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4
Minerals
Pigs have a dietary requirement for certain inorganic elements. These include calcium, chlorine, copper, iodine, iron, magnesium, manganese, phosphorus, potassium, selenium, sodium, sulfur, and zinc. Chromium is now recognized as an essential mineral (National Research Council, 1997), but a quantitative requirement has not been established. Cobalt also is required in the synthesis of vitamin B12. Pigs may also require other trace elements (i.e., arsenic, boron, bromine, fluorine, molybdenum, nickel, silicon, tin, and vanadium) which have been shown to have a physiological role in one or more species (Underwood, 1977; Nielsen, 1984). These elements are required at such low levels, however, that their dietary essentiality has not been proven.
The functions of these inorganic elements are extremely diverse. They range from structural functions in some tissues to a wide variety of regulatory functions in other tissues. Most pigs are now raised in confinement, without access to soil or forage; this rearing environment may increase the need for mineral supplementation. Suggested minimum requirements for the individual elements at various stages of the life cycle are given in tables provided in Chapter 10. Meeting the mineral requirements will be influenced by the bioavailabilities of minerals in feed ingredients. The subject of bioavailability of minerals was included in a recent book, Bioavailability of Nutrients for Animals, edited by Ammerman, Baker, and Lewis (1995).
Several minerals, including antimony, arsenic, cadmium, fluorine, lead, and mercury, can be toxic to swine (Carson, 1986). The toxicities and tolerances of essential and other mineral elements are described in detail in Mineral Tolerance of Domestic Animals (National Research Council, 1980).
Macro Minerals
Calcium and Phosphorus
Calcium and phosphorus play a major role in the development and maintenance of the skeletal system and perform many other physiologic functions (Hays, 1976; Peo, 1976, 1991; Kornegay, 1985). Peo (1991) indicated that adequate calcium and phosphorus nutrition for all classes of swine is dependent upon: (1) an adequate supply of each element in an available form in the diet, (2) a suitable ratio of available calcium and phosphorus in the diet, and (3) the presence of adequate vitamin D. A wide calcium-to-phosphorus ratio lowers phosphorus absorption, resulting in reduced growth and bone calcification, especially if the diet is marginal in phosphorus (Peo et al., 1969; Vipperman et al., 1974; Doige et al., 1975; van Kempen et al., 1976; Reinhart and Mahan, 1986; Hall et al., 1991; Wilde and Jourquin, 1992; Eeckhout et al., 1995; Qian et al., 1996). The ratio is less critical if the diet contains excess phosphorus (Prince et al., 1984; Hall et al., 1991). A suggested ratio of total calcium-to-total phosphorus for grain—soybean meal diets is between 1:1 and 1.25:1. When based on available phosphorus, the ratio is between 2:1 and 3:1 (Jongbloed, 1987; Ketaren et al., 1989; Qian et al., 1996). A narrower calcium-to-phosphorus ratio, whether total or available phosphorus, probably results in more efficient utilization of phosphorus. An adequate amount of vitamin D is also necessary for proper metabolism of calcium and phosphorus, but a very high level of vitamin D can mobilize excessive amounts of calcium and phosphorus from bones (Hancock et al., 1986; Jongbloed, 1987).
A considerable amount of research has been conducted to determine the calcium and phosphorus requirements of weanling pigs (Rutledge et al., 1961; Combs and Wallace, 1962; Combs et al., 1962, 1966; Miller et al., 1962, 1964a,b, 1965b,c,d; Menehan et al., 1963; Zimmerman et al., 1963; Blair and Benzie, 1964; Mudd et al., 1969; Coalson et al., 1972, 1974; Mahan et al., 1980; Mahan, 1982) and growing-finishing swine (Chapman et al., 1962; Libal et al., 1969; Cromwell et al., 1970, 1972b; Stockland and Blaylock, 1973; Doige et al., 1975; Pond et al., 1975, 1978; Fammatre et al., 1977; Kornegay and Thomas, 1981; Thomas and Kornegay, 1981; Maxson and Mahan, 1983; Combs et al.,
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1991a,b). The estimated dietary requirements for calcium and phosphorus for maximum growth rate and feed efficiency of pigs from 3 to 120 kg are given in Chapter 10, Tables 10-5 and 10-6. The requirements for total calcium and total phosphorus are based on a fortified, corn–soybean meal diet and take into account the fact that some of the phosphorus in feedstuffs of plant origin is unavailable. Estimates of the requirements for available phosphorus for the maximum rate and efficiency of gain are also presented in Chapter 10, Tables 10-5 and 10-6. Higher dietary concentrations of calcium and phosphorus may be required if feed intake is low.
The levels of calcium and phosphorus that result in maximum growth rate are not necessarily adequate for maximum bone mineralization. The requirements for maximizing bone strength and bone-ash content are at least 0.1 percentage unit higher than the requirements for maximum rate and efficiency of gain (Cromwell et al., 1970, 1972b; Mahan et al., 1980; Crenshaw et al., 1981; Kornegay and Thomas, 1981; Mahan, 1982; Maxson and Mahan, 1983; Koch et al., 1984; Combs et al., 1991a,b). However, maximization of bone strength by feeding large amounts of calcium and phosphorus to growing pigs does not necessarily improve structural soundness (Pointillart and Gueguen, 1978; Kornegay and Thomas, 1981; Calabotta et al., 1982; Kornegay et al., 1981a,b, 1983; Breman and Aherne, 1984; Lepine et al., 1985; Eeckhout et al., 1995), nor has it been shown to be necessary for good health or longevity (Arthur et al., 1983a,b; Kornegay et al., 1984).
The dietary calcium and phosphorus requirements, expressed as a percentage of the diet, may be slightly higher for gilts than for barrows (Thomas and Kornegay, 1981; Calabotta et al., 1982). Feeding of dietary levels of calcium and phosphorus sufficient to maximize bone mineralization in gilts during early growth and development was shown to improve reproductive longevity in one study (Nimmo et al., 1981a,b) but not in other studies (Arthur et al., 1983a,b; Kornegay et al., 1984). The calcium and phosphorus requirements of the developing boar are greater than those of the barrow and gilt (Cromwell et al., 1979; Hickman et al., 1983; Kesel et al., 1983; Hansen et al., 1987).
Pigs possessing a high lean growth rate do not seem to have a higher dietary requirement for calcium and phosphorus as compared with pigs having a moderate lean growth rate, according to a study by Bertram et al. (1994). However, when the lean growth rate is increased by treating pigs with porcine somatotropin, the dietary requirement, expressed as percentage of the diet, increases due to the reduced daily feed intake resulting from porcine somatotropin treatment (Weeden et al., 1993a,b; Carter and Cromwell, 1998a,b). There is also strong evidence that porcine somatotropin–treated pigs require greater daily amounts of calcium and phosphorus to maximize growth performance, bone mineralization, and carcass leanness than untreated pigs (Carter and Cromwell, 1998a,b).
