extracted or it may be a factor of the actual digestibility of the nutrient itself. Take for instance the feeding of protein. We may feed a cow 2.5 lbs. of protein per day. if the digestibility of that protein is only 65 percent though she is actually only receiving about 1.625 lbs.

The same is true with minerals. Absorption rates may range from 0 to 99.5 percent depending on the source as well as a host of other factors Among the numerous management challenges that cattle producers face, one nutritional challenge is to satisfy the trace mineral requirements of the cow herd. As the genetic progress of the herd improves, mineral supplementation strategies become more complex and are influenced by a variety of factors, including forage mineral bioavailability, trace mineral interactions, stage of production and even breed. Adequate intake and balance are required for proper functioning of metabolic processes including immune response and reproduction. The trace elements most commonly identified as having an impact on productivity of cattle include copper, zinc, and selenium.

What is Bioavailability

When we feed a given mineral a series of chemical changes takes place in the rumen and digestive tract which allows it to be absorbed. The percentage of mineral absorbed as related to the amount fed can be referred to as the biological availability or bioavailability. Depending on the factors mentioned above, bioavailability can vary widely and must be considered when designing a mineral program. If not considered deficiencies can occur even when the producer thinks he is feeding more than adequate amounts of a given mineral.

Trace mineral deficiencies can be classified as either primary or secondary. A primary deficiency is caused by inadequate dietary intake of one or more essential minerals while a secondary deficiency is caused by an interference in absorption, distribution or retention of a mineral. A preexisting disease or a mineral interaction can cause a secondary deficiency. Both deficiencies can occur simultaneously, making the evaluation of mineral status complex.

Trace Minerals: Copper & Zinc

Copper is an essential trace element required for enzyme systems, iron metabolism, connective tissue metabolism and mobilization, plus integrity of the central nervous and immune systems. Copper functions in the immune system through energy production, neutrophil activity and antioxidant enzyme production. It also aids development of antibodies and lymphocyte replication. Reproductive efficiency may be reduced when a Cu deficiency occurs because of metabolic alterations of enzyme systems.

Zinc is actively involved in enzyme systems through metabolism of protein and carbohydrates. Zinc is also required for maintaining responsiveness of the immune system through energy production, protein synthesis, stabilization of membranes against bacterial endotoxins, antioxidant enzyme production and maintenance of lymphocyte replication and antibody production. Virtually every phase of cell growth involves Zn, and a deficiency can impact productivity. For example, zinc serves as an activator of enzymes necessary in steroidogenesis, which regulates secretion of gonadal hormones.

Forage Levels

Forages provide the nutritional base of beef and dairy operations. Supplementation decisions pivot around both the quantity and quality of the forage base. In addition to protein and energy content of the feed resource, mineral concentrations must also be considered. A study several years ago collected and summarized forage mineral data from a survey of 352 cow/calf producers across 18 states. The forages evaluated in this survey were placed in the following categories: alfalfa, brome, bermuda, fescue, sudan, cereal forages, native or prairie grass, grass (brome, timothy, mixed grasses and other grass/hay combinations), silage and other.

The results revealed a deficiency for Zn with only 2.5 percent of the analyzed forages having adequate levels (>30 ppm). Copper values indicated that 14.2 percent of the forages were deficient and 49.7 percent contained marginal levels. Iron and Mo levels in 10 percent of the forages were high enough to cause a Cu deficiency due to antagonistic affects on absorption. Forage Mn concentrations were at adequate levels in 76 percent of the samples.

Another study evaluated mineral concentration of forage grasses in the Northern Great Plains and their adequacy to meet nutritional needs of grazing livestock. Phosphorous, zinc and copper were the minerals most likely to be considered deficient for cattle. Several studies have reported mineral concentrations for range grasses in Montana and mineral values from all four studies were in agreement. Three of the studies were conducted at the same research site during winter grazing periods in 1986, 1987 and 1991. The similarity of values over the various years indicated little fluctuation of elemental values from year to year. Copper (3 ppm) and Zn (25 ppm) levels appeared to be deficient for meeting cattle requirements; however, Mn (79 ppm) was adequate.

Mineral analysis of forage samples determines concentration of specific elements; however, bio-availability to the animal is more difficult to measure. Release of minerals from the forage and interactions with microbial cells, other minerals, or other forage components in the rumen can influence availability for absorption by the animal.

