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Nutrient Requirements of Swine: Eighth revised edition, 1979 (1979)

Chapter: NUTRIENT REQUIREMENTS

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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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Suggested Citation:"NUTRIENT REQUIREMENTS." National Research Council. 1979. Nutrient Requirements of Swine: Eighth revised edition, 1979. Washington, DC: The National Academies Press. doi: 10.17226/19882.
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NUTRIENT REQUIREMENTS Carbohydrates, lipids, proteins, minerals, vitamins, and water are the six classes of nutrients required for mainte- nance, growth, and reproduction of swine. Each nutrient has specific functions and, in addition, carbohydrates, lipids, and proteins are used to supply the energy re- quirements of animals, but with different levels of effi- ciency. ENERGY In nearly all conditions under which swine are fed, car- bohydrates and lipids supply most of the energy needs of the body. These energy requirements are expressed as kilocalories of digestible energy or metabolizable energy needed per kilogram of feed (Tables 5 and 7) and per animal per day (Tables 6 and 8). Digestible energy (DE) is defined as the dietary gross energy intake minus the gross energy excreted in the feces. Metabolizable energy (ME) is defined as the gross energy of the diet minus fecal and urinary gross energy. The loss of energy as gas produced in the digestive tract of nonruminants is usually small and therefore the ME values for swine are not cor- rected for energy lost through the gaseous products of digestion. The ME value of a feedstuff, diet concentration, or daily allowance can be estimated from the DE value using the formula from Asplund and Harris (1969): ME = DE x 96 - (0.202 x crude protein %) 100 The ME content of ingredients used in swine diets gen- erally comprises 90 to 97 percent of DE. A more precise estimate of ME can be obtained by correcting ME for nitrogen gained or lost from the body (ME,,). For the pig, a correction factor of 7.0 kcal/g nitrogen is used for each gram of nitrogen above or below nitrogen equilibrium. This correction is added to the ME for animals in negative nitrogen balance and subtracted when the pig is in posi- tive nitrogen balance. Net energy is used to describe the energy utilized by the animal. A portion of the ME is used for conversion or metabolism of absorbed dietary components into tissue and is defined as heat increment (HI). Thus, net energy (NE) may be defined as ME less the heat increment. Net energy is used by the animal to meet the require- ment for maintenance (NEj and for production (NE^. In the growing pig the efficiency of utilization of ME re- quired for maintenance and production is similar; thus the two components are seldom separated. Net energy, as a fraction of ME, has been shown to vary from 27.2 percent for wheat middlings to 69.0 percent for corn. It has therefore been suggested that NE, though difficult to measure, may be the best measure of the energy available for the maintenance and production of an animal. However, energy utilization is influenced by the level of feed intake; the balance of all the nutrients in the diet; the age, breed, sex, and condition of the animal used in the determination; and the environmental conditions that are present during the determination. At present, NE requirements for maintenance and produc- tion are not available, and, therefore, the energy require- ments of swine (Tables 5-8) and the energy values of the feedstuffs commonly used for swine (Table 9) are pre- sented as DE and ME values. Because the amount of feed consumed daily by growing-finishing pigs fed ad libitum is to a large extent controlled by the energy content of the diet, it follows that the other nutrients are required in some specific ratio to energy. Energy consumed in excess of that required for maintenance, increase of lean body mass, or reproduction leads to deposition of body lipid. Therefore, the lean-to- fat ratio of the pig may be altered by controlling the daily energy intake. The potential for such fat deposition is greatest as the pig approaches maturity. If pregnant sows are fed a corn-soybean meal diet, or another palatable diet of similar energy content, ad libitum, they consume more energy during gestation than they require for main- tenance and for development of the products of concep- tion and will therefore deposit body lipid. Therefore, the energy intake of gestating sows should be limited. Lactat-

Nutrient Requirements of Swine 3 ing sows should be fed at a level to support maintenance plus requirements for milk production. Consequently, DE or ME intake can be adjusted in relation to litter size. Research has shown that in feeding sows, efficiency of utilization of energy can be improved by allowing moder- ate weight gains during gestation and moderate weight losses during lactation, allowing for a small positive in- crease in body weight in each reproductive cycle. During growth, fiber digestibility increases as size and capacity of the digestive tract increase. Thus, the standard DE and ME values, which are normally based on digesti- bility and metabolism studies with growing pigs, may not be applicable to older pigs and breeding animals when high-fiber ingredients are fed. Carbohydrates The proximate analysis, commonly used to determine the nutrient content of a diet or feed ingredient, does not measure carbohydrate directly. Total carbohydrate is the combined nitrogen-free extract (NFE) and crude fiber. In this combined fraction, crude fiber is determined ana- lytically and NFE, which consists primarily of sugars and starches, is then determined by difference. The very young pig does not utilize starch efficiently. When weaned before reaching 2 weeks of age, the carbohydrate fraction of the diet is therefore generally supplied in the form of sugars. For pigs of less than 7 days of age, glucose and lactose are the sugars of choice, and, after 7 days of age, fructose and sucrose can be utilized. Two weeks after birth, enzymes are present for digesting starch. Cellulose is generally the major component of crude fiber with lesser, though variable, amounts of lignin also present. The pig does not produce enzymes for digesting cellulose or lignin; however, some microbial cellulose digestion does occur in the cecum and large intestine. The digestibility of fiber increases with increased capac- ity of the digestive system but varies in relation to chemi- cal complexity of the fiber and level in the diet. It should be considered as relatively indigestible. The inclusion of high levels of fiber in the diet lowers the digestibility of dry matter, protein, and ether extract and thus also de- creases the digestible energy of the diet. Increasing crude fiber in the diets of pigs fed ad libitum tends to reduce growth rate, despite a tendency of pigs to increase feed intake to compensate for the lower energy value of the diet. Any change observed in feed conversion, dressing percentage, and backfat thickness is generally in propor- tion to level and type of fiber added. If, however, energy intake is held constant, the inclusion of fiber should have no effect upon rate or efficiency of gain or upon carcass