Kornegay et al. (1973), Harmon et al. (1974b, 1975), Monegue et al. (1980), Nimmo et al. (1981a,b), Mahan and Fetter (1982), Arthur et al. (1983a,b), Grandhi and Strain (1983), Kornegay and Kite (1983), and Maxson and Mahan (1986) investigated the calcium and phosphorus requirements of breeding swine. During pregnancy, the physiological requirements for calcium and phosphorus increase in proportion to the need for fetal growth and reach a maximum in late gestation. During lactation, the requirements are affected by the level of milk production by the sow. Generally, the requirements for calcium and phosphorus are based on a feeding level of 1.8 to 2.0 kg of feed/day during gestation and 5 to 6 kg of feed/day during lactation. If sows are fed less than 1.8 kg of feed during gestation, the diet should be formulated to contain sufficient concentrations of calcium and phosphorus to meet the daily requirements. The voluntary feed intake of lactating sows may be reduced by high environmental temperatures. In this circumstance, assuming that milk production is not decreased, the lactation diet should be formulated to meet the daily needs of calcium and phosphorus. Adequate calcium and phosphorus intakes are more critical in first parity sows than in mature sows (Giesemann et al., 1992a,b).
The form in which phosphorus exists in natural feedstuffs influences the efficiency of its utilization. In cereal grains, grain by-products, and oilseed meals, about 60 to 75 percent of the phosphorus is organically bound in the form of phytate (Nelson et al., 1968; Lolas et al., 1976), which is poorly available to the pig (Taylor, 1965; Peeler, 1972; Cromwell, 1979). The biological availability of phosphorus in cereal grains is variable (Cromwell et al., 1972a, 1974), ranging from less than 15 percent in corn (Bayley and Thomson, 1969; Miracle et al., 1977; Calvert et al., 1978; Hayes et al., 1979; Stober et al., 1979; Trotter and Allee, 1979a,b; Huang and Allee, 1981; Ross et al., 1983) to approximately 50 percent in wheat (Miracle et al., 1977; Hayes et al., 1979; Trotter and Allee, 1979a; Cromwell et al., 1985; Cromwell, 1992). The greater availability of phosphorus in wheat and wheat by-products (Stober et al., 1980b; Hew et al., 1982) is attributed to the presence of a naturally occurring phytase enzyme in wheat (McCance and Widdowson, 1944; Mollgaard, 1946; Pointillart et al., 1984). The phosphorus in high-moisture corn or grain sorghum is considerably more available than that in dry grain (Trotter and Allee, 1979b; Boyd et al., 1983; Ross et al., 1983). The phosphorus in low-phytic acid corn (modified by the mutant lpa1 gene) is relatively high (77 percent) in its bioavailability (Cromwell et al., 1998).
The phosphorus in oilseed meals also has a low bioavailability (Tonroy et al., 1973; Miracle et al., 1977; Trotter and Allee, 1979a; Stober et al., 1980a; Harrold, 1981; Ross
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et al., 1982; Cromwell, 1992). In contrast, the phosphorus in protein sources of animal origin is largely inorganic, and most animal protein sources (including milk and blood byproducts) have a high phosphorus bioavailability (Cromwell et al., 1976; Hew et al., 1982; Coffey and Cromwell, 1993). The bioavailability of phosphorus in meat and bone meal is variable. Earlier studies indicated that the bioavailability of phosphorus in meat and bone meal was somewhat lower (67%) than in most other animal sources (Cromwell, 1992), but more recent studies showed a relatively high bioavailability (90%) (Traylor and Cromwell, 1998). The phosphorus in dehydrated alfalfa meal is highly available (Cromwell et al., 1983). Steam pelleting has been shown to improve the bioavailability of phytate phosphorus in some studies (Bayley and Thompson, 1969; Bayley et al., 1975) but not in others (Trotter and Allee, 1979c; Corley et al., 1980; Ross et al., 1983). Estimates of relative phosphorus bioavailability in common feed ingredients for pigs are given in Chapter 11, Table 11-1.
Microbial phytase supplementation of high-phytate, cereal grain—oilseed meal diets can result in major improvements in bioavailability of phytate phosphorus (Nasi, 1990; Simmons et al., 1990; Jongbloed et al., 1992; Pallauf et al., 1992a,b; Cromwell et al., 1993b, 1995; Lei et al., 1993a). As a result, the dietary level of phosphorus can be reduced, thereby lowering phosphorus excretion by 30 to 60 percent (see Chapter 8). The magnitude of the response to microbial phytase is influenced by the dietary level of available and total phosphorus (including phytate phosphorus), the amount of supplemental phytase, the calcium-to-phosphorus ratio (or level of calcium), and the level of vitamin D (Jongbloed et al., 1993; Lei et al., 1994; Kornegay, 1996). Microbial phytase also improves the bioavailability of calcium (Lei et al., 1993a; Mroz et al., 1994; Pallauf et al., 1992b; Young et al., 1993; Radcliffe et al., 1995) and zinc (Pallauf et al., 1992a, 1994a,b; Lei et al., 1993b) and has been reported to improve the digestibility of dietary protein (Ketaren et al., 1993; Kornegay and Qian, 1994; Mroz et al., 1994; Kemme et al., 1995; Biehl and Baker, 1996). Pelleting of diets can reduce or destroy phytase activity because of the temperature increases that occur during the pelleting process. Loss of phytase activity has been reported when temperatures exceed 60°C (Jongbloed and Kemme, 1990; Nunes, 1993); such a loss can result in reduced digestibility of phosphorus and calcium (Jongbloed and Kemme, 1990).
The phosphorus in inorganic phosphorus supplements also varies in bioavailability. The phosphorus in ammonium, calcium, and sodium phosphates is highly available (Kornegay, 1972b; Hays, 1976; Clawson and Armstrong, 1981; Partridge, 1981; Tunmire et al., 1983; Cromwell et al., 1987; Cromwell, 1992). The phosphorus in steamed bone meal is less available than that in mono-dicalcium phosphate (Cromwell, 1992). The phosphorus in defluorinated rock phosphate is generally less available than in monocalcium phosphate or monosodium phosphate (Cromwell, 1992; Coffey et al., 1994) but can vary depending on source and processing (Kornegay and Radcliffe, 1997). The phosphorus in high-fluorine rock phosphates, soft phosphate, colloidal clay, and Curaçao phosphate is poorly available (Chapman et al., 1955; Plumlee et al., 1958; Harmon et al., 1974b; Hays, 1976; Peo et al., 1982a,b). Estimates of the bioavailability of phosphorus in phosphorus supplements are given in Chapter 11 (Table 11-8).
Little is known about the availability of calcium in natural feedstuffs. Because of the phytic acid content, the bioavailability of calcium in cereal grain—based diets, alfalfa, and various grasses and hays is relatively low (Soares, 1995). However, most feedstuffs contribute so little calcium to the diet that bioavailability of the calcium is of little consequence. The calcium in calcitic limestone, gypsum, oystershell flour, aragonite, and marble dust is highly available (Pond et al., 1981; Ross et al., 1984), but the calcium in dolomitic limestone is only 50 to 75 percent available (Ross et al., 1984). Particle size (up to 0.5 mm in diameter) appears to have little effect on calcium availability (Ross et al., 1984). Pig data are not available, but on the basis of poultry data, the calcium in dicalcium phosphate, tricalcium phosphate, defluorinated phosphate, calcium gluconate, calcium sulfate, and bone meal is highly available, generally 90 to 100 percent, when compared with the calcium in calcium carbonate (Baker, 1991; Soares, 1995).
Signs of calcium or phosphorus deficiency are similar to those of vitamin D deficiency. They include depressed growth and poor bone mineralization, resulting in rickets in young pigs and osteomalacia in older swine. A common problem of calcium- or phosphorus-deficient sows is a paralysis of the hind legs, called posterior paralysis. The problem occurs most frequently in sows producing high levels of milk toward the end or just after the termination of lactation.
