Supplements

Minerals. Mineral additions to livestock diets are common practice, either to complete diets and supplements or as free-choice ingredients. Salt (NaCl) has been used for a long time; early pioneers sought out salt licks where horses and cattle obtained salt and other minerals. Commonly supplemented are calcium and phosphorus as limestone, calcium carbonate, bone meal, various calcium phosphates, sodium and ammonium phosphates, and phos phoric acid. Many of these sources are mined products from natural deposits, where fluorine is often present and must be reduced (defluorinated phosphate) to acceptable limits to prevent toxicity to livestock.

Other major minerals required by livestock include potassium, magnesium, and sulfur, which are supplemented as potassium chloride and potassium ammonium sulfate, magnesium oxide, flowers of sulfur, sulfate salts, and organic sulfur (methionine).

Livestock require eight trace minerals (cobalt, copper, iron, iodine, manganese, molybdenum, selenium, zinc) and perhaps several others (chromium, fluorine, silicon, nickel, vanadium). The required trace minerals are commonly added to complete diets, trace mineral supplements, and trace mineralized salt. Most are added as inorganic salts. There is increasing interest in the bioavailability of various organic forms of these trace minerals (eg, ethylenedi-amine dihydroiodide; zinc methionine, selenium methionine, and other mineral-amino acid complexes; iron and copper gluconate; and various chelated trace minerals). Some of these trace minerals have wide toxicity to requirement ratios (zinc) whereas others have very narrow ratios (selenium). As a result, trace minerals are seldom added as single ingredients to complete diets or supplements; they are formulated into multimineral premixes and then added as dry or liquid premixes.

Vitamins. All the required vitamins can be supplemented in livestock diets. These vitamins are chemically synthesized and are relatively inexpensive. Vitamin A is commonly added to all diets. The other fat-soluble vitamins—D, E, and K—are used in special situations (vitamin D for animals housed indoors, vitamin E to enhance the immunological competence, and vitamin K where bacterial synthesis in the intestine may be limited).

Water-soluble vitamins are commonly added to the diets of monogastric animals (swine, poultry, dogs, cats). Although these vitamins are synthesized by bacteria in the gastrointestinal tract, only in the case of ruminants where synthesis takes place at the beginning of the tract (rumen) is there opportunity for absorption eliminating the need for supplementation except in special situations. Thiamin, riboflavin, niacin, pyridoxine, pantothenic acid, choline, and B12 are commonly added to diets for monogastric animals; biotin and folic acid (folate) are added in some cases. Vitamin C is required by fish but not required by livestock, although it may be useful in heat stress, especially in laying hens for maintaining egg shell strength.

Amino Acids. Methionine and methionine hydroxy analogue (MHA) have been used for S-amino acid supplementation for several years. Fermentive synthesis of lysine has made this often limiting amino acid an economical addition to many monogastric diets. Arginine, threonine, and tryptophan are also available.

Urea. Ruminant animals can make use of nonprotein nitrogen (NPN) through conversion to microbial protein in the rumen and subsequent digestion in the lower gastrointestinal tract. This conversion results in high-quality animal protein (milk and meat) for human consumption from an inexpensive, nonprotein source. Urea is the major source of NPN used in ruminant diets (355,500 t in 1984; 11). Biuret and various ammoniated feed products are also sources of NPN. Use of urea in ruminant diets is limited to about 1% of the total diet (one-third of the total required protein) for effective conversion to microbial protein. An excess of NPN can result in acute ammonia toxicity due to more rapid microbial conversion to ammonia in the rumen, compared with its incorporation into microbial protein.

Additives. Various other compounds are added to livestock diets for purposes other than nutritional supplements. Sales of animal health products totaled $3.6 billion in 1997 (29). Several antibiotics are added for growth promotion, improved feed utilization, and disease prevention and treatment. Chlortetracycline (aureomycin), oxytetra-cycline (terramycin), penicillin, virginiamycin, monensin, and lasalocid are the primary antibiotics used in livestock diets. Sulfa drugs are also used as bacteriostats. Coc-cidiostats, anthelmentics, and mold inhibitors are also used. These additives result in healthier animals. Use of these compounds is highly regulated by the U.S. Food and Drug Administration (FDA) both in terms of efficacy for a stated claim as well as safety for the target animal and for humans consuming foods from animals. Research costs can total $8 to $15 million to achieve final approval for one of these additives. The U.S. Food Safety and Inspection Service (FSIS) constantly monitors animal food products for the absence of residues. Use of additives constantly changes; the most current reference source on their approved use is the Feed Additive Compendium (30), which is updated annually plus monthly supplements.

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