leanness. Lipids The term lipid is frequently used in a general sense to include both fats and oils. Some questions remain as to whether the pig has a dietary essential fatty acid require- ment. There is limited direct and indirect evidence that the pig is capable of some synthesis of linoleic acid. However, based upon the triene-tetraene ratio of tissue lipids, the dietary requirement for linoleate is small and is supplied adequately by natural ingredients. The concern, therefore, is mainly with lipids as an energy source. Swine can efficiently utilize large quantities of fats and oils in their diet. The efficiency of utilization of lipids is influenced by age of the pig and by the type and molecu- lar weight of the lipids added. Limitations to the level of lipids in swine diets are dictated by economics and the physical problems of mixing, processing, and handling diets containing large amounts of lipids. Upon biological oxidation, lipids yield 2.25 times as much energy as carbohydrates. The inclusion of fats or oils as a replacement for a high carbohydrate source, such as corn, will increase the energy content of the diet. Since swine tend to eat to meet their energy requirement, increasing the caloric density of the diet generally results in a reduction in total feed intake. Thus, the protein, vitamin, and mineral levels of a lipid-supplemented diet should be adjusted upwards to compensate for the ex- pected reduction in feed intake. Approximately the same proportionate reduction in feed intake will result from the addition of 5,10, or 15 percent lipid to the diet. In spite of the decrease in total feed intake, the caloric intake of the pig will be increased. This increased caloric intake may result in increased rate of gain and increased deposition of fat in the carcass. The addition of fat to high-protein diets may not reduce feed intake or increase fat deposi- tion to the same extent as that observed when fat is added to low-protein diets. An improvement in the efficiency of feed conversion is a consistent feature of fat- supplemented diets. However, the relative cost of fat versus carbohydrate sources of energy will determine the economic efficiency of such diets. PROTEIN—AMINO ACIDS Protein Protein is generally referred to as crude protein and in a feedstuff is defined as the nitrogen content x 6.25. The adequacy of dietary protein levels is determined by the capacity of the diet to supply sufficient indispensable (essential) amino acids and, in addition, nitrogen for the synthesis of dispensable (nonessential) amino acids. The dispensable amino acids for swine are needed in normal metabolism, but dietary sources usually are not required. These amino acids are obtained either from normal dietary components or synthesized by making use of amino groups derived from amino acids that are in excess in the diet. Supplemental nonprotein nitrogen, such as urea, has not produced beneficial responses in swine fed practical-type diets. Optimum performance requires that any indispensable amino acid be fed at the proper level and time in the feeding period and with the proper level of energy and other indispensable nutrients. Swine will perform satis-

4 Nutrient Requirements of Swine factorily if these conditions are met, even though there may be some variation in the level of crude protein in the diet. Because they are naturally leaner, gilts and boars require higher levels of crude protein to meet amino acid requirements than barrows. Also, maximal carcass lean- ness may require a greater intake of amino acids than maximal rate of weight gain. Protein levels that are neces- sary to provide the indispensable amino acids for growing swine fed grain-soybean meal-type diets are shown in Table 5. The cereal grains often provide a major portion of the total protein. However, supplementary amino acids, either from natural protein supplements or synthetic amino acids, must be provided to ensure adequate amounts and a proper balance of the indispensable amino acids. The availability of amino acids in the protein of com- mon feed ingredients fed to swine has not been adequately determined. Based upon results obtained with other species, values ranging from 80 to 90 percent are often assumed. Requirements for protein and amino acids listed in Tables 5 through 8 represent, in most instances, the levels of these nutrients required for swine fed grain-soybean meal diets. For example, if a combina- tion of grain and soybean meal furnishes 16 percent protein and 0.70 percent total lysine and is to be fed to swine in a weight range of 20 to 35 kg, it can be assumed that the physiologically available lysine requirement is between 0.56 and 0.63 percent. However, to account for the small portion that is not available, the lysine require- ment (allowance) listed in Table 5, for pigs in this weight range, is 0.70 percent. Amino Acids Requirements for indispensable amino acids by swine of various weights and in different stages of production are shown in Tables 5 through 8. In all cases, the require- ments correspond to the amount of natural isomer (L), the form in which amino acids occur in proteins. When amino acid supplements are provided, DL-methionine can re- place the L form in meeting the need for methionine. D-Tryptophan has a biological activity of about 60 per- cent relative to L-tryptophan for the growing pig. Thus, 0.15 percent DL-tryptophan is equivalent to 0.12 percent L-tryptophan in meeting the needs of the growing- finishing pig for this amino acid. It is assumed that the pig can utilize D-phenylalanine to some extent in meeting the total need for phenylalanine + tyrosine, but the efficiency of D-phenylalanine utilization is not known. Pigs can synthesize arginine at a rate sufficient to meet 60-75 percent of the requirement for normal growth, but the remainder must be provided from a dietary source to fulfill the total need of growing-finishing swine. For gravid and nongravid postpubertal swine, synthesis by the animal completely satisfies the arginine need. Cys- tine can satisfy at least 50 percent of the total need for methionine + cystine (sulfur amino acids), and methionine can meet the need in the absence of cystine. Phenylalanine can meet the total requirement for phenylalanine + tyrosine, since it can be converted to tyrosine. Tyrosine is not converted to phenylalanine, but it can satisfy 50 percent of the total need for the two amino acids. The amino acid requirements of growing-finishing swine, expressed in terms of dietary concentration, in- crease as the levels of dietary protein or caloric density of the diet increases. Because the amino acid concentration of lean tissue remains essentially constant with age, and maintenance needs constitute a small percentage of total needs, it is assumed that the requirements for indispens- able amino acids remain a constant percent of the protein with advancing age and weight. Thus, knowing the requirements for individual amino acids at 16 percent protein for pigs weighing 20 to 35 kg and knowing levels of protein (from grain-soybean meal mixtures) that permit optimal performance in pigs at other stages of growth allow calculation (by linear extrapolation) of require- ments for all weight ranges shown in Table 5. In most cases, requirements determined experimentally have been in close agreement