Excess levels of calcium and phosphorus may reduce performance of pigs (Hall et al., 1991; Reinhart and Mahan, 1986), and the effect is greater when the calcium:phosphorus ratio is increased. Excess calcium not only decreases the utilization of phosphorus but also increases the pig's requirement for zinc in the presence of phytate (Luecke et al., 1956; Whiting and Bezeau, 1958; Morgan et al., 1969; Oberleas, 1983). When the molar ratio of cations (zinc and calcium) was 2:1 or 3:1 with phytate, the formation of an insoluble complex was much greater (Oberleas and Harland, 1996). Excess calcium also increases the requirement for vitamin K (Hall et al., 1991).
Sodium and Chlorine
Sodium and chlorine (chloride) are the principal extracellular cation and anion, respectively, in the body. Chloride is the chief anion in gastric juice.
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The dietary sodium requirement of growing-finishing pigs is no greater than 0.08 to 0.10 percent of the diet (Meyer et al., 1950; Alcantara et al., 1980; Cromwell et al., 1981a; Froseth et al., 1982a; Honeyfield and Froseth, 1985; Honeyfield et al., 1985; Kornegay et al., 1991). The dietary chlorine (chloride) requirement is less well defined but is probably no higher than 0.08 percent for the growing pig (Froseth et al., 1982a; Honeyfield and Froseth, 1985; Honeyfield et al., 1985). A level of 0.20 to 0.25 percent added sodium chloride will meet the dietary sodium and chlorine requirements of growing-finishing pigs fed a corn—soybean meal diet (Hagsten and Perry, 1976a,b; Hagsten et al., 1976). Mahan et al. (1996a,b) recently reported that weanling pigs fed diets containing dried whey or dried plasma (both are relatively high in sodium) responded to added sodium as sodium chloride or sodium phosphate and to added chloride as hydrochloric acid. Their results indicate that early-weaned pigs require more sodium and chlorine than previously thought. Thus, the estimated dietary sodium and chloride requirements have been increased to 0.25 percent of each from 3 to 5 kg, to 0.20 percent of each from 5 to 10 kg, and to 0.15 percent of each from 10 to 20 kg body weight.
The sodium and chlorine requirements of breeding animals are not well established. The results of one study suggested that 0.3 percent dietary sodium chloride (0.12 percent sodium) was not sufficient for pregnant sows (Friend and Wolynetz, 1981). In a regional study, pig birth weights and weaning weights were reduced when sodium chloride was reduced from 0.50 to 0.25 percent during gestation and lactation for two or more parities (Cromwell et al., 1989a). Based upon the sodium content of sow's milk, which is 0.03 to 0.04 percent (Agricultural Research Council, 1981), the dietary sodium requirement should be about 0.05 percentage unit greater during lactation than during gestation. Until more definitive information is available, sodium chloride additions of 0.4 percent to gestation diets and 0.5 percent to lactation diets are suggested.
The availability of sodium and chloride in most feed ingredients is believed to be 90 to 100 percent (Miller, 1980). The sodium in water, which in coastal regions can be as high as 184 mg/L, and in defluorinated phosphate is highly available for pigs (Kornegay et al., 1991).
A deficiency of sodium or chloride reduces the rate and efficiency of growth in pigs. In contrast, swine can tolerate high dietary levels of sodium chloride (National Research Council, 1980), provided they have access to ample non-saline drinking water. If non-saline water is limited or if the level of sodium chloride in water is high, toxicity can result. The high sodium ion concentration is responsible for adverse physiological reactions, apparently because of a disturbance in water balance. The signs of sodium toxicity include nervousness, weakness, staggering, epileptic seizures, paralysis, and death (Bohstedt and Grummer, 1954; Carson, 1986).
Sodium, potassium, and chloride are the primary dietary ions that influence the electrolyte balance and acid-base status of animals. Under most circumstances, dietary mineral balance is expressed as milliequivalents (mEq) of sodium plus potassium minus chloride ions (Na + K—Cl) (Mongin, 1981) and is often referred to as electrolyte balance. Patience and Wolynetz (1990) suggested that calcium, magnesium, sulfur, and phosphorus ions should also be included in the calculation of electrolyte balance. The optimal electrolyte balance in the diet for pigs is 250 mEq of excess cations (Na + K—Cl)/kg of diet according to Austic and Calvert (1981), Golz and Crenshaw (1990), and Haydon et al. (1993); however, optimal growth has been found to occur over the range of 0 to 600 mEq/kg of diet (Patience et al., 1987; Kornegay et al., 1994). If a deficiency of sodium, potassium, or chloride occurs in the diet, then the relationship, Na + K—Cl, does not accurately predict dietary levels for optimum growth (Mongin, 1981).
Magnesium
Magnesium is a cofactor in many enzyme systems and is a constituent of bone. The magnesium requirement of artificially reared pigs fed milk-based semipurified diets is between 300 and 500 mg/kg of diet (Mayo et al., 1959; Bartley et al., 1961; Miller et al., 1965a,c,d). Milk contains adequate magnesium to meet the requirement of suckling pigs (Miller et al., 1965c,d). The magnesium requirement of weanling-growing-finishing swine is probably not higher than that of the young pig. The magnesium in a corn—soybean meal diet (0.14 to 0.18 percent) is apparently adequate (Svajgr et al., 1969; Krider et al., 1975), although some research suggests that the magnesium in natural ingredients is only 50 to 60 percent available to the pig (Miller, 1980; Nuoranne et al., 1980).
The magnesium requirement of breeding animals is not well established. Harmon et al. (1976) fed semipurified diets containing 0.04 and 0.09 percent magnesium to sows during gestation, followed by 0.015 and 0.065 percent magnesium during lactation. They observed no difference in reproductive or lactational performance. However, in a balance study, sows fed the low level of magnesium during lactation were in negative magnesium balance.
In order of appearance, signs of magnesium deficiency include hyperirritability, muscular twitching, reluctance to stand, weak pasterns, loss of equilibrium, and tetany followed by death (Mayo et al., 1959; Miller et al., 1965c). The toxic level of magnesium is not known. The maximum tolerable level for swine is approximately 0.3 percent (National Research Council, 1980).
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Potassium
Potassium is the third most abundant mineral in the body of the pig, surpassed only by calcium and phosphorus (Manners and McCrea, 1964), and is the most abundant mineral in muscle tissue (Stant et al., 1969). Potassium is involved in electrolyte balance and neuromuscular function. It also serves as the monovalent cation to balance anions intracellularly, as part of the sodium potassium pump physiological mechanism.
The dietary potassium requirement of pigs from 1 to 4 kg body weight is estimated to be between 0.27 and 0.39 percent (Manners and McCrea, 1964); from 5 to 10 kg, 0.26 to 0.33 percent (Jensen et al., 1961; Combs et al., 1985); at 16 kg, 0.23 to 0.28 percent (Meyer et al., 1950); and from 20 to 35 kg, less than 0.15 percent (Hughes and Ittner, 1942; Mraz et al., 1958). No estimates are available for finishing or breeding pigs. The content of potassium in most practical diets is normally adequate to meet these requirements for all classes of swine. The potassium in corn and soybean meal is 90 to 97 percent available (Combs and Miller, 1985).
