Generally, the proximate constituents of feedstuffs are considered to contain the following E:
Although E is determined by burning a constituent in an atmosphere of oxygen, the yield of energy, whether via oxidation in biological systems or a furnace, is the same if taken to the same state of oxidation or end products.
Terminology for Discussing Energy Values of Feedstuffs
Gross energy is not particularly descriptive of the energy an animal can derive from a feedstuff. When a feedstuff or combination of feedstuffs (diet) is fed, the digestive process is generally not able to make all the E consumed available to the animal for absorption; thus, there is a loss of energy in the feces. Subtracting the energy excreted in feces from the E consumed yields digestible energy (DE). Digestible energy can be expressed in absolute terms per unit of weight (kcal/g) or as a percentage of gross energy. The term total digestible nutrients (TDN) also is used, but feed energy values are expressed in units of weight instead of calories. TDN is determined by
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Page 2 2 Nutrient Requirements and Signs of Deficiency Energy The term energy, when used to describe diet attributes, actually describes the end product rather than the inherent characteristics of compounds found in feedstuffs. Energy results from the utilization of the absorbed nutrients from metabolic processes such as oxidation and synthesis. It is generally measured as heat of combustion. The specific term used to describe the unit of energy depends on many factors; the most common include calorie and joule. In the United States the calorie is the most common unit for measuring energy in feedstuffs and is used throughout this report. A calorie is the amount of heat necessary to raise one gram of water from 16.5° to 17.5°C. Since the calorie is a very small unit of measurement, energy values for feedstuffs are more commonly expressed as kilocalories (1 kcal = 1,000 calories) and megacalories (1 Mcal = 1,000,000 calories = 1,000 kcal). Internationally, the joule is frequently used (1 calorie = 4.184 joules). The caloric values of individual constituents of feedstuffs are characteristic of their chemical compositions. The energy value of a constituent is measured as the heat released when the substance is completely oxidized to carbon dioxide and water. The amount of energy released is measured in calories and is referred to as the gross energy (E) contained in that constituent. For example, Compound Heat of Combustion, kcal/g Ethanol 7.11 Glucose 3.74 Starch 4.18 Acetic acid 3.49 Propionic acid 4.96 Butyric acid 5.95 Palmitic acid 9.35 Stearic acid 9.53 Glycine 3.12 Tyrosine 5.91 Generally, the proximate constituents of feedstuffs are considered to contain the following E: Feedstuff Component kcal/g Carbohydrate 4.2 Fat 9.4 Protein 5.6 Although E is determined by burning a constituent in an atmosphere of oxygen, the yield of energy, whether via oxidation in biological systems or a furnace, is the same if taken to the same state of oxidation or end products. Terminology for Discussing Energy Values of Feedstuffs Gross energy is not particularly descriptive of the energy an animal can derive from a feedstuff. When a feedstuff or combination of feedstuffs (diet) is fed, the digestive process is generally not able to make all the E consumed available to the animal for absorption; thus, there is a loss of energy in the feces. Subtracting the energy excreted in feces from the E consumed yields digestible energy (DE). Digestible energy can be expressed in absolute terms per unit of weight (kcal/g) or as a percentage of gross energy. The term total digestible nutrients (TDN) also is used, but feed energy values are expressed in units of weight instead of calories. TDN is determined by
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Page 3 summing digestible crude protein, digestible carbohydrates (nitrogen-free extract and crude fiber), and 2.25 × digestible crude fat. Although DE and TDN are frequently used to evaluate feedstuffs and to express nutrient requirements, the use of metabolizable energy (ME) instead of DE or TDN has important advantages for ruminants. Measuring only fecal energy losses does not accurately reflect the energy available to ruminants for use in productive processes. As feedstuffs are exposed to microorganisms in the rumen, a significant part of the E in the feedstuff is metabolized to methane (an end product of fermentation that is very high in energy but of essentially no caloric value to the host animal) that escapes from the rumen in eructated gases. Loss of gross energy as methane varies with the type of diet (high concentrate versus low concentrate) and the level of feeding and ranges from 3 to 10 percent. The energy lost in urine also is not accounted for if only fecal energy is measured. The energy content of urine is rather constant and represents 3 to 5 percent of the E value of a diet. The major factors influencing the fraction of dietary DE in the urine are diet protein level, diet roughage levels, and essential oil content. The last is high in some range plants such as sagebrush (Cook et al., 1952). To determine ME, subtract gaseous and urine energy losses from DE. The conversion of DE to ME generally is estimated as DE × 0.82. This estimation is accurate except for high-grain diets, where higher ratios are observed (Johnson, 1972). Net energy (NE) is the most refined expression of the value of energy in a feedstuff. Although not as commonly used in evaluating feedstuffs and expressing requirements as ME, NE represents the amount of energy available to the animal for maintenance and productive processes. Determination of NE requires one measurement in addition to those required for calculating ME. This is the heat increment (HI), which is the increase in heat produced as a result of digestion and metabolic processes in response to increased ME intake. Thus, HI is the inefficiency of ME use for any given function. Subtracting HI from ME yields NE. This assumes that HI includes both the heat from fermentations in the digestive tract and the heat liberated during nutrient metabolism. Under most conditions, HI is of no value to the animal and frequently is a burden, since it requires that additional energy be dissipated. However, when ruminants are exposed to low environmental temperatures and must increase heat production to maintain normal body temperatures, then HI may be useful in maintaining body temperature. Heat increment varies with diet and physiological function of the animal and can range from 10 to 90 percent of the ME. Net energy is subdivided into that used for maintenance (NEm) and that recovered as some useful product (NEp) (recovered energy [RE], body tissue, milk, or wool). NEm and NEp may be further subdivided. Net energy for maintenance includes the NE for basal metabolism that relates to muscular activity, tissue repair and replacement, and involuntary metabolic processes such as maintenance of ionic gradients. Also included as NEm is the minimal voluntary activity necessary to sustain life. The amount of energy needed to satisfy voluntary activity needs (sometimes called the activity increment) varies widely depending on the availability of feed, water, and shade and the topography of the environment. Extreme examples of management systemsconfinement versus arid rangemay cause the activity increment to be a major factor in determining NEm. During hot or cold weather, the animal uses ME to cool or heat its body; the energy required for this is also part of NEm and is widely variable depending on several environmental factors. Net energy available to the animal in excess of that required for maintenance is used in a variety of productive processes. These include the net energy for growth (NEg), lactation (NEl), reproductive processes (NEy), and production of wool and hair (NEv). Where applicable, the net energy for physical work, in addition to that required by the activity increment, may also be included. The efficiency with which metabolizable energy above maintenance is used as net energy for various functions varies with quality of diet and physiological function. For example, the process of milk production is more efficient than growth as empty body gain. Signs of Deficiency and Toxicity Meeting energy requirements without over- or underfeeding animals is one of the producer's most difficult tasks. Energy deficiency or insufficiency is likely the most widely occurring nutritional deficiency within the sheep industry. Likewise, oversupplying energy to sheep is one of the most wasteful practices. An energy deficiency will manifest itself in a variety of ways depending on its severity. In growing animals an early sign is reduced rate of gain, which progresses to cessation of growth, weight loss, and ultimately death. In reproducing females early signs of energy deficiency are reduced conception rate, reduced reproductive rate (i.e., reduced number of multiple births), and reduced milk production, with progressively worse deficiencies causing reproductive failure, cessation of or lack of initiation of lactation, and death. Similar problems develop in the male, with an initial reduction and eventual cessation in reproductive activity and performance and finally death. With restrictions in energy, wool growth slows; fiber diameter is reduced; total production of wool decreases; and in severe cases wool growth ceases, creating