with extrapolated values. Excep- tions have been lysine for pigs weighing 1 to 5 and 5 to 10 kg, where determined requirements have been somewhat higher than those predicted by extrapolation. Also, the methionine + cystine requirement of swine weighing 60 to 100 kg is lower than that predicted by extrapolation. The levels of amino acids shown in Tables 5 and 6 are adequate to support normal growth and performance, and they apply to diets of the caloric density indicated in Table 5. The lack of adequate quantitative information concerning the effects of caloric density of the diet and metabolizable energy content of feedstuffs for swine of various weights and stages of production precludes set- ting forth amino acid requirements on the basis of energy density of the diet. Where data are available, should caloric density increase or decrease from the values given, amino acid requirements may be adjusted upward or downward, respectively. The requirements for pregnant gilts and sows are based upon amounts required for satisfactory retention of nitro- gen during the late stages of pregnancy and are at least adequate to support development of a normal litter. The requirements for lactation either have been determined experimentally or have been extrapolated from published requirements for maintenance of adult female swine and from amounts calculated to be required to support good milk production. MINERALS At least 13 inorganic elements are known to be required by the pig, including calcium, phosphorus, potassium, sodium, chlorine, magnesium, sulfur, zinc, iron, man- ganese, copper, iodine, and selenium. In addition, vana- dium, chromium, nickel, tin, molybdenum, silicon, and

Nutrient Requirements of Swine 5 fluorine, which have been shown to be required by one or more species, probably are also required by the pig but at such low levels that their dietary essentiality has not been demonstrated. Functions of the inorganic elements are extremely di- verse, ranging from structural functions in some tissues to a wide variety of regulatory functions in many others. The increasing trend toward confinement rearing of pigs, without access to soil or forage, increases the importance of meeting their dietary mineral requirements. Require- ments for the individual elements at various stages of the life cycle are given in Tables 5 through 8 and are dis- cussed below. Calcium and Phosphorus Calcium and phosphorus are of major importance for skeletal development and for many other physiological functions. For maximum performance, minimum dietary levels of each are necessary, as well as the correct ratio of one to the other. Requirements, as shown in Tables 5 through 8, are based upon the feeding of a properly fortified grain-soybean meal diet. The quantitative need for calcium and phosphorus may be modified by other dietary factors, such as vitamin D, magnesium, or the presence of phytic acid in plant materials. Levels that are adequate for maximum gain in body weight are not neces- sarily adequate for maximum bone development. Border- line deficiency may go unnoticed in the growing- finishing animal but cause serious consequences in those saved for breeding purposes. Information on the calcium and phosphorus require- ments of gilts, sows, and boars is very limited. During pregnancy the physiological requirement increases in proportion to the need for fetal growth and reaches a maximum late in the gestation period. Because feeding practices during pregnancy vary greatly, diets should be formulated to meet daily requirements as shown in Table 8. When the total dietary intake of breeding animals is severely restricted the animals may receive too little calcium and phosphorus, even though the dietary concen- tration meets the requirement as shown in Table 7. Ex- cessive intake of calcium or phosphorus may lower per- formance of growing-finishing swine, especially when an excess of calcium interacts with zinc to cause a zinc deficiency. The form in which phosphorus exists in natural feedstuffs influences the efficiency of its utilization. In grains and plant protein supplements, about two-thirds of the phosphorus is in the less available phytate form. Utilization of phytate phosphorus is influenced by phytase present in plant materials and the intake of vita- min D, calcium, and zinc, as well as such factors as the pH of alimentary tract and the ratio of calcium to phosphorus in the diet. Estimates of the availability of total plant phosphorus range from 20 to as high as 60 percent. A wide range of calcium and phosphorus sources simplifies dietary fortification (Table 11). The desired ratio of between 1.0 and 1.5 calcium to 1.0 total phos- phorus in a grain-soybean meal diet can be attained easily by selection of suitable supplements. Signs of calcium and phosphorus deficiency are similar (Table 1) and are not unlike those seen in vitamin D deficiency. Sodium and Chlorine Sodium and chlorine are the principal extracellular cation and anion, respectively, in the body, and chlorine is the chief anion in gastric juice. Recent research has con- firmed that a level of 0.20 to 0.25 percent added sodium chloride will meet the dietary sodium and chlorine re- quirements of the growing-finishing pig fed a grain- soybean meal diet. Little or no information is available on the requirement for breeding-age boars or for gilts and sows in gestation or lactation. Until more definitive in- formation is available, a level of 0.4 percent added sodium chloride is suggested for boars and pregnant ani- mals and 0.5 percent for lactating sows. When a diet deficient in sodium chloride is fed to growing pigs, depressed performance will be evident within a few weeks. Deficiency signs are presented in Table 1. Swine can tolerate high dietary levels of sodium chloride, but, at a high level of intake, it is absolutely necessary to provide ample and readily available drinking water. As little as 1.0 percent dietary sodium chloride has produced toxicity signs and death when water has been restricted. The sodium ion is responsible for the adverse physiological reaction. Signs of toxicity are presented in Table 2. Potassium Potassium is an important mineral involved in electrolyte balance and neuromuscular function and serves as a monovalent cation to balance anions intracellularly, much as sodium functions extracellularly. Experimental esti- mates of the dietary potassium requirement are 0.27 to 0.39 percent for pigs weighing 1 to 4 kg, 0.26 percent for pigs weighing 5 to 10 kg, 0.23 to 0.29 percent for pigs weighing 16 kg, and less than 0.20 percent for pigs weigh- ing 20 to 35 kg. The daily potassium requirement of pigs weighing 45 kg is less than 5 g. Excesses of dietary chloride or sulfate ions increase the potassium require- ment. As dietary potassium is increased, there is an in- creased need for drinking water. Grain-soybean meal diets contain much higher levels of potassium than the requirements listed in Tables 5 and 7, and no potassium deficiency signs have been observed in swine fed these diets. Signs of potassium deficiency in young pigs receiving purified diets were, progressively: anorexia, rough hair coat, emaciation, inactivity, and ataxia (Table 1). Elec- trocardiograms taken during potassium deficiency reveal reduced heart rate and altered electrocardial intervals.