Dietary potassium is interrelated with dietary sodium and chloride. Increasing dietary chloride from 0.03 to 0.60 percent in purified diets depressed growth rate of young pigs when the diet contained 0.1 percent potassium, but it increased growth rate when the diet contained 1.1 percent potassium (Golz and Crenshaw, 1990). The interactive effect of dietary potassium and chloride seems to be an indirect effect on the excretion and retention of additional cations and anions, particularly ammonium and phosphate. The effects on growth are mediated via mechanisms involving renal ammonium ion metabolism (Golz and Crenshaw, 1991).
Potassium has been shown to spare lysine in the chick, but a similar response has not been demonstrated consistently in the pig (Leibholz et al., 1966; Madubuike et al., 1980; Austic and Calvert, 1981; Miller et al., 1981b; Wahlstrom and Libal, 1981; Froseth et al., 1982b,c; Miller and Froseth, 1982; Zimmerman, 1982; Mijada and Cline, 1983). Madubuike and Austic (1989) suggested that this inconsistency may be related to the lysine adequacy of the pig diet.
Signs of potassium deficiency include anorexia, rough hair coat, emaciation, inactivity, and ataxia (Jensen et al., 1961). Electrocardiograms of potassium-deficient pigs showed reduced heart rate and increased electrocardial intervals (Cox et al., 1966). Necropsy of affected pigs revealed no unique gross pathology.
The toxic level of potassium is not well established. Pigs can tolerate up to 10 times the potassium requirement if plenty of drinking water is provided (Farries, 1958). Intravenous infusion of potassium chloride in pigs resulted in abnormal electrocardiograms (Coulter and Swenson, 1970).
Sulfur
Sulfur is an essential element. The sulfur provided by the sulfur-containing amino acids seems adequate to meet the pig's needs for synthesis of sulfur-containing compounds, such as taurine, glutathione, lapoic acid, and chondroitin sulfate. Additions of inorganic sulfate to low-protein diets have not been beneficial (Miller, 1975; Baker, 1977).
Micro/Trace Minerals
Chromium
Chromium is involved in carbohydrate, lipid, protein, and nucleic acid metabolism (Nielsen, 1994). Although the specific function of chromium is unknown, it is believed to work as a cofactor with insulin (White et al., 1993). A glucose tolerance factor containing chromium potentiated insulin activity and was biologically active (Steele et al., 1977). Increased insulin sensitivity was reported for pigs fed chromium picolinate (Amoikon et al., 1995). Chromium added as chromium picolinate was reported by Evock-Clover et al. (1993) to lower serum insulin and glucose concentrations in growing pigs (30 to 60 kg). However, in other studies serum glucose concentrations were not influenced by feeding chromium (Page et al., 1993; Amoikon et al., 1995; Lindemann et al., 1995a). Lindemann et al. (1995a) reported that postfeeding serum insulin values and insulin-to-glucose ratios were lower for fasted gestating sows fed chromium picolinate than for fasted control sows. Amoikon et al. (1995) also reported that when pigs were fed chromium picolinate, the fasting plasma insulin value was reduced; the clearance of glucose after an intravenous glucose tolerance test and insulin challenge test was increased; and the half-life of glucose was decreased.
Chromium, especially inorganic forms, is poorly absorbed from the gastrointestinal tract. The amount of inorganic chromium absorbed ranges from 0.4 to 3 percent according to a review by Anderson (1987). Some organic forms are better absorbed than inorganic forms. The absorption by humans of chromium from chromium picolinate was low; 0.7 to 1.7 percent in one study (Clancy et al., 1994) and 1.5 to 5.2 percent in another (Gargas et al., 1994). Ward et al. (1995) evaluated several forms of chromium (chloride, acetate, oxalate, nicotinate, two sources of picolinate, and nicotinate-glycine-cysteine-glutamate) that were fed to supply 200 ppb chromium, but found that serum metabolites and hormone values were not affected by any of the forms of chromium. Also, chromium chloride (5 or 25 ppb chromium) or chromium picolinate (200 or 400 ppb chromium) did not affect serum metabolites in a study by Mooney and Cromwell (1997).
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Recent interest has focused on the potential use of the organic chromium complex, chromium picolinate, to increase carcass leanness. Positive responses were reported by Page et al. (1993), Lindemann et al. (1995b), Harper et al. (1995), Boleman et al. (1995), and Mooney and Cromwell (1995; 1997). However, others reported no responses in carcass leanness to supplemental chromium in this form (Ward et al., 1995; Harris et al., 1995; Mooney and Cromwell, 1996). The lack of a consistent response may be related to chromium levels of diets, form of chromium, chromium status of pig, and amino acid levels of diet (White et al., 1993; Lindemann et al., 1995b). The total chromium content of a corn—soybean diet can range from 750 to 1,500 ppb, but most of this is probably unavailable.
Larger litters at birth have been reported for sows fed supplemental chromium picolinate (Lindemann et al., 1995a,b). In one large trial, farrowing rate was increased when first and second parity sows were fed 200 ppb chromium as chromium picolinate beginning on the day after breeding through farrowing, but total and live pigs born were not affected by treatments (Campbell, 1996). In a second trial, multiparous sows were fed 200 ppb chromium as chromium picolinate for the first 35 days after breeding; in a third trial they were fed the same amount for 28 days prior to farrowing or for 28 days prior to farrowing through lactation and for 35 days after breeding. The supplementation had little effect on any measure of fertility or fecundity.
Additional research is required to elucidate the role of chromium in swine. The inconsistency of biological responses to chromium could be related to the bioavailability of the chromium found in traditional feed ingredients, the duration of feeding, and the chromium status of the pigs.
Trivalent and hexavalent are the two most common forms of chromium; both are stable. Hexavalent chromium is much more toxic than trivalent chromium, which is believed to be the essential trace mineral (Anderson, 1987; Mertz, 1993). Maximum tolerable dietary levels for domestic animals were set at 3,000 ppm chromium as the oxide and 1,000 ppm as the chloride (National Research Council, 1980). A detailed discussion of tolerance concentration for chromium in animals can be found in Mineral Tolerance of Domestic Animals (National Research Council, 1980). The results of a recent in vitro study with Chinese hamster ovary cells indicate some chromosome damage after treatment with soluble doses of 0.05, 0.10, 0.50, and 1.0 mM of chromium picolinate (Stearns et al., 1996). Chromium nicotinate, nicotinic acid, and trivalent chromium chloride hexahydrate did not produce chromosome damage at equivalent nontoxic doses. These results suggest the need for further investigations of the long-term effects of supplemental chromium. No quantitative estimate of the chromium requirement has been estimated for pigs. A recent review on chromium was published by the National Research Council (1997).
Cobalt
Cobalt is a component of vitamin B12 (Rickes et al., 1948). There is no evidence that pigs have an absolute requirement for cobalt, other than for its role in vitamin B12. Cobalt can substitute for zinc in the enzyme carboxypeptidase and for part of the zinc in the enzyme alkaline phosphatase. Hoekstra (1970) and Chung et al. (1976) have shown that supplemental cobalt prevents lesions associated with a zinc deficiency.
Dietary cobalt can only be used by the intestinal microflora of the pig to synthesize some vitamin B12. Intestinal synthesis assumes greater importance if dietary vitamin B12 is limiting (Klosterman et al., 1950; Robinson, 1950; Kline et al., 1954). The use of supplemental vitamin B12 in practical diets is a routine practice.