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Page 4 a ''break" (weak spot) in the staple of wool. Energy deficiency will cause a reduction in the function of the immune system, resulting in a lowered resistance to disease. Undernourished sheep also will have an increased susceptibility to parasite infestation. On the other hand, an animal consuming more NE than required must find a way to handle the excess. Excesses are stored as adipose tissue and are a valuable reserve until obesity ensues. Signs of NE toxicity are gross excesses in adipose deposits and ultimately a reduction in reproductive performance in both males and females. In pregnant, obese females, NE toxicity manifests itself shortly prior to parturition as ketosis. Maintenance An animal's energy requirement for maintenance is that amount of dietary energy it must consume daily to neither gain nor lose body energy. Experimentally it is the amount of metabolizable energy resulting in zero change in body energy and zero product. Energy maintenance occurs when daily ME intake equals daily heat production. This ME requirement is not independent of kind or quality of diet fed, however, Fasting heat production is most commonly used as a baseline for describing the maintenance requirement of the animal independent of diet. This daily quantity of energy is defined as the net energy required for maintenance (NEm). Measured fasting heat production is used to set maintenance requirements in some systems (ARC, 1980). However, because of the limited data base available and questions about the validity of fasting measurements, particularly on young animals, an extrapolated fasting heat production is used as the reference base for maintenance requirement in the present system (Rattray et al., 1973b). The experimentally derived kilocalorie value of 63 kg0.75 × d-1 has been adjusted from an empty body weight (EBW) basis to a live weight (W) basis assuming a 6.1-kg fill for a 40-kg sheep (ARC, 1980). The resulting daily NEm kilocalorie requirement is approximated as 56 W0.75. Growth Energy requirements for tissue deposition reflect the proportions of lipid, protein, and water deposited. Each kilogram of empty body gain requires between 1.2 Mcal (mainly protein and water) and 8.0 Mcal (mainly fat and water). Changes in the live weight of sheep also reflect changes in the weight of ingesta in the gastrointestinal tract, which can vary from 60 to 540 g/kg of empty body weight. Chemical analyses of the empty bodies of 20- to 50-kg growing sheep representative of genotypes produced in the United States show that caloric densities of empty body weight gains (EBG) vary from 3 to 4 Mcal/kg gain in light-weight lambs to 5.5 to 7.5 Mcal/kg in heavier lambs. If these caloric densities of gain are scaled to the empty body weight of the animal raised to the 0.75 power (EBW0.75), the variation within genotype drops perceptibly. Variation in caloric density from one genotype to another remains considerable, ranging from approximately 300 to 400 when expressed as kcal × EBG × EBW0.75. The requirement for growth across this 20- to 50-kg weight span appears to be closely related to the yearling ram weight of the genotype, which, in turn, is closely related to genotype mature weight (Parker and Pope, 1983). Relating the caloric densities of gains of nine genotypes (Reid et al., 1968; Burton and Reid, 1969; Drew and Reid, 1975b) to a measure of the yearling ram weights (Figure 1) of those genotypes (Parker and Pope, 1983) yields the following equation: where NEg equals Mcal of retained tissue energy per day in empty body gains per kg EBG per EBW0.75, and W equals the yearling ram weight of the genotype. This relationship extrapolated to an average mature ram weight genotype of 115 kg corresponds to an average requirement of 344 kcal × EBG × EBW0.75. Calculation of energy requirements for gain also requires extrapolation from an empty body basis to a live weight basis. Two adjustments are necessary, since requirements are described per kilogram gain per unit body weight. Live weight gains are predicted as 9 percent higher than empty body gains, and empty body weight is multiplied by 1.195 to predict live weight and to adjust for fill at a 30 kg EBW similar to ARC values (1980). Tissue energy retained, which is the net energy for growth (NEg), can now be calculated from live lamb gains and weights. NEg (kcal × d-1) equals 276 LWG × W0.75 for medium mature ram weight (115-kg) genotypes. For every 10 kg mature weight less than 115 kg, the energy requirement increases by 21 kcal × LWG × W0.75, or 7.6 percent. For each 10 kg over 115 kg, a like amount would be subtracted from this requirement for live weight gain (Table 3). Rams deposit less energy than ewes of the same genotype at equal live weights (Bull et al., 1970; Ferrell et al., 1979). These limited data suggest that caloric densities of energy gains in rams can be estimated at 0.82 times those for ewes. Castrated males may also have somewhat lower requirements than females (Kelloway, 1973; ARC, 1980); however, the quantitative differences are not well established (Rattray et al., 1973a) and no adjustment is recommended at this time. Level of diet intake, rate of gain, and concentration of
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Page 5 Figure 1 Relationship between the energy density of empty body weight gain (NEg) and genotypic mature weight (W) as estimated by yearling ram weight. dietary DE have generally had a small long-term effect, if any, on the composition of weight gain in growing lambs after weaning (Reid et al., 1968; Theriez et al., 1982a, b). Very low rates of growth may sometimes result in increased caloric density in body gains (Rattray et al., 1973a; Graham and Searle, 1982), presumably because of the demand for protein resources for wool growth. Very high rates of gain in milk-fed lambs have been associated with greater fat deposition (Black, 1974). On the other hand, high-protein diets in early weaned fast-growing lambs have been associated with depressed fat deposition (Andrews and Ørskov, 1970). The energy requirements for gain in castrated or ewe lambs from medium-sized genotypes are very similar under the present system to the ARC (1980) values for wethers (Figure 2). Increased requirements for small-genotype lambs are similar to increases that ARC (1980) relates to ewe lambs. Gain requirements per kg LWG increase linearly as animals get heavier in the ARC (1980) system and increase slightly curvilinearly in the present system. Pregnancy Sheep utilize metabolizable energy for conceptus development with an efficiency of 12 to 14 percent and for pregnancy (gravid uterus plus mammary gland development) with an efficiency of 16 to 18 percent for diets containing 2.4 to 2.6 Mcal ME/kg DM (Rattray et al., 1973b, 1974). (The efficiency of ME utilization may vary with diets that differ markedly from those used to establish the above values.) The NEy requirements (above NEm and NEg) of ewes bearing single, twin, and triplet fetuses that have a total fetal weight of 5.0, 9.0, and 11.5 kg, respectively, are given in Table 4 for different stages of late pregnancy (Rattray et al., 1974). Actual NE requirements may differ from those listed if the fetuses or placental tissues differ markedly in size or composition from those studied by these workers. Rattray et al. also obtained data indicating that the maintenance requirement of ewes and the efficiency of utilization of ME for maternal maintenance and gain were not changed by pregnancy. The extra heat production that occurs in pregnancy appears to be primarily fetal in origin. Total feed requirements of pregnant sheep can be obtained by summing the various diet amounts needed to meet each NE requirement (e.g., feed for NEm + feed for NEg + feed for NEy for a pregnant ewe lamb or feed required for NEm + feed for NEy for a pregnant adult ewe). Fetal growth and pregnancy requirements are substantial in the last 6 weeks of pregnancy and average approximately 0.5 × maintenance for single-bearing ewes and 1.0 × maintenance for twin-bearing ewes. Total feed requirements would thus increase to 1.5 to 2.0 × maintenance for this physiological phase. Lactation Few estimates of the utilization of ME for lactation in sheep are available. Sheep have a relatively short lactation period, and the actual quantity and composition of milk produced by animals suckling young are difficult to determine. Gardner and Hogue (1964) have estimated that 65 to 83 percent of ME is converted to milk energy during 12 weeks of lactation. Higher values were obtained for ewes suckling twins than for ewes with single
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Page 6 Figure 2 Energy density of live weight gains of large, medium, and small genotypes compared with ARC data (1980). lambs. The average of these values is slightly above that calculated for dairy cattle. NE Value of Feedstuffs The NE value of feedstuffs in meeting the NEm and NEg requirements of animals has been established from the following. The widest data base and the one most applicable to production situations was developed from the relationships established by Garrett (1980) relating NEm and NEg to the ME concentration of the diet: The data reported by Rattray et al. (1973b) are applicable to pelleted diets. Pelleting changes the relationship between NE and ME, particularly when predicting NEg (Blaxter and Boyne, 1978). Ovine and bovine partial efficiencies of ME use for maintenance and gain can be interchanged (Blaxter and Wainman, 1964; Garrett et al., 1959; Rattray et al., 1973c; ARC, 1980). The NEy value of the diet can be estimated as 0.17 ME. Gut Fill Variation The large and variable ingesta fraction of sheep weight and gain complicates requirement definitions. Gut fill varies from 6 percent of live weight in milk-fed lambs to 30 or 35 percent in forage-fed lambs soon after weaning. Gut fill assumptions used in this publication are typical for mixed-grain/forage-fed animals, unshrunk but before the morning meal. Gut fill will be higher on forage diets and lower on very high concentrate diets. Animal handling procedures, feed processing, and quality will cause considerable variation. Environment Ambient temperature, thermal radiation, humidity, air movement, contact surfaces, and precipitation may all have a positive or negative effect on a sheep's energy requirement, depending on where they put the animal in relation to its thermoneutral zone. For instance, environmental temperatures above or below the thermoneutral zone will increase energy needs. More specific information is available in Effect of Environment on Nutrient Requirements of Domestic Animals (NRC, 1981). Wool is a very effective insulation against cold and heat; however, reports on the insulating effect of wool against heat are rare (Curtis, 1981). Several scientists (Blaxter, 1966; Ames, 1969; and Brink and Ames, 1975) have reported the influence of fleece on lower critical temperature (LCT) in sheep. Length of fleece and level of feeding (fasting, maintenance, or full feed) interact in influencing LCT. At a given level of feeding, the shorter the fleece the higher the LCT. Thus, shearing increases energy needs when the environmental temperature is below the LCT. NRC (1981) reports LCTs of 25° to 31°C for shorn sheep and -3°C for sheep with full fleece. Diet digestibility by shorn sheep generally declines approximately 0.001 units per degree centigrade fall in ambient temperature; however, unshorn sheep show no digestibility change between -10° and +20°C (Christopherson and Kennedy, 1983).