6 Nutrient Requirements of Swine Necropsy of potassium-deficient pigs reveals no unique pathology. Magnesium Magnesium is a cofactor in many important enzyme sys- tems and is a constituent of bone. The magnesium re- quirements of growing-finishing and adult swine are not known. Young, artificially reared pigs were shown to have a requirement of between 325 and 500 mg magnesium per kilogram of diet. Grain-soybean meal diets generally con- tain sufficient magnesium (1 g or more per kilogram of diet) to prevent deficiency signs. Similarly, milk contains adequate magnesium to protect suckling pigs. Signs of deficiency, in order of appearance, are: hyperirritability, muscular twitching, reluctance to stand, stepping syndrome, weak pasterns, loss of equilibrium, and tetany followed by death (Table 1). Iron Iron is required for the formation of hemoglobin, myoglo- bin, ferritin, hemosiderin, transferrin, and all iron- containing enzymes. Pigs are born with about 50 mg of iron in their body, most of which (80 percent) is present in erythrocytes as hemoglobin. Necessary iron retention in the nursing pig to maintain levels of hemoglobin and storage iron range from 7 to 16 mg daily or 21 mg per kilogram of body-weight gain. Sow's milk contains an average of 1 mg of iron per liter and thus, if not supple- mented, will result in the rapid development of anemia. The feeding of high levels of different iron compounds to gestating and lactating sows has not effectively increased the iron content of the milk. The oral iron requirement of baby pigs fed milk or purified liquid diets is 60 to 150 mg per kilogram of milk solids. It appears that the require- ment of baby pigs receiving a dry (casein-based) diet is about 50 percent higher per unit of dry matter than when fed a similar diet in homogenized liquid form. Germ-free baby pigs have a dietary iron requirement similar to that of conventionally reared baby pigs. Numerous studies have shown the effectiveness of a single intramuscular injection of 100 to 200 mg of iron, as iron dextran or iron dextrin complexes, given in the first 3 days of life. Over 90 percent of the injected iron is utilized over the next few weeks as an anemia preventative. The postweaning dietary iron requirement is about 80 ppm and diminishes in later growth and maturity, since there is a smaller increase in blood volume with increasing body weight. Feed ingredients in a balanced diet usually sup- ply enough iron to meet postweaning requirements. Some calcium and phosphorus sources, such as feed grade defluorinated phosphate and dicalcium phosphate, contain from 0.6 to 1.0 percent of iron, of which about 60 percent is available to the pig. Availability of iron from different sources (Table 11) varies greatly. Ferrous sulfate and ferric ammonium ci- trate are effective in preventing iron-deficiency anemia, but an iron compound with low solubility, such as ferric oxide, is ineffective. Ferrous carbonate is much less effec- tive than ferrous sulfate. Dietary iron is effective in detoxifying gossypol- containing diets. The addition of iron from soluble sources to the diet equal to the weight of free gossypol improves growth rate, reduces toxicity, and helps prevent accumulation of gossypol in the liver. Blood hemoglobin concentration is a rapid, reliable indicator of the iron status of the pig. The following hemoglobin levels can be used as indicators of the iron status of pigs between birth and 8 weeks of age: Blood Hemoglobin (g/100 ml) 10 or above 9 8 7 or below 6 or below 4 or below Comment Normal level; adequate iron and optimum performance Minimum level required for average performance and the dividing line between nor- mality and anemia Borderline anemia; iron treat- ment needed Anemic condition that has been shown to retard growth rate Severe anemia accompanied by marked reduction of per- formance Severe anemia that can be ex- pected to result in increased mortality rate Iron-deficiency anemia is of the hypochromic- microcytic type. Anemic pigs show signs of poor growth, listlessness, rough hair coat, wrinkled skin, and paleness of mucous membranes. Fast-growing anemic pigs may die suddenly of anoxia. A characteristic sign is labored breathing after minimal activity or a spasmodic jerking of the diaphragm muscles from which the term "thumps" arises. Necropsy findings include an enlarged and fatty liver, thin watery blood, ascites, marked dilation of the heart, and an enlarged firm spleen (Table 1). In 3- to 10-day-old pigs the toxic oral dose of iron from ferrous sulfate is approximately 600 mg/kg of body weight. Clini- cal signs of toxicity are observed within 1 to 3 hours after iron is fed (Table 2). Zinc Zinc is a component of many metalloenzymes and the hormone insulin and is thereby involved in protein, car- bohydrate, and lipid metabolism. The dietary zinc requirement is influenced by many diet-related factors, including phytic acid or plant phy- tates, calcium, copper, cadmium, cobalt, histidine, as well as type and level of protein. The requirement of growing pigs receiving semipurified diets containing isolated soy- bean protein, or natural corn-soybean meal diets contain- ing the recommended calcium level, is about 50 ppm.