A level of 400 ppm cobalt was toxic to the young pig and may cause anorexia, stiff-leggedness, humped back, incoordination, muscle tremors, and anemia (Huck and Clawson, 1976). Cobalt concentration in the kidney and liver increased linearly and growth decreased linearly over a 4- to 5-week period as 0, 150, and 300 ppm cobalt were added to a basal diet containing <2 ppm cobalt (Kornegay et al., 1995). The maximum tolerance level for weanling pigs is <150 ppm of diet. Selenium, vitamin E, and cysteine provide some protection against toxicity from excessive levels of dietary cobalt (Van Vleet et al., 1977; Southern and Baker, 1981), but growth-stimulating levels of copper may aggravate the growth depression caused by cobalt (Kornegay et al., 1995).
Copper
The pig requires copper for the synthesis of hemoglobin and for the synthesis and activation of several oxidative enzymes necessary for normal metabolism (Miller et al., 1979). A level of 5 to 6 ppm in the diet is adequate for the neonatal pig (Okonkwo et al., 1979; Hill et al., 1983a). The requirement for later stages of growth is probably no greater than 5 to 6 ppm. Definitive information on requirements during gestation and lactation are scarce. Lillie and Frobish (1978) suggested that 60 ppm of copper fed to sows improved pig weights at birth and at weaning, but this response may have resulted from the pharmacological effect of high dietary copper. Kirchgessner et al. (1980) found that pregnant sows fed 2 ppm of copper had reduced ceruloplasmin and farrowed more stillborn pigs than sows fed 9.5 ppm of copper. In a balance study, Kirchgessner et al. (1981) estimated the copper requirement of pregnant sows at 6 ppm.
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Copper salts with high biological availabilities include the sulfate, carbonate, and chloride salts (Miller, 1980; Cromwell et al., 1998). The copper in cupric sulfide and cupric oxide is poorly available to the pig (Cromwell et al., 1978, 1989b; Sazzad et al., 1993). Organic complexes of copper appear to have equal bioavailability to copper sulfate in several trials (Bunch et al., 1965; Zoubek et al., 1975; Stansbury et al., 1990; Coffey et al., 1994; Apgar et al., 1995). However, in two trials reported by Coffey et al. (1994) and Zhou et al. (1994a), growth performance was greater in pigs fed growth promotion levels of copper from a copper lysine complex than those fed copper sulfate.
A deficiency of copper leads to poor iron mobilization; abnormal hemopoiesis; and poor keratinization and synthesis of collagen, elastin, and myelin. Copper deficiency signs include a microcytic, hypochromic anemia; bowing of the legs; spontaneous fractures; cardiac and vascular disorders; and depigmentation (Hart et al., 1929; Elvehjem and Hart, 1932; Teague and Carpenter, 1951; Follis et al., 1955; Carter et al., 1959; Carnes et al., 1961; Hill et al., 1983a).
Copper may be toxic when dietary levels in excess of 250 ppm are fed for extended periods of time (National Research Council, 1980). Toxicity signs include depressed hemoglobin levels and jaundice, which are the results of excessive copper accumulation in the liver and other vital organs. Reduced dietary levels of zinc and iron or high levels of dietary calcium accentuate copper toxicity (Suttle and Mills, 1966a,b; Hedges and Kornegay, 1973; Prince et al., 1984).
When fed at 100 to 250 ppm, copper (as copper sulfate) stimulates growth in pigs (Barber et al., 1955b; Braude, 1967, 1975; Wallace, 1967; Cromwell et al., 1981b; Kornegay et al., 1989; Cromwell, 1997). The growth response to copper in young pigs is independent of, and in addition to, the growth response to other antibacterial agents (Stahly et al., 1980; Roof and Mahan, 1982; Edmonds et al., 1985; Cromwell 1997). The response to high levels of copper may be enhanced by added fat (Dove and Haydon, 1992; Dove, 1993a, 1995). The continuous feeding of high copper levels to sows for up to six consecutive gestation—lactation cycles did not have any apparent negative effects on reproductive performance, in spite of rather large increases in liver and kidney copper concentrations (Cromwell et al., 1993a). In fact, birth and weaning weights were greater in pigs from sows fed high copper. Improved weight gain of suckling pigs was also observed by Lillie and Frobish (1978), but other studies in which copper was fed during late gestation and lactation (Thacker, 1991) or during lactation (Roos and Easter, 1986; Dove, 1993b) showed no response to added copper in weight gain of suckling pigs.
The mechanisms through which beneficial effects from copper are observed are unknown. The growth-stimulating action of dietary copper has been attributed to its antimicrobial actions (Fuller et al., 1960); however, evidence supporting this hypothesis is lacking. A correlation between the availability of copper and the growth-promoting action of copper has been observed (Bowland et al., 1961; Cromwell et al., 1989b). Zhou et al. (1994b) reported that both body weight gain and serum mitogenic activity were stimulated in young pigs given intravenous injections of copper histidinate every other day for 18 days. Because the gastrointestinal tract was bypassed in this study, these results suggest that copper can act systemically to promote growth. Feeding 250 ppm copper stimulated lipase and phospholipase A activities and led to an improvement of dietary fat digestibility in weaning pigs (Luo and Dove, 1996). High dietary levels of copper increase fecal copper excretion, but the form of copper in pig feces is poorly bioavailable to chickens and sheep (Prince et al., 1975; Izquierdo and Baker, 1986).
Iodine
The majority of the iodine in swine is present in the thyroid gland, where it exists as a component of mono-, di-, tri-, and tetraiodothyronine (thyroxine). These hormones are important in the regulation of metabolic rate. Hart and Steenbock (1918), Kalkus (1920), and Welch (1928) demonstrated that hypothyroidism existed in swine raised in the northwestern United States and the Great Lakes region because of iodine-deficient feedstuffs produced on low-iodine soil.
The dietary iodine requirement is not well established. The requirement is increased by goitrogens, which are present in certain feedstuffs, including rapeseed, linseed, lentils, peanuts, and soybeans (McCarrison, 1933; Underwood, 1977). A level of 0.14 ppm of iodine in a corn—soybean meal diet is adequate to prevent thyroid hypertrophy in growing pigs (Cromwell et al., 1975). A level of 0.35 ppm of added iodine prevented iodine deficiency in sows (Andrews et al., 1948).
Calcium iodate, potassium iodate, and pentacalcium orthoperiodate are nutritionally available forms of iodine and are more stable in salt mixtures than are sodium iodide or potassium iodide (Kuhajek and Andelfinger, 1970). The incorporation of iodized salt (0.007 percent iodine), at a level of 0.2 percent of the diet, provides sufficient iodine (0.14 ppm) to meet the needs of growing pigs fed grain—soybean meal diets.
A severe iodine deficiency causes pigs to be stunted and lethargic and to have an enlarged thyroid (Beeson et al., 1947; Braude and Cotchin, 1949; Sihombing et al., 1974). Sows fed iodine-deficient, goitrogenic diets farrow weak or dead pigs that are hairless, show symptoms of myxedema, and have an enlarged, hemorrhagic thyroid (Hart and Steenbock, 1918; Slatter, 1955; Devilat and Skoknic, 1971).
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A dietary iodine level of 800 ppm depressed growth, hemoglobin level, and liver iron concentration in growing pigs (Newton and Clawson, 1974). During lactation and the last 30 days of gestation, as much as 1,500 to 2,500 ppm of iodine was not harmful to sows (Arrington et al., 1965).