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Page 7 A distinct seasonal shift in the maintenance energy requirement of sheep, probably modulated by photoperiod, has been noted in thermoneutral environments (Blaxter and Boyne, 1982). The sine wave fluctuation increases to a peak (14 percent above average) in July and decreases a similar magnitude in winter. Parallel fluctuations in voluntary dietary intake also occur. Level of energy intake or rate of gain to which a sheep has been accustomed has been shown to shift maintenance requirements (Koong et al., 1982). Fast growing, ad libitum-fed sheep have a fasting heat production 30 to 40 percent higher than those of matched weight and age but accustomed to zero gain and low intakes. High basal metabolism appears related to high vital organ mass (i.e., 30 to 40 percent higher liver and small intestine organ weights). These adaptations to low energy intakes are likely to be important to animal survival during periods of scarce feed supplies. Also, low metabolism and renewed increases in vital organ mass likely are important contributors to compensatory gains when animals are refed after a period of scarce feed supply. Management Considerations Ewes that begin pregnancy in a very thin condition and weigh, for example, 60 kg (132 lb) are about as large physiologically (i.e., capacity of digestive tract, body fluids, and body surface) as they are when fat and weigh 70 to 75 kg (154 to 164 lb). When thin, they must be fed more energy and protein during gestation than the tables suggest for their particular weight. Feeding at levels suggested for a ewe 10 kg heavier would enable the thin ewe to regain some of the weight lost due to the stress of lactation and/or inadequate feed. Conversely, an overly fat ewe whose weight suggests she is physiologically larger than she actually is can be fed less during the first 3.5 months of gestation without affecting lamb and wool production. Consideration of an ewe's initial body condition and its effect on subsequent nutrient needs is as vital as consideration of size and age. Figure 3 shows the daily and cumulative weight changes of a 60-kg ewe during various stages of production. Depending on the desired response of the animals, their existing body condition, their appetite, and environmental conditions, the amount of feed given to sheep may be varied from the levels recommended in Tables 1 and 2. If diets are more concentrated than those indicated in Table 2, the level of dry matter fed may be reduced accordingly. Regardless of the concentration of energy in the diet, however, amounts of feed given should provide the suggested daily requirements of energy, protein, minerals, and vitamins. Figure 4 gives the approximate daily DE requirements of 65- to 70-kg ewes at various stages of production. Figure 3 Daily and cumulative weight changes of a 60-kg ewe during maintenance, gestation, and lactation. EwesFirst 15 To 17 Weeks Of Gestation The requirements given in Tables 1 and 2 are intended to provide for maintenance, wool growth, and a small daily gain. If ewes are fat, a submaintenance diet is permissible during the first 3.5 months of gestation (non-critical period) to avoid overly fat ewes at lambing time. No allowance has been made for flushing the ewe to increase lamb production (see the subsection on Flushing, p. 30). The nutrients required for wool production depend on the genetic potential of the sheep to produce wool. The energy required for wool production represents a small fraction of the total energy consumed. EwesLast 4 Weeks Of Gestation In early pregnancy, fetal growth is every small, and the total feed requirement of the ewe is not significantly different from the feed requirement during periods of maintenance. During the last 4 to 6 weeks of gestation, ewes need more energy to meet increased nutrient demands for fetal growth and the development of the potential for high milk production. The nutrient levels recommended in Tables 1 and 2 are adequate for normal fetal and mammary development in single- and twin-bearing ewes. Excessive energy intake may lead to fattening with resultant birth difficulties in single-bearing ewes. Excessively low energy intakes can result in impaired milk
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Page 8 Figure 4 Approximate daily digestible energy (DE) requirements of 65- to 70-kg breeding ewes at various production stages. production capability, reduced mothering instinct, and lower birth weights, leading to reduced viability in the lambs. Either low energy intakes or excessive fattening may result in pregnancy toxemia in the ewe (see the section on Pregnancy Disease, p. 28). EwesLactation In Tables 1 and 2, energy requirements are estimated for four groups of lactating ewes: those in the first 6 to 8 weeks of lactation, suckling singles; those in the last 4 to 6 weeks of lactation, suckling singles; those in the first 6 to 8 weeks of lactation, suckling twins; and those in the last 4 to 6 weeks of lactation, suckling twins. A ewe nursing twin lambs produces 20 to 40 percent more milk than a ewe nursing one lamb. Within the genetic capability of the ewe, milk production responds to nutrient intake of the ewe and demand for milk by the lamb or lambs. Requirements for the last 6 to 8 weeks of lactation are based on the assumption that milk production during that period is approximately 30 to 40 percent of the production during the first 8 weeks. Thus, nutrient intake during the last 6 to 8 weeks of lactation may be reduced. For example, the weaning weight of lambs nursing ewes fed 20 percent less total digestible nutrients for 6 weeks postpartum than suggested (NRC, 1975) was no different from that of lambs nursing ewes fed 115 to 120 percent of NRC-suggested requirements (Jordan and Hanke, 1977). In preparing these tables, it has been anticipated that ewes will lose a small amount of weight during early lactation. The amount of weight loss varies greatly, depending on management factors (e.g., the quality and amount of feed available), the number of lambs suckled, the environment, and the ewe's genetic background. Under some range conditions, ewes lose weight in winter (during pregnancy) and gain weight during lactation when grazing high-quality summer ranges. Replacement Lambs Separate requirements are presented in Tables 1 and 2 for replacement ewes and rams. Ram lambs have the potential to grow at a faster rate than ewe lambs, especially after they reach 40- to 50-kg body weight. Mature size for the breed will influence energy requirements. Smaller breeds tend to grow more slowly, whereas larger breeds grow more rapidly and have higher nutrient requirements. Nutrients needed for gain by ewe and ram lambs have been compared. The performance of the ewe lambs fits the equations for maintenance and gain used in developing Tables 1 and 2. Ram lambs gain more rapidly than the equations suggest, have a higher feed intake, and use feed more efficiently for body weight gain. Gains in the body weight of intact males are higher in water and protein and lower in fat than in females. Producers breeding ewe lambs to yearlings should feed at levels that will result in Finn cross ewe lambs weighing a minimum of 43 kg (95 lb) and other breeds weighing 50 kg (110 lb) at breeding. During the gestation period, sufficient additional feed should be provided to meet pregnancy requirements and weight gains of 0.12 to 0.16 kg daily. During lactation these ewes still require additional feed to ensure adequate milk production and continued growth. Usually this means providing up to 1 kg of grain per ewe daily in addition to a full feed of forage. Weaning at around 6 weeks coupled with sufficient feed postweaning will permit recovery of lactation weight loss and resumption of normal growth rates in preparation for subsequent breeding. Protein The lamb is born with a nonfunctional rumen that requires dietary protein be provided through milk or a
