United States Department of Agriculture, Agricultural Research Service, Clay Center, Nebraska, U.S.A.
Nutrient needs of tissues of ruminants are similar to those of nonruminants. Tissues of ruminants require oxygen, water, energy, amino acids, fatty acids, minerals, and fat-and water-soluble vitamins. Dietary needs of ruminants are simpler and often cheaper than for nonruminants because of anaerobic microbial metabolism in the rumen. Microbial metabolism of dietary intake also increases the complexity of relating dietary intake to nutrients available to the animal.
Water is required by the animal for regulation of body temperature and acts as a solvent necessary for transport of nutrients, metabolites, and waste products. The requirement for water reflects needs for accretion in body tissues (e.g., growth, pregnancy) and milk production plus that lost from the animal. Water is lost from the animal by excretion as urine or feces, from the lungs as water vapor during respiration, and from skin by evaporation. Losses vary considerably and depend in part on activity, air temperature, diet, and water consumption. Because feeds contain water, and oxidation of nutrients produces water, not all water needs must be provided by drinking.
Energy is defined as the potential to perform work and is required to perform the ''work'' of living. Energy requirements depend on the additive needs of individual cells and vary according to physiological needs imposed upon those cells. Energy is derived from the metabolism of carbohydrates, proteins or amino acids, and fats and can be supplied from the diet, or if dietary supply is inadequate, from body tissues (fat, protein, glycogen). Carbohydrates are the primary dietary source of energy of ruminants. Dietary protein, peptides, and amino acids contribute up to about 20% to the energy supply. Fat is low (2 4%) in diets typically consumed by ruminants, and is, thus, not a major contributor to energy supplies. Fat may be added to diets of feedlot cattle or lactating cows to increase the energy density of the diet, but dietary fat contents of greater than 8 10% may have adverse effects on rumen microbial metabolism.
Cellulose, hemicellulose, and starch are the major carbohydrates utilized by ruminants. Many species of bacteria in the rumen produce cellulase enzymes capable of hydrolyzing the b 14 linkages between the glucose units in cellulose and others hydrolyze the b 14 linkages in hemicellulose. Many species of microbes, as well as a amylases present in pancreatic secretions of all animals, hydrolyze a 14 linkages of starch. The symbiotic relationship between ruminants and rumen microbes allow utilization of forages and other feeds, especially those containing complex carbohydrates such as cellulose that are unusable or poorly utilized by nonruminants. Volatile fatty acids (VFA; acetate, proprionate, butyrate, etc.) are primary metabolic end-products of carbohydrate (and protein) hydrolysis by anaerobic microbes in the rumen and serve as the major energy source of ruminants. One of the major metabolic differences between ruminants and nonruminants is the reliance of ruminants on VFAs as the major substrates for oxidative metabolism and energy storage.
Little glucose is available for absorption from the digestive tract of ruminants. However, glucose is required by nervous tissue, muscle, adipose, mammary gland, and gravid uterus. Glucose requirements of ruminants are met through gluconeogenesis, primarily from proprionate, amino acids (e.g., alanine, glutamine, aspartate, glutamate), glycerol, and lactate. In spite of the lower blood glucose concentrations and extra metabolic steps required to provide glucose, requirements of ruminants appear to be similar to nonruminants.
It is generally assumed that tissue requirements for amino acids of ruminants are similar to those of nonruminants. However, this assumption has not been rigorously tested. Amino acids are required for synthesis of protein and other essential compounds and provide the carbon skeleton for a major proportion of glucose needed by the ruminant. Lysine, arginine, histidine, isoleucine, leucine, methionine, phenylalanine, threonine, tryptophan, and valine must be supplied from the digestive tract, but specific requirements have not been well defined. Requirements have been estimated based on rate of accretion and amino acid composition of whole body protein.