Iron
Iron is required as a component of hemoglobin in red blood cells. Iron also is found in muscle as myoglobin, in serum as transferrin, in the placenta as uteroferrin, in milk as lactoferrin, and in the liver as ferritin and hemosiderin (Zimmerman, 1980; Ducsay et al., 1984). It also plays an important role in the body as a constituent of several metabolic enzymes. Pigs are born with about 50 mg of iron, most of which is present as hemoglobin (Venn et al., 1947). A high level of iron fed to sows during late gestation (Brady et al., 1978) or parenteral administration of iron dextran to sows in gestation (Rydberg et al., 1959; Pond et al., 1961; Ducsay et al., 1984) does not substantially increase placental transfer of iron to fetuses.
The suckling pig must retain 7 to 16 mg of iron daily, or 21 mg of iron/kg of body weight gain to maintain adequate levels of hemoglobin and storage iron (Venn et al., 1947; Braude et al., 1962). Sow's milk contains an average of only 1 mg of iron per liter (Brady et al., 1978). Thus, pigs receiving only milk rapidly develop anemia (Hart et al., 1929; Venn et al., 1947). Feeding of high levels of various iron compounds, including iron sulfate and iron chelates, to gestating and lactating sows does not increase the iron content of milk to an extent that iron deficiency can be prevented. These levels can, however, prevent iron deficiency in suckling pigs that have access to the sow's feces (Chaney and Barnhart, 1963; Veum et al., 1965; Spruill et al., 1971; Brady et al., 1978; Sansom and Gleed, 1981; Gleed and Sansom, 1982).
The iron requirement of young pigs fed milk or purified liquid diets is 50 to 150 mg/kg of milk solids (Matrone et al., 1960; Ullrey et al., 1960; Manners and McCrea, 1964; Harmon et al., 1967; Hitchcock et al., 1974). Miller et al. (1982) suggested a requirement of 100 mg of iron/kg of milk solids for pigs raised in a conventional or germ-free environment. The iron requirement of pigs fed a dry, casein-based diet is about 50 percent higher per unit of dry matter than for those fed a similar diet in liquid form (Hitchcock et al., 1974).
Numerous studies have shown the effectiveness of a single intramuscular injection of 100 to 200 mg of iron, in the form of iron dextran, iron dextrin, or gleptoferron given in the first 3 days of life (Barber et al., 1955a; McDonald et al., 1955; Maner et al., 1959; Rydberg et al., 1959; Ullrey et al., 1959; Zimmerman et al., 1959; Linkenheimer et al., 1960; Wahlstrom and Juhl, 1960; Kernkamp et al., 1962; Parsons, 1979; Pollmann et al., 1983). The intestinal mucosa of the newborn pig actively absorbs iron (Furugouri and Kawabata, 1975, 1976, 1979). Oral administration of iron from bioavailable inorganic or organic sources within the first few hours of life also will meet the iron needs of the suckling pig. However, early administration, before gut closure to large molecules, is crucial (Harmon et al., 1974a; Thoren-Tolling, 1975). An excessive level (more than 200 mg) of injectable or oral iron should be avoided because unbound serum iron encourages bacterial growth and results in increased susceptibility to infection and diarrhea (Weinberg, 1978; Klasing et al., 1980; Knight et al., 1983; Kadis et al., 1984).
The postweaning dietary iron requirement is about 80 ppm (Pickett et al., 1960). In later growth and maturity, this requirement diminishes as the rate of increase in blood volume slows. Natural feed ingredients usually supply enough iron to meet postweaning requirements. Feed-grade defluorinated phosphate and dicalcium phosphate, which contain from 0.6 to 1.0 percent iron, also supply substantial amounts of iron. The iron in defluorinated phosphate is about 65 percent as available to the pig as the iron in ferrous sulfate (Kornegay, 1972a).
Availability of iron from different sources varies greatly (Zimmerman, 1980). Ferrous sulfate, ferric chloride, ferric citrate, ferric choline citrate, and ferric ammonium citrate are effective in preventing iron deficiency anemia (Harmon et al., 1967; Ammerman and Miller, 1972; Ullrey et al., 1973; Miller et al., 1981a). Iron compounds with low solubility, such as ferric oxide, are ineffective (Ammerman and Miller, 1972). The biovailability of iron in ferrous carbonate is lower and more variable than that of iron in ferrous sulfate (Harmon et al., 1969; Ammerman et al., 1974). Iron from iron methionine was 68 to 81 percent as bioavailable as that in iron sulfate (Lewis et al., 1995). Soybean meal contains 175 to 200 ppm of iron, and the bioavailability of iron in soybean meal has been estimated to be 38 percent, based on hemoglobin depletion—repletion assays in chicks (Biehl et al., 1997).
The hemoglobin concentration of blood is a reliable indicator of the pig's iron status, and it is easy to determine. Hemoglobin levels of 10 g/dL of whole blood are considered adequate. A hemoglobin level of 8 g/dL suggests borderline anemia, and a level of 7 g/dL or less represents anemia (Zimmerman, 1980). The type of anemia resulting from iron deficiency is hypochromic-microcytic anemia. Anemic pigs show evidence of poor growth, listlessness, rough hair coats, 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; marked dilation of the heart; and an enlarged firm
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spleen. Anemic pigs are more susceptible to infectious diseases (Osborne and Davis, 1968).
In 3- to 10-day-old pigs, the toxic oral dose of iron from ferrous sulfate is approximately 600 mg/kg of body weight (Campbell, 1961). Clinical signs of toxicity are observed within 1 to 3 hours after iron is fed (Nilsson, 1960; Arpi and Tollerz, 1965). Lannek et al. (1962) and Patterson et al. (1967, 1969) have found that injectable iron (100 mg as iron dextran) is toxic to pigs from vitamin E—deficient dams. A dietary level of 5,000 ppm of iron produces rachitic lesions, which may be prevented by increasing the level of dietary phosphorus (O'Donovan et al., 1963; Furugouri, 1972).
Manganese
Manganese functions as a component of several enzymes involved in carbohydrate, lipid, and protein metabolism. Manganese is essential for the synthesis of chondroitin sulfate, a component of mucopolysaccharides in the organic matrix of bone (Leach and Muenster, 1962).
The dietary requirements for manganese are quite low (Johnson, 1944) and not well established. The manganese status of the sow affects the manganese status of the neonates, because manganese readily crosses the placenta (Newland and Davis, 1961; Gamble et al., 1971). Leibholz et al. (1962) reported that as little as 0.4 ppm of manganese is sufficient for young pigs. With manganese-depleted dams, however, the requirement for the neonates is 3 to 6 ppm (Kayongo-Male et al., 1975). A corn—soybean meal diet should contain ample manganese for normal growth and bone formation in growing-finishing pigs (Svajgr et al., 1969).
Long-term feeding of a diet containing only 0.5 ppm of manganese results in abnormal skeletal growth, increased fat deposition, irregular or absent estrous cycles, resorbed fetuses, small, weak pigs at birth, and reduced milk production (Plumlee et al., 1956).
