In the last issue we began a review of the effects of some of the more critical trace elements as pertaining to the nutritional needs of the grazing beef animal. The scope here is somewhat limited and it should be emphasized that many other trace minerals are required in the animal's diet. They become critical when they are available in amounts insufficient to meet the animal's needs. The trace minerals reviewed in the previous issue and in this are those which have been proven time and time again to be especially critical in the cow's physiology. It has also been observed that if these elements are in short supply the effects to the animal's health and performance are significant.
Some of the terminology here may be a bit more technical than the reader is accustomed to in this format but is necessary to convey the message as it relates to health and physiological processes.
Selenium (Se) requirements for grazing livestock falls within a relatively narrow range, from <5 ppm to >.l ppm with deviations either above or below this range resulting in considerable problems for the animal. The toxic effects of Se have been noted for many generations with the first recorded observations as early as 1295 by Marco Polo in grazing livestock in China. Selenium is now known to be one of the few elements absorbed by forage plants in sufficient amounts to create a toxicity hazard to grazing animals. It was not until 1957 that Se was identified as essential by Schwartz and Folz in work demonstrating Se's role in preventing liver degeneration in rats.
One of the primary metabolic requirements for Se is for the production of glutathione peroxidase, a Se-containing enzyme necessary for the prevention of oxidative damage of cellular and subcellular membranes, i.e. an
antioxidant, as we commonly hear the term used in human medicine. The enzyme apparently attacks and destroys peroxides before they can damage the membranes. The production of peroxides in the body is a normal metabolic process and a constant source of glutathione peroxidase is required to counteract this activity. A Se deficiency reduces the amount of active enzyme, allowing greater amounts of peroxides to go unchecked. Selenium requirements are often categorized with those of Vitamin E which has a similar antioxidative activity in cellular membranes as that of glutathione peroxidase.
Other biochemical roles of Se which have been identified include constituency of structural metaloproteins in spermatazoa mitochondria (reproductive process) as well as incorporation into purines and pyrimidines of RNA (protein and other tissue proliferation). Other important roles of Se include adequate immune response in livestock and Se may affect prostaglandin synthesis and essential fatty acid metabolism. It becomes obvious that Se plays an extensive role in the metabolic functions of the body.
The concentration of Se within plant tissues and the relative availability of the mineral is largely dependent on two factors; soil concentration and the form in which Se is bound. As previously discussed, Se is one of the few TM which may build up to toxic levels within the plant having taken these reserves from the soil. The areas which tend to exhibit elevated Se levels are small and highly centralized. Areas low and truly deficient in Se are much more extensive and tend to pose a fairly substantial problem.
Several methods exist that are primarily used for Se supplementation. These include:
1) A free-choice supplement
2) Se fertilization
3) Se injections
4) Se ruminal pellets or boluses.
The first method, use of a Se-fortified mineral mixture appears to be the most practical, especially when compared to fertilization and administration of injections and intrauminal devices which once again increase labor requirements as well as additional handling of cattle. In almost every range grazing situation, the use of free-choice mineral supplements appears to be the most cost and labor effective, especially when properly placed around water or other supplemental feed sources where cattle tend to congregate.
The concentration of Zn in forages appears to be highly variable and considerable uncertainty exists concerning actual zinc requirements of ruminant animals. These two factors, when considered collectively, create a substantial challenge to the individual attempting to meet the needs of cattle grazing range and pastures. In 1970, Reid and coworkers reported significant effects of plant species, cutting, growth stage and season on Zn concentrations in pasture and hay crops.
Zinc is essential to all animals and plays significant roles in the metabolic activity of the grazing ruminant. Zinc functions in enzyme systems and is largely involved in nucleic acid metabolism, protein synthesis and carbohydrate metabolism. Smith et al. reported a need for Zn for mobilization of Vitamin A from the liver. Zinc is found in all body tissues which are high in protein or calcified material. The absorption of the metal appears to be directly dependent on the body's physiological need.
Early effects of Zn deficiency include reduced feed intake, reduced growth rate and feed efficiency followed by skin disorders. If left untreated, other more serious conditions may be manifested including inflammation of nose and mouth, unthrifty appearance, stiffness of joints with soft edematous swelling of the feet in front of the fetlocks as well as a host of other deficiency related symptoms. Many of these symptoms appear to be related to the role of zinc in protein synthesis and energy metabolism.
Zinc availability in plant tissues appears to be dependent on a number of factors, including soil, plant species, stage of maturity, yield, pasture management and climate. Low levels of Zn within the soil tends to reduce plant stores, thus decreasing Zn availability to grazing animals. Zinc tends to increase in forages found on poorly drained soils and soils with increased pH levels. Underwood reported in 1981 that Zn levels tend to decrease as plants mature. This may be partially due to the diluting effect as noted with Co in the previous issue but may also be related to decreased digestibility of more lignified plant material. Recent research has shown that in the case of Zn, higher incidences of an actual deficiency in certain locales is noted as opposed to created deficiencies of minerals such as copper. Copper, as you will recall, can prove deficient in certain instances due to interference with absorption by other minerals such as sulfur, molybdenum, iron, etc.
Once again, the most effective means of supplementing Zn, especially from a cost and labor saving standpoint appears to be through free-choice mineral supplementation, with Zn supplied in the form of Zn sulfates, oxides and carbonates (inorganic sources) and organic sources such as amino acid complexes, chelates and a host of other recognized complexes based on mineral combinations with organic molecules. Research has shown the form plays an important role in overall bioavailability (amount absorbed) as related to the amount available to the ruminant. Other means of supplying Zn to cattle is the injection of Zn salts (sulfates) and the treatment of deficient soils with Zn-containing fertilizers, the practicality of both having been previously discussed.
The minerals discussed are only a minute sampling of the TM required under grazing situations by cattle. The points being expressed in the discussions, however, become apparent as one reviews literature in this area. While it has been thought that the availability of most minerals is highly dependent on soil availability and contributory factors which lead to plant uptake and subsequent provision to the grazing animal there is some thought that actual uptake by the plant in situations where supplemental levels of various minerals have been added to the soil may or may not have a significant effect. From here the importance of plant species, stage of growth and maturity, as well as selectivity in grazing by the animal becomes obvious, especially when considering delivery to the digestive system. Interactions between TM become an important consideration as well due to the inhibitory effect of one mineral on the digestion and absorption of another. Finally, when the decision is made to supplement one or more TM, delivery methods must be scrutinized closely to determine the system which will best meet the needs of the animal while still proving economical. Collectively, these factors illustrate the fact that TM and the use of supplementation is highly variable and must be researched carefully in order to properly provide for the needs of the grazing animal.
Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, TX. He can be contacted at Route 4 Box 89 Sulphur Springs, TX 75482, by phone at (903) 885-7992 or by e-mail at firstname.lastname@example.org.