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Page 9 milk replacer until the rumen becomes functional. The rumen develops some degree of functionality by 2 weeks of age, primarily as a result of the consumption of dry feed (Poe et al., 1971, 1972). During early rumen development, creep feed should be provided to supplement milk or milk replacer. By 6 to 8 weeks of age, the functioning rumen has developed into a culture system for anaerobic bacteria, protozoa, and fungi. These microbes digest feedstuffs and synthesize protein to extents that allow efficient production without milk. Ruminal microorganisms can utilize either protein or nonprotein nitrogen to synthesize microbial protein. The microbial protein, along with undigested feed protein, passes from the rumen-reticulum through the omasum to the abomasum and small intestine where it is subjected to digestive processes similar to those of the nonruminant. Microbial protein reaching the small intestine usually accounts for 40 to 80 percent of the total protein reaching this area of the digestive tract (Owens and Bergen, 1983). The requirements given in Tables 1, 2, and 5 apply to functioning ruminants and were determined factorially with the basic formula: Crude protein required in g/d = (where PD = protein deposited, MFP = metabolic fecal protein, EUP = endogenous urinary protein, DL = dermal loss, and NPV = net protein value). Protein deposited in gain was estimated by applying the following equation (NRC, 1984): PD in g/d = daily gain in kg × (268 - 29.4 × ECOG) when energy content of gain with NEg values taken from Table 3. Protein deposited was set at 2.95 g/d for early gestation and 16.75 g/d for the last 4 weeks of gestation (ARC, 1980) for ewes with single lambs and increased proportionately for higher lambing rates. A milk production of 1.74 kg/d for ewes nursing a single lamb and 2.60 kg/d for those nursing twins and a crude protein content of 47.875 g/liter of milk (ARC, 1980) were used to determine PD need for lactation. Ewe lambs were assumed to produce 75 percent as much milk as mature ewes. Metabolic fecal protein in g/d was assigned a value of 33.44 g/kg DM intake (NRC, 1984). Endogenous urinary protein in g/d was calculated as 0.14675 × body weight in kg + 3.375 (ARC, 1980). Dermal loss in g/d was estimated to be 0.1125 × kgW0.75 (ARC, 1980). Crude protein in wool in g/d of ewes and rams was assigned a value of 6.8 g, assuming an annual grease fleece weight of 4 kg. For lambs, crude protein in wool in g/d was calculated as 3 + (0.1 × protein retained in the fleece free body) (ARC, 1980). A net protein value of 0.561 was used based on a true digestibility of 0.85 (Storm and Ørskov, 1982) and a biological value of 0.66 (NRC, 1984). Expression of requirements as crude protein is consistent with the dairy (NRC, 1978) and beef (NRC, 1984) reports, but contrary to ARC (1980). The potential advantages of digestible protein are negated by the use of standard factors for conversions between crude protein and digestible protein by both NRC (1975) and ARC (1980). Development of a more-comprehensive system of expressing protein requirements of ruminants is being intensively studied, but a consensus has not developed. Some of the key issues are summarized later in this section. Numerous reviews are available that treat these issues more extensively (NRC, 1976, 1984, 1985; ARC, 1980; Huber and Kung, 1981; Ørskov, 1982; Owens, 1982; Owens and Bergen, 1983; Chalupa, 1984). Microbial Nitrogen Requirements Although a variety of anaerobic microorganisms inhabit the rumen, bacteria are most active in protein digestion and synthesis of microbial protein. Bacteria degrade dietary protein in the rumen to simpler nitrogen compounds such as ammonia, amino acids, and peptides and incorporate these materials into cellular protein. Ammonia also is derived from dietary nonprotein nitrogen sources such as urea. Ammonia is the nitrogen source preferred by the bacteria in the rumen for cellular protein synthesis (Bryant and Robinson, 1963; Hungate, 1966). Lack of ammonia in the rumen may limit microbial growth when the intake of protein or the ruminal degradation of the dietary protein is low. Although the concentration of ruminal ammonia-nitrogen required for optimal microbial growth is unclear, concentrations above 5 to 10 mg per 100/ml of ruminal fluid have not consistently increased bacterial protein production (Satter and Slyter, 1974; Pisulewski et al., 1981; Leng and Nolan, 1984). Nonprotein Nitrogen Substitution of dietary nonprotein nitrogen (NPN) for plant and animal protein sources often will lower the cost of the complete diet fed to sheep. Urea is the most common source of NPN fed. It is useful only when it is needed to provide a source of ammonia to ruminal bacteria. Since urea is rapidly hydrolyzed to ammonia by bacteria in the rumen, strict management techniques are essential when high levels of urea or other NPN sources are fed to prevent decreased feed intake and ammonia toxicity. Urea is utilized most efficiently when thoroughly mixed in high-concentrate, low-protein diets that are fed continuously. Urea concentrations should not exceed 1 percent of the dietary dry matter or one-third of the total dietary protein. High-concentrate diets provide more energy than high-roughage diets for bacterial protein synthesis from ammonia (Owens and Bergen, 1983). The resulting decrease in ammonia absorption reduces the
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Page 10 likelihood of ammonia toxicity (Bartley et al., 1976). If NPN is substituted for dietary protein, special emphasis should be given to the supplementation of potassium, phosphorus, and sulfur, which are absent in urea. Particular attention should be given to sulfur supplementation, because wool contains a high percentage of sulfur-containing amino acids. In attempts to reduce the threat of ammonia toxicity and/or improve the utilization of ruminal ammonia, other forms of NPN (biuret, triuret, and complexes of urea with formaldehyde or molasses) have been developed (Nikolic et al., 1980). These compounds have slower ammonia release, which should more nearly parallel energy availability and increase bacterial protein synthesis (Johnson, 1976). However, these slow release forms of NPN have not consistently improved nitrogen utilization (Owens and Bergen, 1983). Ruminal Degradation and/or Bypass of Dietary Protein Dietary protein is either digested in the rumen or escapes undigested to the omasum and abomasum. If it is not digested in the rumen, it is described as "bypass" or "escape" protein (Owens and Bergen, 1983). Bypass protein is either digested postruminally or excreted in the feces. Dietary protein degraded in the rumen yields ammonia (Chalupa, 1975), which can then be incorporated into microbial protein. Chalupa (1975), Satter and Roffler (1975), and ARC (1980) have classified protein sources on the basis of the extent to which they bypass ruminal degradation (percentage of dietary protein that reaches the small intestine undigested). Low-bypass sources (‹ 40 percent) include casein, soybean meal, sunflower meal, and peanut meal; medium-bypass sources (40 to 60 percent) include cottonseed meal, dehydrated alfalfa meal, corn grain, and brewers dried grains; and high-bypass sources (› 60 percent) include meat meal, corn gluten meal, blood meal, feather meal, fish meal, and formaldehyde-treated proteins. Feed processing conditions, animal variations, dietary alterations, and changes in microbial population affect extent of dietary protein bypass, but these effects have not been well quantitated. When high-bypass protein sources are fed, supplementation with NPN will be needed to maintain adequate ruminal ammonia levels for microbial protein synthesis. Increased bypass of dietary protein does not always increase production, because bypassed protein may be poorly digested postruminally, the balance of amino acids available for absorption from the small intestine may be poor, or other nutrients may limit production (Young et al., 1981; Owens and Bergen, 1983). Conversely, if microbial protein is the only protein reaching the small intestine, animal production may not be maximal (Satter et al., 1977). Presentation to the small intestine of a mixture of microbial protein and complementary dietary protein is desired. Striving to optimize this mixture will undoubtedly be the subject of much research activity in the future, as it has been in the past. Amino Acids Amino acids available for absorption from the small intestine are supplied by microbial and/or bypassed dietary protein. The tissues of sheep require the same amino acids as those of the nonruminant (Black et al., 1957; Downes, 1961). In sheep, however, the relationship of dietary amino acid supply with tissue requirements has been difficult to define because of the intervention of the protein digestive and synthetic functions in the rumen. Also, amino acid requirements are difficult to quantitate because of variability in requirements for various productive functions. For example, wool growth responds to sulfur amino acid supplementation (Reis and Schinckel, 1963), whereas other functions do not. Hogan (1975) concluded that the amino acid composition of protein deposited in the tissues and that secreted in milk, plus the maintenance requirement, should equal the total needed by the animal. Owens and Bergen (1983) further concluded that the quantity, as well as the ratios, of amino acids required by the animal varies with both the productive function and the level of production. Dietary amino acids are normally rapidly degraded in the rumen. To increase bypass, Neudoerffer et al. (1971) and Digenis et al. (1974) coated dietary amino acids so they would be ruminally stable but available for absorption postruminally, suggesting that the combination of amino acid and NPN supplementation may be feasible in the future. Protein Deficiency and Toxicity Ammonia deficiency in the rumen reduces the extent and efficiency of rumen function. Deficiencies or imbalances of amino acids at the tissue level result in decreased protein synthesis, as well as reduced feed intake and lower efficiency of feed utilization. Growth rate and milk and wool production all react to inadequate protein intake. Extreme deficiency results in severe digestive disturbances, loss of weight, anemia, edema, and reduced resistance to disease. Increased feed intake after that protein was deficient (NRC, 1984). Excess protein becomes an expensive and inefficient source of energy, but rather large excesses can be fed without producing acute toxicity (Fenderson and Bergen, 1976). Excesses of NPN or highly soluble protein may
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Page 11 produce ammonia toxicity (Bartley et al., 1981). Affected animals may display nervousness, incoordination, labored breathing, bloating, severe tetany, respiratory collapse, and ultimately death. Minerals Although the body contains many mineral elements, only 15 have been demonstrated to be essential for sheep. Seven are major mineral constituents: sodium, chlorine, calcium, phosphorus, magnesium, potassium, and sulfur. The other eight are trace elements: iodine, iron, molybdenum, copper, cobalt, manganese, zinc, and selenium. Additional elements under investigation with other species may eventually prove to be essential for sheep. Fluorine is discussed (p. 22) because of its toxicity to sheep. The multiplicity of interactions among minerals makes it difficult to determine the requirements of sheep for specific minerals, because a lack or abundance of one mineral may render others deficient or toxic. Tables 6 and 7 present the mineral requirements of sheep and the toxic levels when known. In both tables, values are estimates based on available experimental data. Sodium and Chlorine (Salt) Sodium (Na) and chlorine (Cl) serve many functions in the body. The maintain osmotic pressure, regulate the acid-base balance, and control water metabolism in tissues. Sodium occurs primarily in extracellular fluids and bones. Chlorine is found within cells, in the body fluids, in gastric secretions such as hydrogen chloride, and in the form of salt (Underwood, 1981). Animals that are deprived of adequate salt may try to satisfy their craving by chewing wood, licking dirt, or eating toxic amounts of poisonous plants. Inadequate salt may result in inappetence, growth retardation, inefficiency of feed use, and increased water consumption (Hagsten et al., 1975; Underwood, 1981). In addition, the concentration of sodium falls and that of potassium rises in the parotid saliva of sheep on low-sodium diets (Morris and Peterson, 1975). Signs of sodium deficiency occur without a significant decline in either plasma or milk sodium concentrations until a condition of extreme deficiency is reached (Morris and Peterson, 1975; Underwood, 1981). Several feeding and metabolism studies have been conducted to determine the sodium and/or salt requirement of sheep. McClymont et al. (1957) reported that the addition of 1.2 to 2.6 g of sodium per day (as sodium chloride) to the diet of very thin wethers fed a low-sodium grain diet increased growth rate. They concluded that the sodium requirement was greater than 0.9 g/d (0.06 percent of the diet). From balance data, Devlin and Roberts (1963) estimated the sodium requirements for maintenance of wether lambs to be 1.01 g/d (0.18 percent of the diet). Hagsten et al. (1975) concluded that the dietary salt requirement for growing lambs ranged between 0.33 and 0.43 percent of the air-dry ration (90 percent dry matter). They further stated that since most sheep rations contain approximately 0.2 percent salt, a supplemental level of 0.2 percent is adequate. Based on the maintenance of a normal Na+:K+ ratio in the parotid saliva, Morris and Peterson (1975) concluded that a dietary sodium level of 0.09 percent met the requirements of lactating ewes. Apparently no feeding trials have been conducted in which the requirement for chlorine can be assessed independently of sodium; thus, the chlorine requirement is unknown. When adding salt to mixed feeds, it is customary to add 0.5 percent to the complete diet or 1.0 percent to the concentrate portion. Range operators commonly provide 220 to 340 g of salt per ewe per month as a salt lick. Drylot tests show lambs consume approximately 5 to 10 g of salt daily (Denton, 1969). Mature ewes in confinement consume 15 to 30 g of salt daily when it is offered free choice (Jordan and Hanke, 1982). Salt may safely be used to limit free-choice supplement intake if adequate water is available. Such mixtures are usually 10 to 50 percent salt depending on the desired amount of ration to be consumed. Trace-mineralized salt should not be used for this purpose because of the possibility of excessive intake of various trace minerals, particularly toxic levels of copper. In many areas (commonly arid), feed and water may contain enough salt to meet the animal's requirements, and supplemental salt need not be offered. On the basis of research conducted by Meyer and Weir (1954) and Meyer et al. (1955), the maximum tolerable level of dietary salt for sheep was set at 9.0 percent (NRC, 1980). Jackson et al. (1971), however, reported a linear decrease in weight and energy gains of growing-finishing lambs as salt content increased from 1.8 to 7.6 percent of the diet. Calcium and Phosphorus Calcium (Ca) and phosphorus (P) are closely interrelated, particularly in the development and maintenance of the skeletal system. Approximately 99 percent of the body's calcium and 80 percent of its phosphorus are found in bones and teeth. Diets lacking in calcium or phosphorus may result in abnormal bone development, a condition known as rickets in young animals and osteomalacia in adults. The 1 percent of calcium and 20 percent of phosphorus not present in skeletal tissues are widely distributed in body fluids and soft tissues, where they
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Page 12 serve a wide range of essential functions (Underwood, 1981). Signs of calcium deficiency due to a low intake of calcium develop slowly because the body draws on calcium in bone. Blood levels of calcium are normally not good indicators of calcium intake or status, as these levels are hormonally controlled (Care et al., 1980). Blood calcium levels below 9 mg/dl of plasma (hypocalcemia), however, suggest chronic low calcium intake or utilization at a rate that exceeds calcium mobilization from bone (as during lactation). In extreme cases, which may develop in lambs on high-grain diets, low intakes of calcium may result in tetany or precipitate an outbreak of urinary calculi in intact or castrated male sheep. Sheep efficiently utilize phosphorus, partly by recycling considerable amounts in parotid and other salivary secretions. The phosphorus concentration of parotid saliva, rumen fluid, and serum is related to phosphorus intake (Tomas et al., 1967). In some cases, sheep recycle more phosphorus per day through parotid saliva than is required in the diet to maintain normal concentrations in body pools. This salivary phosphorus can moderate variations in rumen phosphorus due to diet, particularly at low phosphorus intakes (Cohen, 1980). A phosphorus deficiency may be manifested by slow growth, deprived appetite, unthrifty appearance, listlessness, low level of phosphorus in the blood (less than 4 mg/dl of plasma), and development of rickets (Beeson et al., 1944; Preston, 1977). Calcium and phosphorus utilization are influenced by vitamin D. Dietary calcium is absorbed according to the nutritional requirements of the animal, and on a