On the basis of manganese retention, Kirchgessner et al. (1981) estimated the manganese requirement of pregnant sows at 25 ppm. Total litter weight at birth was less for sows fed a low-manganese, basal corn—soybean meal diet (10 ppm manganese) than for sows fed the basal diet plus 84 ppm manganese (Rheaume and Chavaz, 1989). Colostrum and milk from sows fed supplemental manganese contained a higher concentration of manganese, but retention of manganese was only numerically higher. Christianson et al. (1989, 1990) reported that birth weight of pigs was greater when sows were fed 10 or 20 ppm manganese than when they were fed 5 ppm. Also, return to estrus was improved by feeding 20 ppm manganese. On the basis of these recent studies, the manganese requirements for gestation and lactation have been increased to 20 ppm of the diet.
Although the toxic level of manganese is not well defined, depressed feed intake and reduced growth rates have been observed when pigs were fed 4,000 ppm of manganese (Leibholz et al., 1962). A dietary level of 2,000 ppm of manganese resulted in reduced hemoglobin levels (Matrone et al., 1959), and 500 ppm of manganese reduced growth rate and resulted in limb stiffness in growing pigs (Grummer et al., 1950).
Selenium
Selenium is a component of the enzyme glutathione peroxidase (Rotruck et al., 1973), which detoxifies lipid peroxides and provides protection of cellular and subcellular membranes against peroxide damage. Thus, the mutual sparing effect of selenium and vitamin E stems from their shared antiperoxidant roles. High levels of vitamin E, however, do not completely eliminate the need for selenium (Ewan et al., 1969; Bengtsson et al., 1978a,b; Hakkarainen et al., 1978). Selenium has been shown to have a function in thyroid metabolism, because iodothyronine 5'-deiodinase has been identified as a selenoprotein (Arthur, 1994).
The dietary requirement for selenium ranges from 0.3 ppm for weanling pigs to 0.15 ppm for finishing pigs and sows (Groce et al., 1971, 1973a,b; Ku et al., 1973; Mahan et al., 1973; Ullrey, 1974; Young et al., 1976; Glienke and Ewan, 1977; Wilkinson et al., 1977a,b; Mahan and Moxon, 1978a,b, 1984; Piatkowski et al., 1979; Meyer et al., 1981). The requirement for selenium is influenced by dietary phosphorus level (Lowry et al., 1985b) but not dietary calcium level (Lowry et al., 1985a). Several forms of selenium, including selenium-enriched yeast, sodium selenite, and sodium selenate, are effective in meeting the dietary requirement (Mahan and Magee, 1991; Suomi and Alaviuhkola, 1992; Mahan and Parrett, 1996; Mahan and Kim, 1996). The selenium status of the dam influences reproductive performance and the selenium requirement of suckling and weanling pigs (Van Vleet et al., 1973; Mahan et al., 1977; Piatkowski et al., 1979; Chavez, 1985; Ramisz et al., 1993). Total body retention of selenium, as well as serum and tissue levels of selenium in growing, finishing, and reproducing gilts and their suckling progeny, increased as the dietary level of selenium increased (0.1 to 0.3 or 0.5 ppm); the amount of selenium retained and stored was usually greater at the various selenium levels when the effects of a selenium-enriched yeast source were compared with those produced by sodium selenite (Mahan, 1995; Mahan and Kim, 1996; Mahan and Parrett, 1996). In reproducing gilts, serum glutathione peroxidase activity was not improved beyond 0.1 ppm selenium, and the increase in activity was similar for selenium-enriched yeast and sodium selenite (Mahan and Kim, 1996). In growing-finishing pigs, serum selenium concentration and serum glutathione peroxidase activity reached a plateau at a dietary level of 0.1
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ppm selenium for selenium-enriched yeast and sodium selenite, but the magnitude of the response was lower for the yeast than for the sodium selenite, which suggests that the selenium-enriched yeast product was less biologically available than sodium selenite (Mahan and Parrett, 1996). About 50 percent of the selenium in the selenium-enriched yeast product was suggested to be selenomethionine, with the remainder in one of several seleno-amino acids or as their analogs (Mahan, 1995).
Certain soils of the United States and Canada are low in selenium. When diets consist exclusively of ingredients grown in such regions, selenium will be deficient unless supplemental selenium is added (Grant et al., 1961; Trapp et al., 1970; Ewan, 1971; Groce et al., 1971; Sharp et al., 1972a,b; Ku et al., 1973; Mahan et al., 1973, 1974; Diehl et al., 1975; Doornenbal, 1975; Piper et al., 1975; Wilkinson et al., 1977b; Bengtsson et al., 1978b). Environmental stress may increase the incidence and degree of selenium deficiency (Michel et al., 1969; Mahan et al., 1975).
In 1974, the U.S. Food and Drug Administration (FDA) approved the addition of 0.1 ppm of selenium to all swine diets. In 1982, the FDA approved the addition of 0.3 ppm of selenium to diets for pigs up to 20 kg, because 0.1 ppm of added selenium does not always prevent deficiency signs in weanling pigs (mahan and Moxon, 1978b; Meyer et al., 1981). The current regulation allows up to 0.3 ppm of selenium in the diet for all pigs (Food and Drug Administration, 1987a,b). As reviewed by Ullrey (1992), concerns about environmental pollution by selenium have led to efforts to reduce the level to 0.1 ppm, but the level of 0.3 ppm has been maintained.
The primary biochemical change in selenium deficiency is a decline in glutathione peroxidase activity (Thompson et al., 1976; Young et al., 1976; Fontaine and Valli, 1977). Hence, the level of glutathione peroxidase in the plasma is a reliable index of the selenium status of pigs (Chavez, 1979a,b; Wegger et al., 1980; Adkins and Ewan, 1984). Sudden death is a prominent feature of the selenium deficiency syndrome (Ewan et al., 1969; Groce et al., 1971, 1973a,b). The gross necropsy lesions of selenium deficiency are identical to those of vitamin E deficiency. These include massive hepatic necrosis (hepatosis dietetica); edema of the spiral colon, lungs, subcutaneous tissues, and submucosa of the stomach; bilateral paleness and dystrophy of the skeletal muscles (white muscle disease); mottling and dystrophy of the myocardium (mulberry heart disease); impaired reproduction; reduced milk production; and impaired immune response (Eggert et al., 1957; Orstadius et al., 1959; Lindberg and Siren, 1963, 1965; Michel et al., 1969; Trapp et al., 1970; Sharp et al., 1972a,b; Ruth and Van Vleet, 1974; Ullrey, 1974; Fontaine et al., 1977a,b,c; Nielsen et al., 1979; Sheffy and Schultz, 1979; Peplowski et al., 1980; Spallholz, 1980; Larsen and Tollersrud, 1981; Simesen et al., 1982).
When fed to growing swine as sodium selenite, sodium selenate, selenomethionine, or seleniferous corn, selenium does not produce toxicity at levels of less than 5 ppm. In some cases, however, a level of 5 ppm (Mahan and Moxon, 1984) and levels from 7.5 to 10 ppm (Wahlstrom et al., 1955; Trapp et al., 1970; Herigstad et al., 1973; Goehring et al., 1984a,b) have produced toxicity. Signs of toxicity include anorexia, hair loss, fatty infiltration of the liver, degenerative changes in the liver and kidney, edema, occasional separation of hoof and skin at the coronary band (Miller, 1938; Miller and Williams, 1940; Wahlstrom et al., 1955; Orstadius, 1960; Lindberg and Lannek, 1965; Herigstad et al., 1973), and symmetrical, focal areas of vacuolation and neuronal necrosis (Stowe and Herdt, 1992). Dietary arsenicals help to alleviate selenium toxicity (Wahlstrom et al., 1955).