Nutrient Requirements of Swine 7 Boars have a slightly higher zinc requirement than gilts, and both boars and gilts have a higher requirement than barrows. When dietary calcium level is excessive, the zinc requirement is increased. A zinc level of 33 ppm in corn-soybean meal gestation and lactation diets of sows through five parities was adequate for optimal gestation performance, but not for lactation. Because of the absence of phytic acid or plant phytates, the zinc requirement of baby pigs receiving a casein-glucose diet is reduced to about 15 ppm. Signs of zinc deficiency in growing pigs include parakeratosis and reduced appetite with reduced rate and efficiency of gain. Markedly reduced levels of serum zinc, alkaline phosphatase, and albumin are also found. Gilts receiving zinc-deficient diets during gestation and lacta- tion produce fewer and smaller pigs that at birth have a reduced serum and tissue zinc level. Testicular develop- ment of the zinc-deficient growing boar and thymic de- velopment of the zinc-deficient baby pig are retarded (Table 1). Zinc may be increased to 1,000 ppm in the diet without producing signs of toxicity, but a dietary zinc level of 2,000 ppm from zinc carbonate produced the following toxic signs: growth depression, arthritis, hemorrhage in axillary spaces, gastritis, and enteritis (Table 2). Manganese Manganese functions as a component of several enzymes involved in carbohydrate, lipid, and protein metabolism and is essential for synthesis of chondroitin sulfate, which is a component of mucopolysaccharides in the organic matrix of bone. Minimum requirements for manganese are not well defined. Growth is normal when pigs are fed a purified diet containing as little as 1.5 mg/kg of diet. Continued feeding of a lower level of 0.5 mg/kg of diet interferes with normal development and reproduction. Signs of manganese deficiency have included abnormal skeletal growth with altered ratio of fat to lean body tissue; ab- sence of, or irregular, estrual cycles; poor mammary de- velopment and lactation; resorption of fetuses; and small, weak pigs at birth (Table 1). There is some evidence that diets containing com- monly used feed ingredients should contain higher levels of manganese than the amount required in purified diets and that excessive dietary levels of calcium and phos- phorus may reduce manganese absorption. For gestation and lactation a minimum of 10 mg/kg of diet is suggested. While the toxic level of manganese is not well estab- lished, depressed feed intake and reduced rate of gain have been observed when pigs were fed a diet containing 4,000 mg/kg (Table 2). Copper Copper is essential for the synthesis and activation of several oxidative enzymes necessary for normal metabolism in the pig. A deficiency of copper leads to poor iron mobilization, abnormal hematopoiesis, kera- tinization, and synthesis of collagen, elastin, and myelin. Low fertility has been associated with copper defi- ciency in several animal species, but not in swine. A level of 6 ppm of copper in the diet for the very young pig seems to be adequate, and the requirement is probably no greater for later stages of growth. Definitive information on the requirements for pregnancy and lactation is lack- ing. Signs of deficiency include leg weakness and ataxia. A subclinical deficiency is associated with reduced serum copper and ceruloplasmin and a microcytic hypochromic anemia (Table 1). While availability does vary, the copper requirement can be effectively met by the use of supple- mental copper sulfate, copper carbonate, or copper oxide. Copper toxicity has been reported at levels above 250 ppm, particularly when iron and zinc levels are limiting or when calcium is in excess. Toxicity signs are presented in Table 2. Iodine Iodine is a component of thyroxine and triiodothyronine, which are important in the regulation of metabolic rate. The dietary iodine requirement of the pig is increased by goitrogens and excessive levels of arsenic, fluoride, cobalt, calcium, and sodium chloride. Experimental esti- mates of the requirement for normal growth and thyroid size range from 0.05 to 0.14 ppm, and, in the absence of excesses of the above interrelated minerals, a dietary iodine level of 0.14 ppm seems to be adequate for all stages of the life cycle. Calcium and potassium iodate and pentacalcium orthoperiodate are nutritionally available forms and are more stable in salt and feed mixtures than sodium or potassium iodides. If salt contains 0.007 per- cent iodine, the requirement can be met by incorporating 0.2 percent salt in the diet. Signs of iodine deficiency observed in pigs born to sows on goitrogenic diets are hairlessness, thickened skin, and myxedema. Most of the pigs are born alive, but are weak and usually die within a few hours. At necropsy, the thyroid is enlarged and hemorrhagic (Table 1). Iodine may be increased to 400 ppm in the diet of the growing pig without depressing growth rate or the effi- ciency of feed utilization. Dietary iodine levels of 800 ppm or more depress growth rate, feed intake, hemoglo- bin level, and liver iron concentration, while bringing about an increase in serum iron and thyroid weight (Table 2). Selenium The dietary requirement for selenium is between 0.1 and 0.2 mg/kg of diet and is inversely related to vitamin E level. Selenium is an essential component of the enzyme glutathione peroxidase, which functions in peroxide re- duction. Thus, the mutual sparing effect of selenium and vitamin E stems from their shared antiperoxidant roles. High levels of vitamin E will not eliminate the need for selenium. Certain soils of the United States and Canada

8 Nutrient Requirements of Swine are low in selenium, and the feeding of diets formulated from ingredients grown in such regions often results in a selenium deficiency. Incidence and degree of selenium deficiency may be increased by environmental stress. The primary biochemical change in selenium defi- ciency is a decline in glutathione peroxidase activity. Serum transaminases, lactic dehydrogenase, and creatine phosphokinase may be elevated as a result of tissue dam- age. Sudden death is a prominent feature of the selenium-deficiency syndrome. Gross necropsy lesions of selenium deficiency are identical to those of vitamin E deficiency (Table 1). These include massive hepatic ne- crosis (hepatosis dietetica) and edema of the spiral colon, lungs, subcutaneous tissues, and submucosa of the stomach. Bilateral paleness and dystrophy of the skeletal muscles (white muscle disease) are often found. Occa- sionally, mottling and dystrophy of the myocardium (mulberry heart disease) are also observed. Selenium, when fed to growing swine as sodium sele- nite, sodium selanate, selenomethionine, or seleniferous corn, does not produce toxicity at levels as high as 5 mg of selenium per kilogram of diet. Levels from 7.5 to 10 mg/ kg may produce toxicity. Signs of toxicity include anorexia, hair loss, fatty infiltration of the liver, degenerative changes in liver and kidney, swelling, and occasional separation of hoof and skin at the coronary band (Table 2). Dietary arsenicals help to alleviate selenium toxicity. Cobalt Cobalt does not appear to be a dietary requirement except as a part of the vitamin B,2 molecule. Since the intestinal flora of the pig is capable of synthesizing vitamin B,2, a minimum level of dietary cobalt is required for this pro- cess to occur. Such