Ruminants have the unique ability to subsist and produce without dietary protein or amino acids due to synthesis of microbial protein from a wide variety of nitrogen (N) sources within the rumen. The sources of N that microbes utilize for protein synthesis include dietary protein and nonprotein N (NPN), as well as N recycled to the rumen via saliva or diffusion (primarily as urea). Most ruminal bacteria can use ammonia N as a source of N, but much of the N used by bacteria is derived from amino acids or peptides, if available. Ruminants can grow, reproduce, and lactate with only NPN as a source of N, but additional sources of amino acids are required to achieve maximal productivity. Rumen microbes, as well as dietary protein that escape (bypass) degradation in the rumen, supply the intestine with protein for digestion and absorption as amino acids. Microbial N composes about 40% of the nonammonia N entering the intestine on high-energy diets with high protein levels, about 60% with low protein diets, and 100% with purified, NPN-supplemented diets. Biological values of microbial protein range from about 65 to 90, with an ideal value of 100.
The quantity and quality of protein reaching the small intestine is modulated by the effects of degradation and synthesis in the rumen. Both quality and quantity of protein available to the animal may be improved by microbial metabolism if a diet containing a low level or low quality of protein is fed. Microbial action may decrease the quantity and quality of available protein when a diet containing a high level of high-quality protein is fed. The amino acid profile of microbial protein is relatively constant and well balanced relative to tissue needs, and thus is utilized very efficiently. However, dietary protein escaping ruminal degradation may be less well balanced. As with nonruminants, a poorly balanced supply of amino acids results in increased catabolism of amino acids. Unless used for synthesis of protein or other essential compounds, amino acids are catabolized with the N being converted to urea and the carbon skeleton being oxidized or used for storage. A poorly balanced amino acid supply results in inefficient use of N and is energetically costly.
At least 17 minerals are required by ruminants. Macro-minerals (those required in large amounts) include calcium, magnesium, phosphorus, potassium, sodium, chlo rine, and sulfur. Required microminerals (those required in small amounts) are chromium, cobalt, copper, iodine, iron, manganese, molybdenum, nickel, selenium, and zinc.[1-3] Other minerals including arsenic, boron, lead, silicon, and vanadium have been shown to be essential for one or more animal species, but there is no evidence to indicate these minerals are of practical importance in ruminant diets. Two features of ruminant nutrient requirements are noteworthy. Phytate phosphorus is not well utilized by nonruminants, but as a result of microbial fermentation, is utilized readily by ruminants. Cobalt functions as a component of vitamin B12. Ruminants are not dependent on a dietary source of vitamin B12, but cobalt is required for its synthesis by rumen microbes. Many of the essential minerals are usually found in typical feeds, while others must be provided by dietary supplementation for optimal animal performance. Supplementation in excess of requirements increases mineral excretion. In addition, several essential minerals (e.g., copper and selenium) are toxic at high levels, while others, although not toxic per se, interfere with absorption of other essential minerals when included in the diet in excessive amounts.
Ruminants require fat-soluble vitamins (A, D, E, and K) and water-soluble vitamins (B complex), but typically only have a dietary requirement for vitamins A and E. Vitamin A is essential for normal growth and reproduction, maintenance of epithelial tissues, and bone development, and is a constituent of the visual pigment rhodopsin present in the rod cells of the retina. Vitamin A (retinol) per se does not occur in plants, but its precursors, carotenes, occur in various forms. Beta-carotene is the most widely distributed. High-quality forages provide carotenes in large amounts, but tend to be seasonal. Carotenes are rapidly destroyed by sunlight and air. Conversion of carotenes to retinol occurs in intestinal mucosal cells, but efficiency of conversion tends to be lower in ruminants than in nonruminants. Functions of vitamin E include serving as an antioxidant and in the formation of cellular membranes. Vitamin E occurs in feedstuffs as a-tocopherol. Vitamin E requirements depend on dietary concentrations of antioxidants, sulfur-containing amino acids, and selenium. Because vitamin D is synthesized by ruminants exposed to sunlight, or fed sun-cured forages, these animals rarely require vitamin D supplementation. Physiological needs of Vitamin K and the B vitamins (e.g., B12, thiamin, niacin, riboflavin, pyridoxine, pantothenic acid, biotin, and choline) have been clearly demonstrated, but requirements are normally easily met by microbial synthesis in the rumen.
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