Minerals in forage consist of three fractions: 1) highly soluble and rapidly released; 2) slow release as cell walls and protein components are degraded and, 3) no release. The major portion of Cu in forages appears to be contained in the rapid release fraction; however, Zn has been shown to have the lowest percentage release compared to Ca, Mg, K, P and Cu. Increasing the NDF content of the diet has also been shown to decrease apparent absorption of Mn, Zn, Fe, and Cu.

Interactions between Minerals

Copper deficiency has been identified as a serious problem for grazing ruminants. A deficiency may be due to low concentrations of Cu in forage and can be further exaggerated when Mo, Fe and S levels are elevated.

Molybdenum and selenium interfere with Cu absorption by forming thiomolybdates that bind Cu, resulting in compounds that cannot be absorbed by the animal. Decreased liver Cu levels can be caused by excessive Fe in the diet. In the same study, Mo appeared to have an additive action with Fe by decreasing liver Cu, while S and Fe had independent effects on Cu. The antagonistic mechanism of Fe is not well understood. Excessive dietary Zn can also negatively effect Cu absorption of copper.

Immune System

Deficiencies of protein, energy, vitamins and minerals are known to compromise immune function. Trace mineral deficiencies in beef cattle have been shown to alter various components of the immune system. A 1981 study reported viral and bacterial challenges increased serum ceruloplasmin and plasma copper in copper-repleted cattle indicating a major protective role of copper in infectious diseases. Copper deficiency in ruminants resulted in decreased neutrophil function and altered responsiveness to viral infection.

It has been indicated that zinc had an indispensable role in the development and maintenance of immunocompetence. Zinc has been shown to have a positive impact on immunity in stocker and feeder cattle with limited research in beef cows. Zinc supplementation enhanced recovery rate in IBR-virus-stressed cattle. Zinc methionine has also been shown to increase antibody titer against bovine herpesvirus-1.

Improved antibody titers for IBR-challenged yearling heifers when Cu, Zn, Mn and Co were supplemented, were reported. Supplementing first-calf gestating beef heifers with amino acid-complexed forms of Cu, Zn, Mn and Co enhanced cell-mediated immune response when compared to heifers supplemented with sulfate forms of the trace minerals or heifers offered supplement with no additional Cu, Zn, Mn, and Co.

Reproduction Responses

Intake of bioavailable minerals is necessary in postpartum cows for proper involution of the uterus, display of estrus, ovulation, conception and maintenance of a new fetus. Doyle and co-workers reported a decrease in length of time from the beginning of the breeding season to conception for cows supplemented with trace minerals compared to cows fed supplement without trace minerals or those receiving no supplement. In a Cu deficient status, productivity may be reduced due to metabolic alterations of enzyme systems. Delayed or suppressed estrus and embryo death have been identified as common symptoms of Cu deficiency in beef cattle. Infertility associated with a Cu deficiency may also be a result of excessive dietary Mo intake. One study reported heifers receiving a diet with marginal Cu levels and high Mo exhibited delayed puberty, lower ovulation rates and lower conception rates compared to heifers consuming a diet containing high levels of Fe.

Zinc deficiency can adversely affect reproductive processes in females from estrus to parturition. Inadequate Zn levels in gestating cows may result in abortion, fetal mummification, lower birth weight or altered myometrial contractility with prolonged labor. Other effects reported include impaired growth, delayed puberty and decreased appetite in Zn deficient bull calves. A loss of appetite results in lowered mineral ingestion, which further decreases feed utilization due to hindered nutrient metabolism.

Evaluating Mineral Status

Sub-clinical or marginal mineral deficiencies may have an economic impact on the beef producer. Trace mineral imbalances can be the result of dietary levels, water source, production demands, breed differences and mineral interactions. Sub-clinical trace mineral deficiencies in cattle may be a larger problem than an acute deficiency because specific clinical symptoms are not obvious enough to allow the producer to recognize a deficiency. Cattle with a sub-clinical status continue to reproduce or grow, but may have decreased feed efficiency and a depressed immune system.

It is important that if you notice any of the symptoms described that you get with your vet, nutritionist or extension personnel to assist with the evaluation of your herd and the mineral program you are using. Remember that this is not an overnight process and that because of the complexity of the issue can be difficult. However, you will find that in many cases simple adjustments to your program can have a profound impact on your productivity and profitability.

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