synthesis assumes greater importance if dietary vitamin B,2 is limiting. There is also some evidence that cobalt may have a "sparing action" on zinc in zinc deficiency. No dietary requirement for cobalt has been established. A level of 400 mg of cobalt per kilogram of diet is toxic to the young pig and may cause anorexia, stiff-leggedness, humped back, incoordination, muscle tremors, and anemia (Table 2). VITAMINS Vitamins are organic compounds required in small amounts for normal growth and reproduction and for maintaining the health of swine. Some vitamins are es- sential in metabolism but may not be required in the diet, since they can be synthesized readily from other food constituents. An example of this is the production of niacin from tryptophan. Bacteria in the intestinal tract also are capable of producing vitamins, such as vitamin K and vitamin B ,2, which can then be made available to the animal. The vitamins are generally divided into fat- soluble vitamins A, D, E, and K and water-soluble B vitamins and vitamin C. Fat-Soluble Vitamins Each of the four fat-soluble vitamins occurs naturally in a variety of vitamin or provitamin forms in feedstuffs and each is produced in synthetic forms that have a high degree of vitamin activity. The increasing trend toward rearing pigs in confinement without access to forage or sunlight increases the importance of meeting the dietary vitamin requirements presented in Tables 5 through 8 and discussed below. Vitamin A The vitamin A requirement of swine can be met by either vitamin A or by provitamin A in the form of beta-carotene. Beta-carotene is the standard for provita- min A, and for the rat 1 IU of vitamin A activity is equal to 0.6 /ig of beta-carotene. The conversion of carotene to vitamin A in the pig is less efficient. Based upon liver storage, the biopotency of 1 mg of beta-carotene ranges from 200 to 500 iu of vitamin A activity compared to 1,667 IU of vitamin A activity for the rat. Feed ingredients contain a mixture of carotenoids, some of which have less biological activity than beta-carotene. For swine, 1 mg of the total carotenoid material in corn provides about 250 IU of vitamin A activity. International standards for vitamin A are based upon its utilization by the rat and are as follows: 1.0 IU of vitamin A activity = 1.0 U.S.P. unit = vitamin A activity of 0.300 jig of crystalline vitamin A alcohol (retinol), which corre- sponds to 0.344 jug of vitamin A (retinyl) acetate or 0.550 fig of vitamin A (retinyl) palmitate. Vitamin A is essential for the normal maintenance and function of the eyes and of the epithelial tissues of the respiratory, reproductive, nervous, and genitourinary sys- tems. Recommended dietary requirements for vitamin A or beta-carotene are shown in Tables 5 through 8. Taken individually, night blindness, elevated pressure of cere- brospinal fluid, and reduced growth rate are poor criteria for assessing the vitamin A status of swine, but collec- tively they provide a reliable indicator. Deficiency signs are described in Table 1. Swine have the ability to store vitamin A in the liver, which is then available during periods of stress or low intake. The excessive intake of nitrate or nitrite may increase the dietary vitamin A re- quirement. Vitamin D Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are both equally effective in meeting the vitamin D requirements of swine. One international unit (iu) of vitamin D is defined as the biological activity of 0.025 /u,g of cholecalciferol. Cholecalciferol is converted to 25-hydroxy D3 by the liver and is the major circulatory metabolite of vitamin D3, but for conversion to the biolog- ically active form of the vitamin it must undergo further hydroxylation to 1,25-dihydroxy D3 in the kidney. This is the hormonal form that stimulates intestinal calcium and phosphate transport, bone calcium mobilization, and renal calcium reabsorption. Other vitamin D3 metabolites may also be of importance in calcium and phosphorus homeostasis.

Nutrient Requirements of Swine 9 The dietary vitamin D2 requirement of the baby pig, reared on a purified diet containing casein as the protein source, is 100 IU/kg of diet. When casein is replaced by isolated soybean protein, which has rachitogenic activity, the vitamin D2 requirement is higher. The minimum requirement of growing pigs receiving a grain-soybean meal diet is 200 IU/kg. Grains, grain by-products, and protein feedstuffs are practically devoid of vitamin D. Therefore, unless swine are exposed to the ultraviolet rays of the sun, the diet should be fortified with this vitamin. The principal manifestation of vitamin D deficiency is a disturbance of calcium and phosphorus absorption and metabolism with insufficient calcification of bones. In young growing pigs, the result is rickets, while in mature swine diminished mineral content with softening of the bone (osteomalacia) results. Serum calcium and phos- phorus are reduced and serum alkaline phosphatase activ- ity is increased. In severe vitamin D deficiency, serum magnesium may also be low and pigs may exhibit signs of calcium and magnesium deficiency, including tetany (Table 1). Weanling pigs that were supplemented with 250,000 IU of vitamin D3 daily for 4 weeks had reduced body-weight gain and reduced weight of liver, radius, and ulna. Pigs receiving this level of vitamin D3had periods of inappe- tance, and, at necropsy, pathological calcification was observed in the aorta, heart, kidney, and lung (Table 2). Vitamin E Vitamin E activity is a function of several tocopherols, and acts primarily as a lipid-soluble an- tioxidant. The dietary requirement for vitamin E is de- pendent upon levels of dietary selenium and polyunsatu- rated fatty acids, as well as the presence or absence of other antioxidants. Extremes of temperature and exercise may increase the requirement. The tocopherols differ considerably in their biological activity with d-a- tocopherol being most active. One international unit (iu) of vitamin E activity is defined as the biological activity of 1 mg d/-o-tocopheryl acetate. In the presence of adequate selenium, a total of 10 to 15 iu of vitamin E per kilogram of diet is adequate for grain-soybean meal diets. The level necessary to prevent deficiency signs, however, may be considerably higher in the absence of adequate selenium and/or in the pres- ence of high levels of prooxidants. Signs of deficiency are the same as those encountered in selenium deficiency. Elevated serum transaminases, lactic dehydrogenase, and creatine phosphokinase are found early in the development of the deficiency state. Hepatic necrosis, muscular dystrophy, and edema, as well as sudden death, are among other features shared with selenium deficiency (Table 1). A toxicosis due to high intakes of vitamin E has not been demonstrated in swine. Levels as high as 100 IU/kg diet have been fed to growing pigs and over 300 lU/kg diet have been fed to boars and sows without toxic effects. Vitamin K The pig requires vitamin K for the formation of prothrombin and other plasma proteins essential for normal clotting of blood. The best natural sources of the vitamin include the legumes and other green forage mate- rials. Synthesis of vitamin K2 by the gastrointestinal mi- croflora is an important factor influencing the dietary re- quirement. A hemorrhagic condition in the growing pig, believed to be a deficiency of vitamin K, has been associated