low-calcium diet the efficiency of absorption is increased. The efficiency of absorption also is increased in adult animals during pregnancy and lactation (Care et al., 1980; Scott and McLean, 1981; Braithwaite, 1983a). Differences have been observed in absorption of phosphorus within and between breeds of sheep. These differences appear to be partly heritable and vary as much as twofold (Field et al., 1983; Field, 1984). Adaptation to a low-phosphorus diet is due to an increase in the efficiency of intestinal absorption and a reduction in the salivary secretion of phosphorus (Care et al., 1980). Calcium and phosphorus requirements were calculated using a factorial approach. First, a net requirement was calculated from estimates of the storage and excretion of these elements during growth, pregnancy, and lactation and of endogenous losses. The dietary requirement then was calculated by dividing the net requirement by the coefficient of absorption. Daily dietary requirements were converted to dietary concentrations (percent of diet) by dividing by daily DM intakes. Endogenous fecal losses of calcium were assumed to vary in a linear relationship with DM intake, as described by Braithwaite (1982, 1983a). These values varied from 11.6 mg Ca/kg body weight per day for maintenance of mature ewes consuming 15.6 g DM/kg body weight per day to 43.2 mg Ca/kg body weight per day for a 10-kg, rapidly growing, early-weaned lamb consuming 60 g DM/kg body weight per day. Total endogenous losses of phosphorus were assumed to be 20 mg/kg body weight per day for maintenance, early gestation, and growth. However, a higher value (30 mg/kg body weight per day) was used to calculate phosphorus requirements during the last 4 weeks of gestation and during lactation. ARC (1980) used a constant value of 14 mg P/kg body weight per day to calculate phosphorus requirements for all stages of production. The higher values used in this publication reflect the fact that there may be inevitable losses of phosphorus associated with the higher phosphorus intakes required to meet the demands of late gestation and lactation as suggested by Braithwaite (1984a), as well as evidence that the phosphorus levels recommended by ARC (1980) may be inadequate for pregnancy and lactation (Braithwaite, 1983b, 1984b) and growth (Field et al., 1982). The calcium and phosphorus contents of gain were 11 and 6 g/kg empty body gain, respectively (ARC, 1980; Grace, 1983). Although it is recognized that numerous factors influence the birth weight of lambs, including breed, size, and age of ewe; breed of sire; season and type of birth; sex of lamb; and nutrition of the ewe (Neville et al., 1958; Jamison et al., 1961; Shelton, 1968; Rastogi et al., 1982; Stritzke and Whiteman, 1982), only the size of the ewe at mating and the type of birth (single or twins) were considered in estimating calcium and phosphorus requirements for gestation. It was assumed that single lambs were 22.6 percent and twins were 36.1 percent of the ewe's metabolic weight at the time of mating (Donald and Russell, 1970). Net calcium and phosphorus values for the gravid uterus were calculated as described in ARC (1980). The calcium and phosphorus contents of ewes' milk, used to calculate nutritional requirements, were 0.18 and 0.14 percent, respectively. The milk production values used in estimating net calcium and phosphorus requirements for lactation in mature ewes were 1.74 kg/d, first 6 to 8 weeks of lactation suckling singles; 1.11 kg/d, last 4 to 6 weeks of lactation suckling singles; 2.60 kg/d, first 6 to 8 weeks of lactation suckling twins; and 1.67 kg/d, last 4 to 6 weeks of lactation suckling twins. The milk production of ewe lambs was 1.30 kg/d, first 6 to 8 weeks of lactation suckling singles, and 1.95 kg/d, first 6 to 8 weeks of lactation suckling twins (Langlands, 1973; Pert et al., 1975; Doney et al., 1979). The values used for absorption of dietary calcium were 0.4 for maintenance, 0.5 for gestation, and 0.6 for lactation and rapidly growing lambs. These values were
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Page 15 percent of the dry matter. Therefore, the possibility of potassium deficiency is slight under most feeding conditions. Nevertheless, attention should be given to potassium supply when lambs are fed high-grain diets and when sheep are grazing mature range forage during winter or drought periods. Potassium levels in mature range forage have been reported to decrease to less than 0.2 percent. Under such grazing conditions beef cattle have responded favorably to the addition of potassium to the range supplement (Clanton, 1980). The maximum tolerable level of potassium for sheep is approximately 3 percent of the diet DM (NRC, 1980). Magnesium absorption was depressed 24.4 and 61.2 percent when diets containing 2.4 and 4.8 percent potassium, respectively, were fed to wethers. Increasing the level of potassium in the diet also depressed serum magnesium levels (Greene et al., 1983a). The negative effect of high levels of potassium on magnesium utilization can help precipitate magnesium tetany in sheep on diets marginal in magnesium (Field, 1983b). Increasing the level of dietary potassium from 0.7 to 3.0 percent linearly decreased energy and weight gains in lambs (Jackson et al., 1971). Sulfur The signs of sulfur (S) deficiency are similar to the signs of protein deficiency (loss of appetite, reduced weight gain or weight loss, and reduced wool growth). In addition, they include excessive salivation, lacrimation, and shedding of wool. In extreme cases, emaciation and death may occur (Goodrich et al., 1978). Because sulfur functions in the synthesis of the sulfur-containing amino acids (methionine and cysteine) and B-vitamins (biotin and thiamin) during microbial digestion in the rumen, rumen microorganisms that are deficient in sulfur do not function normally. Addition of sulfur in such cases increases feed intake, digestibility, and nitrogen retention (Bray and Hemsley, 1969; Bird, 1974; Guardiola et al., 1983). Sulfur levels of 0.15 to 0.20 percent (DM basis) appear adequate for normal rumen function (Goodrich et al., 1978). Sulfur has functions in the body in addition to those concerned with protein structure. Sulfate sulfur is an important constituent of the chondroitin sulfates and of the mucins of the gastrointestinal tract (including saliva), the reproductive tracts, and other duct systems (Moir, 1979; Goodrich and Thompson, 1981). Because wool is high in sulfur, this element is closely related to wool production. Much information has been obtained in recent years about sulfur metabolism in the rumen, sulfur losses, sulfur requirements of microoganisms, and the recycling of sulfur and nitrogen (Goodrich et al., 1978; Bull, 1979; Moir, 1979). This information generally supports the recommendation that a dietary nitrogen-sulfur ratio of 10:1 be maintained. The percentages of sulfur required in diet dry matter are 0.14 to 0.18 for mature ewes and 0.18 to 0.26 for young lambs. Practically all common feedstuffs contain more than 0.1 percent sulfur. Mature grass and grass hays (especially those grown on granitic soils), however, are sometimes low in sulfur and may not furnish enough for optimal performance. Where forages are low in sulfur, or where diets contain relatively large quantities of urea, weight gains and growth of wool can be increased by feeding a sulfur supplement, such as sulfate sulfur, elemental sulfur, or sulfur-containing proteins or amino acids. Most grains contain 0.10 to 0.15 percent sulfur, so it is conceivable that lambs on high-concentrate diets could lack adequate sulfur. Although inorganic compounds are generally more convenient and economical for supplemental feeding, sulfur availability is greatest from methionine followed by sulfate sulfur and then elemental sulfur. Sulfur from sodium sulfate is around 80 percent as available as sulfur from methionine, and sulfur from elemental sulfur is about half as available as that from sodium sulfate (Johnson et al., 1970). Available data do not allow the establishment of a safe upper limit for the different sulfur sources for sheep, but it appears that 0.4 percent is the maximum tolerable level for dietary sulfur as sodium sulfate (NRC, 1980). At levels slightly above 0.4 percent, there is a decrease in DM intake and rumen motility. At higher levels, complete anorexia, ruminal stasis, impaction, and a foul odor of hydrogen sulfide on the breath of sheep are observed. Since the availability of elemental sulfur is only 50 percent that of sodium sulfate, a correspondingly higher level of elemental sulfur would be required to induce signs of sulfur toxicosis (Johnson et al., 1970). Sulfur forms insoluble complexes with copper and molybdenum and decreases their utilization (Suttle and McLauchlan, 1976; Grace and Suttle, 1979; Suttle, 1983a). It also decreases selenium retention (Pope et al., 1968). Iodine Iodine (I) is necessary for the synthesis of the thyroid hormones, thyroxine and triiodothyronine (Underwood, 1977). In newborn lambs, the most common sign of iodine deficiency is enlargement of the thyroid gland. If the condition is not advanced, lambs may survive. Other signs are lambs born weak, dead, or without wool (Underwood, 1981). Signs of iodine deficiency in mature sheep seldom take the form of a change in the animal's appearance. Through the impairment of physiological functions, however, deficiency may result in reduced yield of wool and reduced rate of conception (Potter et al., 1980; Underwood, 1981). Iodine requirements of sheep have been estimated
