Principles Of Animal Nutrition And Feeding

The proper feeding of livestock is a matter of supplying them with the correct amount of nutrients essential for reproduction, growth, or lactation. Nutrients are chemical elements and compounds required by the cells of the animal's body in support of the three basic functions: (1) structural matter for building and maintaining the body; (2) a source of energy for work, thermogenesis (heat production), and fat deposition; and (3) regulating body processes or the synthesis of body regulators. An auxiliary function of nutrients would be their use in milk production.

The Nutrients

Nutrients required for livestock, just like for humans, can be categorized into six functional or chemical classes: carbohydrates, proteins, fats, minerals, vitamins, and water. All carbohydrates or saccharides are related structurally and chemically and contain the same amount of gross energy. Carbohydrates are comprised primarily of hexose (C6H1206) and pentose (C5H10O5) molecules. Tetrose and triose molecules are present in small quantities but are generally not important in animal nutrition. Carbohydrates are usually categorized as monosaccharides, disac-charides, and polysaccharides based on how many hexose and pentose molecules are linked together. Common monosaccharides, also called simple sugars, consist of glucose, fructose, and galactose. Disaccharides consist of two monosaccharides linked together with hydroxyl groups of each sugar unit. The common monosaccharides include sucrose (table sugar), maltose, and lactose. Polysaccharides have the empirical formula (C6H10O5)n and contain large polymers of monosaccharides. Polysaccharides important in livestock nutrition include starch, glycogen, and fiber (hemicellulose, cellulose, and lignin).

Carbohydrates are important energy sources in livestock feeds, comprising 65-80% of plant dry weight. Carbohydrate as a class of nutrients is usually divided into two groups: nitrogen free extract (NFE) and fiber. Fiber is what remains after a feed has been boiled in dilute acid or alkali and roughly approximates the amount of carbohydrate poorly digested in the animal's intestinal tract. NFE represents the soluble, readily digestible carbohydrate portion of a feedstuff. Corn grain, eg, contains only 2.2% fiber and 70% NFE. Good-quality alfalfa hay contains 26% fiber and 46% NFE. Cellulose is not an efficient energy source for nonruminant livestock (swine and poultry) but can be readily digested by the bacteria of the rumen. Because of this characteristic, ruminant livestock occupy an important niche in utilizing a potentially wasted feed resource.

Lipids are water-insoluble organic molecules that can be extracted from plant and animal tissues with nonpolar solvents such as benzene and ether. Lipids contain 2.25 times more energy per unit weight than either carbohy drate or protein. The term lipid is used in a general sense and used interchangeably with fat and oils. Livestock rations are generally low in fat, with most grains and roughages containing less than 5% lipid. Fats may sometimes be added to a ration, especially for swine and poultry, and high producing dairy cattle when higher energy rations are desired.

Lipids are composed of carbon, hydrogen, and oxygen, as are carbohydrates, but contain less oxygen. Lipids have several important biological functions, including storage and transport forms of energy and components of cell surface membranes. The fatty acid composition of body fat may be altered by the composition of dietary fats. This is especially true for swine and poultry.

Protein constitutes the most expensive portion of livestock feed. Protein primarily consists of 20 a amino acids linked together by peptide bonds. Both plant and animal tissues contain a diversity of proteins with variable amounts of amino acids. Ten amino acids cannot be synthesized by animal tissues or can be synthesized only to a limited extent. These 10 amino acids are referred to as dietary essential amino acids (Table 1). The primary function of dietary protein in livestock rations is to supply amino acids, the building blocks of proteinaceous body tissues. Ruminant livestock do not have dietary amino acid requirements as such, but depend on ruminal bacterial protein synthesis to supply these nutrients. Because of this, ruminant animals are able to utilize nonprotein nitrogen, such as urea, to replace part of the natural protein in the ration. The use of rumen bypass protein supplements, such as corn gluten meal and blood meal, has been shown to increase growth in beef cattle and milk production in dairy cattle (14,15). Swine rations are usually balanced for the first two or three most-limiting amino acids as well as for balancing the ration for crude protein.

Sixteen or more minerals are required by livestock. Of these, seven are classified as macrominerals (Ca, P, K, Na, S, Mg, Fe), with the remaining constituting microminerals (Co, Cu, F, I, Fe, Mn, Mo, Se, Zn). A description of the minerals' functions and deficiency characteristics in livestock nutrition has been reviewed in Ref. 16.

Vitamins are dietary-essential organic molecules that are required in minute quantities by livestock. Vitamins are usually classified into two groups: water soluble and fat soluble (Table 2). Approximately 15 vitamins are known to function in animal metabolism, but only several of these are needed in the ration of livestock because synthesis of certain vitamins occurs in the animal. Animal nutritionists formulating livestock rations are especially concerned about vitamins A and D for ruminants and certain B vitamins for nonruminants. Ruminants do not generally require B vitamin supplementation because the bacteria

Table 1. Dietary Essential Amino Acids Required by Swine and Poultry

Table 2. Fat-Soluble and Water-Soluble Vitamins Required by Livestock

Fat-soluble

Water-soluble

Vitamin A Vitamin D Vitamin E Vitamin K

Thiamine

Riboflavin

Niacin

Pyridoxine

Biotin

Pantothenic acid Vitamin B12 Choline Folic acid Inositol

Para-aminobenzoic acid

Table 1. Dietary Essential Amino Acids Required by Swine and Poultry

Arginine

Lysine

Tryptophan

Histidine

Methionine

Valine

Isoleucine

Phenylalanine

Glycine (poultry)

Leucine

Threonine

Proline (poultry)

of the rumen synthesize sufficient quantities. Vitamin E is normally present in ample quantities in natural feeds, but may be supplemented to ruminants and nonruminant livestock receiving processed grain diets. Vitamin E supplementation of cattle diets also has been shown to stabilize the red color of fresh beef and help prevent oxidative rancidity (17). Livestock normally do not need ascorbic acid supplementation where adequate quantities are synthesized in the tissues. Ref. 16 outlines a review of vitamins in livestock nutrition.

Digestion

Digestion involves the physical and chemical preparation of feed in the gastrointestinal tract to absorption-ready nutrients. The digestive systems of livestock are anatomically and functionally similar, with the very important distinction of the ruminant having a large, four-compartment stomach (Fig. 3). The abomasum or the true stomach is functionally similar to the stomach of monogastrics. The rumen, the largest of the four compartments, with a capacity approaching 50 gal, functions as a fermentation vat and contains billions of bacteria and protozoa. The presence of the bacteria and the enzymes that they secrete enables the ruminant to efficiently use cellulose from fibrous plants and feedstuffs. The microorganisms also synthesize amino acids to make bacterial protein and supply the ruminant animal with essential amino acids.

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