with the consumption of mold-contaminated diets. Signs of deficiency have been produced in newborn pigs housed in wire-bottomed cages and fed a synthetic diet contain- ing an antibiotic and a sulfa drug. Lack of the vitamin increases prothrombin time and clotting time and may result in internal hemorrhage. Signs of deficiency are eliminated by the addition of vitamin K or water-soluble synthetic forms of the vitamin to the diet (Table 1). The most common synthetic materials with vitamin K activity for dietary use are menadione sodium bisulfite, menadione sodium bisulfite complex, and menadione dimethyl pyrimidinol bisulfite. Such water-soluble forms have biological activity related primarily to their menadione content. The occasional appearance of signs of vitamin K deficiency under field conditions has led to the common addition of vitamin K to swine diets, particu- larly where pigs are reared in confinement. If there is evidence of a deficiency, or if there is need to supply vitamin K activity for preventative purposes, it is suggested that the diet be supplemented with 2.0 mg of menadione per kilogram. Water-Soluble Vitamins Deficiencies of niacin, pantothenic acid, riboflavin, choline, and vitamin B,2 may occur in pigs fed unsupple- mented grain-soybean meal diets. Thiamin, vitamin B6, biotin, and folacin are contained in many feed ingredients at levels that furnish more than the pig's requirement. Ascorbic acid (vitamin C) is produced by the pig and, under most conditions, is not required in the diet. Water- soluble vitamin requirements are presented in Tables 5 through 8 and discussed below. Thiamin Thiamin is essential for swine, but virtually all diets contain an abundant quantity of this B vitamin. Grains are particularly rich in thiamin. Signs of deficiency in swine have been produced only under experimental conditions. Raw fish contains a thiaminase that renders thiamin inactive, and bracken fern contains an an- tithiamin substance of nonenzymatic nature. Thiamin functions in intermediary metabolism primar- ily in its coenzyme form, thiamin pyrophosphate (TPP), and as such is involved in decarboxylation reactions. In either crystalline form or in food, thiamin is heat labile. Thus, drying of grains and cooking of soybeans lower the concentration of available thiamin in these ingredients. Signs of thiamin deficiency are presented in Table 1. Riboflavin Riboflavin functions in the pig in two coen- zymes essential in oxidation-reduction reactions, flavin

10 Nutrient Requirements of Swine mononucleotide (FMN) and flavin-adenine dinucleotide (FAD). The riboflavin requirements of the baby pig receiv- ing semipurified liquid or dry diets, and of the weanling pig receiving natural diets, is 3 mg/kg of diet. When expressed as a concentration in the diet, the requirement is reduced as body weight increases. Intestinal bacterial synthesis of riboflavin reduces dietary needs, and more of the vitamin is required when fat is added to the diet. The requirement is most closely related to energy expenditure and is consistently 700 to 800 mg per megacalorie of dietary metabolizable energy. The minimum dietary riboflavin requirement of the sow for gestation and lacta- tion is about 3 mg/kg. Signs of riboflavin deficiency in the young growing pig include slow growth, vomiting, cataracts, abnormal stiff- ness of gait, seborrhea, and alopecia. A normocytic anemia and myelinic degeneration of nerve tissue have been reported. In sows, riboflavin deficiency results in poor reproduction and lactation performance (Table 1). Niacin Niacin is required by all living cells, and it is an essential component of important enzyme systems in- volved in lipid, carbohydrate, and protein metabolism. As nicotinamide, it is a component of the coenzymes nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP). In the diet, nicotinamide can substitute for nicotinic acid on an equal weight basis. Signs of deficiency in the pig include anorexia and decreased rate of gain, followed by diarrhea, occasional vomiting, and an exfoliative type of dermatitis and loss of hair (Table 1). Diets based upon cereal grains are low in available niacin; thus crystalline nicotinic acid is generally added to satisfy requirements. Two factors must be considered in evaluating the ade- quacy of niacin in diets: 1. Niacin occurs in cereal grains in a bound form, which is largely unavailable to the pig. This fact is not revealed in the conventional niacin assays, which merely indicate the total content of the vitamin. 2. Because of the conversion of tryptophan to niacin by the pig, the tryptophan level of the diet is important in determining the niacin requirement. Each 50 mg of tryp- tophan, in excess of the tryptophan requirement, will yield 1 mg of niacin. Pantothenic Acid Pantothenic acid is distributed widely in feed ingredients of plant and animal origin. It is re- quired by the pig as a component of coenzyme A, an important enzyme in carbohydrate and fatty acid metabolism. The vitamin is commercially available as the calcium salt (calcium pantothenate), which is used as the supple- mental form. Products marketed are commonly a mixture of the d and / forms of calcium pantothenate. Since only the d isomer has biological activity, in formulating diets or premixes the guarantee should be stated in terms of this form only. Pantothenic acid deficiency is most commonly seen in the young pig and is revealed as leg stiffness and locomotor incoordination, which sometimes gives the ap- pearance of "goose-stepping." It should be pointed out that while a deficiency of pantothenic acid will cause goose-stepping, other conditions, not well defined, also may lead to the development of a similar gait. Signs of pantothenic acid deficiency may also include slow growth and poor condition of the hair and skin. In the sow, following a prolonged inadequate intake of this vitamin, the above signs of deficiency may appear in pigs shortly after birth. Such signs in the newborn will be evident before reproductive function is impaired (Table 1). Vitamin B ,2 Vitamin B,2 is required for the maturation of red blood cells and is involved in numerous other metabolic functions. Feed ingredients of animal origin contain substantial, but highly variable, quantities of the vitamin. Synthesis of vitamin B,2 by intestinal bacteria serves to supplement dietary sources. Vitamin B,2 contains the trace element cobalt, and vitamin B,2 synthesis by intestinal flora is dependent upon the presence of this mineral in the feed. This is the only established function of cobalt as an essential nu- trient. In the growing pig a deficiency of vitamin B ,2 reduces growth. In the reproducing animal, litter size and pig survival are reduced (Table 1). When a sufficient quan- tity of the vitamin is not supplied by ingredients of animal origin, a supplemental level is recommended at all stages of the life cycle (Tables 5-8). There is some evidence that the reproductive perfor- mance of sows may be improved by the inclusion of higher than recommended levels of vitamin B,2. The response is evidenced by an increase in the number and weight of pigs at birth. Response to such elevated levels is not consistent and may relate to variable synthesis by intesti- nal bacteria or to differences in utilization of the vitamin. Choline Choline does not strictly qualify as a vitamin because it is required in the diet at levels far greater than those of the other vitamins, and because it is actually a structural component of fat and nerve tissue. Moreover, choline is not known to participate in any enzyme system. Nonetheless, because of its biological function in cell structure (component of phospholipid), lipid transport, and nerve impulse transmission (acetyl choline), choline is generally considered along with the vitamins. Choline is an important source of labile methyl groups that function in a variety of one-carbon transfer reactions referred to as transmethylation, and the major portion of the dietary choline required is necessary for this function. In the pig, methionine (also a methyl donor) can com- pletely replace that portion of the choline needed for transmethylation. Thus, at methionine levels in excess of the physiological requirement, 4.3 mg methionine pro- vides the same methylating capacity as 1 mg of choline. Choline deficiencies have been encountered in baby