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Page 16 from heat production (Underwood, 1977), thyroxine secretion rate (Henneman et al., 1955; Singh et al., 1956; Falconer and Robertson, 1961; Robertson and Falconer, 1961; Falconer, 1963), and serum triiodothyronine levels (Barry et al., 1983). Based on heat production the minimum iodine requirement is between 0.05 and 0.10 mg/kg diet DM. Values based on the rate of thyroxine secretion vary from 0.05 to 1.25 mg/kg diet DM. Levels of 0.18 to 0.27 mg I/kg diet DM are necessary to maintain serum triiodothyronine levels in growing lambs. The previous NRC (1975) publication for sheep reported the iodine requirement as 0.10 to 0.80 mg/kg diet DM in diets not containing goitrogens, the higher level being indicated for pregnancy and lactation. These levels are also being recommended in this revision. When goitrogens such as the glucosinolates found in kale (Brassica oleracea) or other thioglycosides found in cruciferous plants are present, the dietary iodine should be increased (Underwood, 1977; Barry et al., 1983). Areas in the United States deficient in iodine are the northeastern section of the country and the Great Lakes and Rocky Mountain regions (Underwood, 1981). Serious losses of lambs can be prevented in these areas by feeding iodized salt to ewes during gestation. Iodized salt generally is formulated by adding 0.0078 percent of stabilized iodine to salt (Perry, 1982). Stabilization is necessary to prevent losses from exposure to sunlight or moisture. Iodized salt should not be used in a mixture with a concentrate supplement to limit feed intake, since the animals may consume an excessive amount of iodine. Signs of iodine toxicosis are depression, anorexia, hypothermia, and poor body weight gain (McCauley et al., 1973). According to NRC (1980) the maximum tolerable level of iodine for sheep is 50 mg/kg diet DM. McCauley et al. (1973), however, reported that levels of 267 mg iodine (as ethylenediamine dihydroiodide) and 133 mg iodine (as potassium iodide) per kilogram had no effect on live weight gain and feed intake of lambs during a 22-day treatment period. Iron Iron (Fe) deficiency in animals is characterized by poor growth, lethargy, anemia, increased respiration rate, decreased resistance to infection, and in severe cases high mortality (Underwood, 1981). A primary iron deficiency in grazing sheep is very unlikely because of the iron content of pasture plants and the contamination of plants by soil (McDonald, 1968). Loss of blood resulting from parasite infestation, however, can produce a secondary iron-deficiency anemia (Silverman et al., 1970). Experimentally, iron-deficiency anemia has also been produced in milk-fed lambs (Thomas and Wheeler, 1932) and in lambs raised on slotted wooden floors and fed a semipurified diet (Lawlor et al., 1965). Anemia in suckling lambs can be prevented by administering intramuscular injections of iron-dextran or by offering a commercial oral iron compound free choice in the creep area. Two injections, 150 mg of iron each, given 2 to 3 weeks apart are preferable to a single injection (Holz et al., 1961; Mansfield et al., 1967). The addition of 13 mg Fe/kg diet DM was reported to increase blood hemoglobin levels and total red cell volume in artificially reared lambs given a liquid diet of skimmed milk plus fat (Brisson and Bouchard, 1970). In another study, acute iron-deficiency signs were observed in lambs fed a semisynthetic diet containing 10 mg Fe/kg diet. A 25-mg Fe/kg diet did not support maximum growth, but 40 mg Fe/kg seemed adequate to meet the dietary requirement (Lawlor et al., 1965). Hoskins and Hansard (1964) estimated the gross requirements of ewes to be at least 34 mg iron per day during the final stages of pregnancy. This value is equivalent to about 20 mg iron Fe/kg diet DM. Based on the limited information available, 30 mg/kg would appear adequate to meet the dietary iron requirements for all classes of sheep. Signs of chronic iron toxicity are reductions in feed intake, growth rate, and efficiency of feed conversion. In acute toxicosis animals exhibit anorexia, oliguria, diarrhea, hypothermia, shock, metabolic acidosis, and death (NRC, 1980). Feeding 1,600 mg Fe/kg of diet as either ferrous sulfate or ferric citrate reduced feed intake below maintenance in lambs (Standish and Ammerman, 1971). The ferrous sulfate diet was less palatable than the ferric citrate diet. In another study (Lawlor et al., 1965) an unexplained diarrhea occurred among lambs receiving diets containing 210 and 280 mg Fe/kg. A maximum tolerable level of 500 mg Fe/kg of diet has been suggested for sheep (NRC, 1980). Molybdenum Although molybdenum (Mo) occurs in low concentrations in all tissues and fluids of the body and is a component of three metalloenzymes, unequivocal evidence of molybdenum deficiency in sheep, unrelated to copper, has not been reported (Underwood, 1977; 1981). A significant growth response to added molybdenum and an improvement in cellulose digestibility were reported in one study with lambs fed a semipurified diet containing 0.36 mg Mo/kg (Ellis et al., 1958). This observation, however, was not substantiated in three subsequent experiments with semipurified and practical-type pelleted diets (Ellis and Pfander, 1960). The minimum dietary requirements for molybdenum are not known but appear to be extremely low. Although the 1975 edition of this report stated the requirement as
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Page 17 > 0.5 mg Mo/kg diet DM, sheep regularly graze pastures containing less molybdenum with no adverse effects other than increased copper retention in the tissues (Underwood, 1981). The major concern about the level of molybdenum in the diet involves its interaction with copper and sulfur. Molybdenum forms insoluble complexes with copper and sulfur and decreases the utilization of dietary copper (Suttle, 1975, 1983a; Suttle and McLauchlan, 1976). Copper absorption is inhibited most by 4 to 6 mg Mo/kg diet DM. Higher levels of molybdenum inhibit sulfide production and may give rise to a recovery in copper absorption (Suttle, 1983a). The rates of absorption, retention, and excretion of molybdenum are inversely related to the level of dietary sulfur (Grace and Suttle, 1979; NRC, 1980). Sheep appear more resistant to molybdenosis than cattle and tolerate plasma molybdenum levels of 0.1 to 0.2 mg/dl (approximately 20 to 40 times the normal plasma level), providing dietary sulfate is at least 0.1 percent (NRC, 1980). High levels of molybdenum induce a copper deficiency, and the signs of molybdenosis in sheep are the same as those described for copper deficiency (''stringy" wool, lack of pigmentation in black sheep, anemia, bone disorders, and infertility). Several of the western states have extensive areas where forage plants have 10 to 20 mg/kg or more of molybdenum (Kubota, 1975). Sheep start to scour a few days after being turned on pasture with a high molybdenum content (5 to 20 mg/kg on a DM basis). The feces become soft, the fleece becomes stained, and the animals lose weight rapidly. When the dietary copper level falls below normal (5 to 8 mg/kg) or the dietary sulfate level is high (0.40 percent), molybdenum intake as low as 1 to 2 mg/kg may prove toxic. Molybdenum toxicity is controlled by increasing the copper level in the diet by 5 mg/kg. Copper A condition known as neonatal ataxia or "swayback" is characteristic of copper (Cu) deficiency in young lambs. Most often ataxia is