by: Stephen B. Blezinger
Ph.D., PAS
Seldom do many weeks go by when I do not get questions concerning mineral programs and their necessity at different times of the year. Often as we approach times of the year when forage availability becomes greater, many cattlemen feel that they no longer need to provide mineral supplementation since the grass is so succulent and plentiful. Only in very rare circumstances do we find mineral requirements completely met by the forage base (or even the supplemental grain base as well unless formulated accordingly). Additionally, we find that 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, balance and absorption of the trace elements are required for proper functioning of metabolic processes including immune response, reproduction, milk production and growth. The trace elements most commonly identified as having an impact on productivity of cattle include copper, zinc, and selenium.
What is Bioavailability Anyway?
Let's take a second and discuss what is meant by the term �bioavailability.� One thing to remember is that no nutrient is ever completely digested and absorbed. For instance, we may feed a cow 2.5 lbs of protein in a day. That protein, depending on the source may only be 65 percent digestible which means that of the 2.5 lbs. fed, only 1.625 lbs of it will actually be broken down and absorbed into the intestinal tissues for utilization in the animal. The rest will return to the environment as organic fertilizer.
Similarly, when we are feeding a mineral supplement, the minerals within the product will seldom be completely absorbed. Depending on the source or type of mineral the amount of a given element which is absorbed as related to the amount fed can vary widely (from 0 to 95 percent). Simply put, bioavailabilty is the amount of a given mineral actually absorbed as a percentage of the amount fed. Bioavailabilty is affected by a large number of factors. Some of these can include:
1) Type of mineral source - inorganic (oxide, sulfate, carbonate, etc.) or organic (amino acid complex, proteinate, etc.)
2) Presence of antagonistic compounds in the diet.
3) Forage digestibility
When an animal absorbs an insufficient amount of a given mineral this is referred to as a deficiency. 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 with 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. Some estimates are that Zn is involved in between 200 and 300 enzyme systems in the body. 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 for steroid/hormone synthesis, which regulates secretion of gonadal (ovaries, testes) 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. Several years ago a forage study was completed that included forage mineral data from 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 widespread 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.
A study in 1992 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 other 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 (6).
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 Trace Minerals
Copper deficiency has been identified as a serious problem for grazing ruminants. A deficiency may be due to low levels of Cu in forage and can be further exaggerated when Molybdenum (Mo), Iron (Fe) and Sulfur (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. It has also been found that excessive dietary Zn can also negatively affect Cu status through the absorption process.
Immune system
Deficiencies of protein, energy, vitamins and minerals are known to have a negative effect on immune function. Trace mineral deficiencies in beef cattle have been shown to alter various components of the immune system. A study in 1981 reported viral and bacterial challenges increased serum ceruloplasmin and plasma copper in copper deficient cattle indicating a major protective role of copper in infectious diseases. Copper deficiency in ruminants resulted in decreased neutrophil function and reduced response to viral infection.
It has been shown 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.
Effects on Reproduction
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 (1988) 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 nerve activity with prolonged labor. Another trial found 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
Obviously, sub-clinical or marginal mineral deficiencies may have a significant 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.
If you find that some of the symptoms discussed above exist in your herd it is important that you consult your vet, nutritionist or extension personnel for assistance in determining what the mineral status may be in your herd. In many cases this may not be an overnight process. It will, however, prove beneficial to improvement of overall performance.
Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, TX. He can be reached at 667 CR 4711 Sulphur Springs, TX 75482, by phone at (903) 352-3475 or by e-mail at [email protected].
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