Nutrient Requirements of Swine 11 pigs fed a high fat synthetic milk diet containing 0.8 percent methionine or less. Gestating sows fed corn- soybean meal diets have responded to supplemental choline with increased litter size at birth. Signs of choline deficiency in the growing pig are depressed growth rate, unthriftiness, and fatty infiltration of the liver (Table 1). Vitamin B6 Vitamin B6 exists in three forms in feedstuffs: pyridoxine, pyridoxal, and pyridoxamine. Pyridoxal phosphate is the coenzyme form of the vitamin and is essential for the biological activity of decar- boxylases, dehydrases, synthetases, transaminases, and racemases involved in amino acid metabolism. Vitamin Bg-containing enzymes are involved in the synthesis and catabolism of all amino acids. In experimentally produced vitamin Bg deficiency, the conversion of tryptophan to niacin is blocked. Because of its wide distribution in natural feed ingre- dients, vitamin B6 is seldom deficient in swine diets. Consequently, it is not generally added in supplemental form. For young pigs the dietary requirement is 1.5 mg/kg and for the growing-finishing pig 1.1 mg/kg (Table 5). Biotin Biotin is a functional component of enzymes re- quired in carboxylation reactions such as acetyl-CoA car- boxylase, an enzyme needed in fatty acid synthesis. The small amount required by the pig is usually supplied by the diet and is augmented by microbial synthesis of the vitamin in the digestive tract. Only after the feeding of diets containing high levels of raw egg white, containing avidin, which forms a complex with biotin, or, following the inclusion of high levels of sulfa drugs to eliminate microbial synthesis, can a deficiency be produced. Clini- cal signs of deficiency include a dermatosis and spasticity of the hind legs (Table 1). Factors that may influence the dietary need for biotin include the possible sparing effect of vitamin B,2, the destruction of biotin caused by rancidity, and the variabil- ity of bound and free biotin in feed ingredients. Perfor- mance of pigs has generally not been improved by biotin supplementation of grain-soybean meal diets. It is noteworthy, however, that wheat is low in available biotin, containing only about half the quantity present in corn, sorghum, and barley. Folacin Folacin has an essential role in the normal metabolic function of body cells. As a constituent of the folate coenzymes, it is involved in the incorporation of single carbon units into larger molecules. Deficiency signs have been reported in young pigs only when fed diets containing folacin antagonists or high levels of sulfa drugs. Weakness, poor growth, and a normocytic anemia were evidence of the deficiency (Table 1). The folacin content of ingredients commonly used in swine diets, plus bacterial synthesis in the intestinal tract, seems to be sufficient to meet the requirement of the pig. Ascorbic Acid The pig synthesizes ascorbic acid at a level that meets requirements for normal growth and skeletal development. High levels are found in sow's colostrum and milk and in the blood of newborn pigs. In some experiments supplemental levels of ascorbic acid have been associated with improved growth rate. Reasons for such a response are not known. It has been suggested that under conditions of environmental stress there may be need for a dietary source of the vitamin. In poultry some evidence for such a requirement is reported. Very high levels of ascorbic acid in the growing-finishing pig diet (0.5 percent) have been shown to decrease the ab- sorption or retention of copper and to increase iron ab- sorption. WATER Swine receive water from three sources, namely, metabolic water from the breakdown of carbohydrate, fat, and protein; water that is a component of feedstuffs; and water that is drunk. The latter makes up a large portion of the normal intake, although all that is required may be supplied by liquid feeds such as whey. Water is involved in many physiological functions necessary for maximum animal performance. Among these are temperature regulation, transport of nutrients and wastes, metabolic processes, lubrication, and milk production. The water requirements of swine are variable and gov- erned by many factors. Water accounts for as much as 80 percent of body weight at birth and declines to approxi- mately 50 percent in a finished market animal. The need for water is increased when fecal water excretion is high (diarrhea). Likewise, excessive urinary excretion, as may be caused by high salt or protein intake, markedly in- creases the water requirement. High ambient tempera- ture, fever, and lactation are other conditions that in- crease water requirement. The minimum requirement is that amount needed to balance water losses plus the amount needed for the formation of new tissue or prod- ucts. Water requirement has a relationship to feed intake and to body weight. Under normal conditions swine will consume 2.0 to 5.0 kg of water per kilogram of dry feed or 7 to 20 kg of water per 100 kg of body weight daily. The wide range of consumption per unit of body weight is influenced by age, with the young animal having the higher requirement. Older swine approach the higher level of water intake when fed a highly palatable liquid feed. A weight ratio of 3 parts water to 1 part of dry feed is usually recommended for liquid feeding systems, but when the ambient temperature exceeds 35°C the pig may desire more water and refuse a portion of the solids if no supplemental water is available. Water can serve as a vehicle for dewormers, medicinals, oral vaccines, or as a carrier for water-soluble nutrients when administered through a properly controlled dis- pensing system. Water may contain harmful levels of

12 Nutrient Requirements of Swine agricultural or industrial chemicals if the supply origi- Temperature of the water will affect volume intake, nates from shallow wells or if wells collect surface runoff. Additional energy is required to warm liquids consumed An example is nitrate contamination. An informative pub- at temperatures below that of the body. Lactating sows lication is Nutrients and Toxic Substances in Water for must have unlimited access to water if they are to milk Livestock and Poultry, National Academy of Sciences, adequately, and suckling pigs need water in addition to 1974. that in sows' milk for optimum performance. Free access to water located near feed dispensers is desirable.

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