Zinc
Zinc is a component of many metalloenzymes, including DNA and RNA synthetases and transferases, many digestive enzymes, and is associated with the hormone, insulin. Hence, this element plays an important role in protein, carbohydrate, and lipid metabolism.
Many diet related factors influence the dietary requirement for zinc (Miller et al., 1979), including phytic acid or plant phytates (Oberleas et al., 1962; Oberleas, 1983), calcium (Tucker and Salmon, 1955; Hoekstra et al., 1956; Lewis et al., 1956, 1957a,b; Luecke et al., 1956, 1957; Stevenson and Earle, 1956; Bellis and Philp, 1957; Newland et al., 1958; Whiting and Bezeau, 1958; Berry et al., 1961; Hansard and Itoh, 1968; Morgan et al., 1969; Norrdin et al., 1973; Oberleas, 1983), copper (Hoefer et al., 1960; O'Hara et al., 1960; Ritchie et al., 1963; Kirchgessner and Grassman, 1970), cadmium (Pond et al., 1966), cobalt (Hoekstra, 1970), ethylenediamine tetraacetic acid (EDTA) (Owen et al., 1973), histidine (Dahmer et al., 1972a), and protein level and source (Smith et al., 1962; Dahmer et al., 1972b).
The zinc requirement of young pigs consuming a casein—glucose diet is low (15 ppm) because such a diet does not contain plant phytates (Smith et al., 1962; Shanklin et al., 1968). For growing pigs fed semipurified diets that contain isolated soybean protein or corn—soybean meal diets (both diets contain significant amounts of phytate) that contain the recommended level of calcium, the zinc requirement is about 50 ppm (Lewis et al., 1956, 1957a,b; Luecke et al., 1956; Stevenson and Earle, 1956; Smith et al., 1958, 1962; Miller et al., 1970). Boars have a higher zinc requirement than gilts; and gilts have a higher requirement than barrows (Liptrap et al., 1970; Miller et al., 1970). The zinc requirement is increased when excessive levels of calcium are fed (Lewis et al., 1956; Forbes, 1960; Hoefer et al., 1960; Pond and Jones, 1964; Pond et al., 1964;
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Oberleas, 1983). The zinc requirement of breeding animals is not well established, but may be higher than for growing pigs due to fetal growth, milk synthesis, tissue repair during uterine involution, and sperm production in boars. A level of 33 ppm of zinc in a corn–soybean meal diet for sows through five parities was adequate for optimal gestation performance, but not for lactation (Hedges et al., 1976). Kirchgessner et al. (1981) estimated the zinc requirement of pregnant sows at 25 ppm in a balance study. A low level of dietary zinc (13 ppm) during the last 4 weeks of pregnancy prolongs the duration of farrowing (Kalinowski and Chavez, 1984).
The classic sign of zinc deficiency in growing pigs is hyperkeratinization of the skin, a condition called parakeratosis (Kernkamp and Ferrin, 1953; Tucker and Salmon, 1955). Zinc deficiency reduces the rate and efficiency of growth and levels of serum zinc, alkaline phosphatase, and albumin (Hoekstra et al., 1956, 1967; Luecke et al., 1957; Theuer and Hoekstra, 1966; Miller et al., 1968, 1970; Prasad et al., 1969, 1971; Ku et al., 1970). Gilts fed zinc-deficient diets during gestation and lactation produce fewer and smaller pigs, which have reduced serum and tissue zinc levels (Pond and Jones, 1964; Hoekstra et al., 1967; Hill et al., 1983a,b,c). The zinc concentration in the milk from these dams is also reduced (Pond and Jones, 1964). Zinc deficiency retards testicular development of boars and thymic development of young pigs (Miller et al., 1968; Liptrap et al., 1970).
Bioavailabilities of zinc from zinc salts vary when these are included in the diet and can be influenced by the type of dietary ingredients used (Miller, 1991). The zinc in zinc sulphate, zinc carbonate, zinc chloride, and zinc metal dust is highly available (100 percent). Bioavailability estimates are expressed as a percentage of a recognized standard and do not refer to percentage absorbed or retained. Absorbed and retained zinc as a percentage of intake is usually much less than 50 percent of the intake. Zinc is less available from zinc oxide (50 to 80 percent) and is poorly available from zinc sulfide (Miller, 1991). Zinc from organic complexes appears to have approximately equal bioavailability to the zinc in zinc sulfate (Hill et al., 1986; Swinkels et al., 1996; Hahn and Baker, 1993; Wedekind et al., 1994; Cheng and Kornegay, 1995; Cheng et al., 1995; Schell and Kornegay, 1996). Zinc from grains and plant protein has low availability (Miller, 1991), but the availability is enhanced by microbial phytase addition to the diet (Kornegay, 1996).
Zinc toxicity in growing pigs fed a corn–soybean meal diet supplemented with 2,000 to 4,000 ppm zinc from zinc carbonate was manifested by depression, arthritis, hemorrhage in axillary spaces, gastritis, and death. However, a dietary zinc level of 1,000 ppm was not toxic (Brink et al., 1959). Growing pigs fed 2,000 to 4,000 ppm of zinc from zinc oxide did not show symptoms of zinc toxicity (Cox and Hale, 1962; Hsu et al., 1975; Hill et al., 1983b). However, pigs became lame and unthrifty within 2 months when they were fed a diet containing 1,000 ppm of zinc from zinc lactate (Grimmett et al., 1937). In contrast, pigs fed a diet containing 1,000 ppm of zinc from zinc sulphate for 7 months showed no signs of zinc toxicity (Kulwich et al., 1953). High dietary calcium reduces the severity of zinc toxicity (Hsu et al., 1975). A 5,000-ppm dietary level of zinc as zinc oxide through two parities reduced litter size and pig weight at weaning and caused osteochondrosis in sows (Hill and Miller, 1983; Hill et al., 1983a). Pigs from sows fed high levels of dietary zinc have reduced tissue levels of copper and rapidly develop anemia when fed a low-copper diet (Hill et al., 1983b,c). The toxicity of zinc depends upon the zinc source, dietary level, the duration of feeding, and the levels of other minerals in the diet.
A report that reduced postweaning scouring and increased weight gain resulted when the starting diet was supplemented with 3,000 ppm of zinc from zinc oxide for 14 days (Poulsen, 1989) stimulated a great deal of interest in the pharmacological use of zinc. Several recent studies have confirmed this finding and have shown improved weight gain even in the absence of scouring (Kavanagh, 1992; Hahn and Baker, 1993; Carlson et al., 1995; LeMieux et al., 1995; McCully et al., 1995; Smith et al., 1995a,b; Hill et al., 1996). Levels of zinc varied from 2,000 to 6,000 ppm and were fed for up to 5 weeks in some studies. A recent study (Ward et al., 1996) compared zinc oxide and zinc methionine; they reported that supplementing starter diets with 250 ppm zinc from zinc methionine gave equal improvements in performance to 2,000 ppm zinc from zinc oxide. Some studies, however, have failed to observe beneficial effects of pharmacological levels of zinc (Fryer et al., 1992; Tokach et al., 1992; Schell and Kornegay, 1996). A recent large regional study showed that high dietary levels of zinc (3,000 ppm, as zinc oxide) and copper (250 ppm, as copper sulfate) were both efficacious, but were not additive in terms of growth promotion when they were added in combination to diets for weanling pigs (Hill et al., 1996).
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Representative terms from entire chapter:
weanling pigs
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