apparent immediately after birth, but it may be delayed several weeks. Signs of ataxia, generally seen in suckling lambs, include muscular incoordination, partial paralysis of the hindquarters, and degeneration of the myelin sheath of the nerve fibers. Lambs may be born weak and may die because of their inability to nurse, a condition that occurs when the central nervous system develops during a time of maternal copper deficiency (Howell, 1970; Underwood, 1977; Miller, 1979a). Sheep suffering from copper deficiency have "steely" or "stringy" wool, lacking in crimp, tensile strength, affinity for dyes, and elasticity. Lack of pigmentation of the wool of black sheep also occurs and appears to be a sensitive index of copper deficiency (Underwood, 1977). The condition is similar to that noted in black sheep on high levels of molybdenum. Anemia, bone disorders (osteoporosis in lambs and spontaneous bone fractures in adult sheep), and infertility have also been associated with copper deficiency in sheep (Underwood, 1977). Copper requirements of sheep are so dependent on dietary and genetic factors that it is difficult to state requirements without specifying the conditions for which they apply. Concentrations of sulfur and molybdenum are the major dietary factors influencing copper requirements. These minerals form insoluble complexes with copper, thereby reducing its absorption and increasing dietary levels needed to meet requirements. Sulfur appears to exert an independent effect on the availability of copper, but the effect of molybdenum is sulfur dependent (Suttle, 1975; Underwood, 1981; Suttle and Field, 1983). The relationship for the effects of sulfur and molybdenum on the true availability (A) of dietary copper for sheep fed semipurified diets is described by equation (1): This relationship is based on the data from 10 repletion experiments with sheep fed semipurified diets varying from 0.8 to 4.0 g S/kg and from 0.5 to 1.5 mg Mo/kg (Suttle and McLauchlan, 1976). The relationship for summer pasture is given by equation (2): where S and Mo are herbage concentrations of sulfur and molybdenum in g/kg and mg/kg, respectively (Suttle, 1983a). This equation differs substantially from that describing the effects of sulfur and molybdenum in semipurified diets and should be used to estimate the absorption of copper from pasture. Sulfur and molybdenum concentrations did not exceed 4g/kg and 6 mg/kg diet DM, respectively, in the data from which equation 2 was derived, and the equation should not be used to extrapolate to higher concentrations. High concentrations of zinc (Campbell and Mills, 1979), iron, and calcium (Miller, 1979a) have also been shown to decrease copper absorption. Differences in copper metabolism within and among breeds also cause variation in the minimum copper requirements of sheep (Wiener, 1979; Woolliams et al., 1982; Wiener and Woolliams, 1983; Field, 1984). These differences, which are partly heritable, appear to be due to differences in absorption and are reflected in differences in blood and liver copper concentrations and in the incidence of copper deficiency (swayback) and toxicity exhibited by different breeds of sheep (Wiener and Woolliams,
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Page 18 1983). In fact, it has been shown that dietary amounts of copper that are adequate for some breeds are deficient for others and possibly toxic to some (Wiener and Woolliams, 1983). Finnish Landrace ewes have lower copper concentrations in their blood than Merino ewes, and the values for Merino ewes are lower than for some British breeds (Hayter and Wiener, 1973). Although it is impossible to give exact requirements for copper, several estimates have been made of the amounts of copper that should be provided in the diet of sheep. In the 1975 revision of this report, 5 mg/kg diet DM was suggested for sheep fed diets with normal levels of sulfur and molybdenum. Merino sheep are less efficient in absorbing copper from feedstuffs than British breeds and therefore need an additional 1 to 2 mg/kg in their diet. Using a factorial approach, the ARC (1980) estimated the requirements of sheep for copper as follows: for growing lambs ranging from 5 to 40 kg live weight, 1.0 to 5.1 mg Cu/kg diet DM; for maintenance of adult sheep, 4.6 to 7.4 mg Cu/kg diet DM; for gestation, 6.2 to 7.5 mg Cu/kg diet DM; and for lactation, 4.6 to 8.6 mg Cu/kg diet DM. These recommendations do not take into account individual or genetic differences but do suggest adjustment factors for diets not containing normal levels of sulfur (2.5 g/kg DM) and molybdenum (2 to 3 mg/kg DM). More recently Suttle (1983c) recalculated the ARC (1980) estimates using a new value for the net copper requirement for growth and lower estimates of copper absorption that varied depending on the molybdenum content of diet as follows: Recommended Copper Allowance Recommended Cu Allowance (mg/kg diet DM) Mo Content of Diet (mg/kg) Growth Pregnancy Lactation ‹ 1.0 8-10 9-11 7-8 › 3.0 17-21 19-23 14-17 Available data (Grace, 1975; Stevenson and Unsworth, 1978) suggest a variable availability of copper from natural sources. Availability from all forage diets ranged from 10 to 35 percent (Grace, 1975), whereas lower values were reported when high-concentrate diets and straw-based low-concentrate diets were fed (Stevenson and Unsworth, 1978). Copper is found in adequate amounts over most of the United States, but deficient areas have been reported in Florida and in the coastal plains region of the Southeast. Also, in several of the western states there are areas where an excess of molybdenum induces copper deficiency (Kubota, 1975). (For additional discussion of molybdenum and copper interrelationships, see the section on Molybdenum on p. 16.) Copper can be provided conveniently in deficient areas by adding copper sulfate to salt at a rate of approximately 0.5 percent. Stores of copper in the body serve as a reserve for as long as 4 to 6 months when animals are grazing copper-deficient forage. The differential between copper requirement and copper toxicity is very narrow. Errors in feed mixing frequently result in mortality due to copper toxicity. Complete manufactured feeds for sheep in the United States may contain 25 to 35 ppm copper. When vitaminmineral preparations are added to feeds, the copper content of the diet may be excessive (Buck and Sharma, 1969). These levels of copper can be extremely harmful if the molybdenum level of the diet is low. In fact, if the molybdenum level is extremely low (‹ 1 ppm), forage with a normal copper content of 8 to 11 ppm can produce toxicity. The normal concentration of copper in whole blood is 0.7 to 1.3 ppm and in liver (fresh basis) 12 ppm (Pope, 1971). The concentration of copper in liver gives a reliable indication of the copper status of sheep. The concentration in the kidney cortex provides an even better criterion for diagnosing copper poisoning. In most cases of copper poisoning, concentrations of copper, on a DM basis, exceed 500 ppm in the liver and 80 to 100 ppm in the kidney cortex (Pope, 1971). Hemolysis, jaundice (easily detected in the eyes), and hemoglobinuria are characteristic signs of toxicity and result in very-dark-colored liver and kidneys (Todd, 1969). In treating copper toxicity, both molybdenum and sulfate should be administered. Dietary inorganic sulfate alone has less effect on uptake or reduction of copper in the liver and on utilization of copper for synthesis of ceruloplasmin (Ross, 1966). High dietary concentrations of zinc protect against copper intoxication. A diet of 100 ppm of zinc on a DM basis reduces liver copper storage (Pope, 1971). An effective treatment for copper toxicity in lambs is to drench each lamb daily with 100 mg of ammonium molybdate and 1 g of sodium sulfate in 20 ml of water. Adding equivalent amounts of molybdenum and sulfur to the daily feed is equally effective. Either treatment usually requires a minimum of 5 to 6 weeks (Ross, 1966, 1970). The Food and Drug Adminstration does not recognize molybdenum as safe, and the law prohibits adding it to feed for sheep unless prescribed by a veterinarian. Copper toxicity can be prevented by reducing or eliminating extraneous sources of copper in the diet. Cobalt The only known function of cobalt (Co) in sheep nutrition is to promote synthesis of vitamin B12 in the rumen. Thus, signs of cobalt deficiency are actually signs of vitamin B12 deficiency. These are lack of appetite, lack
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Page 19 of thrift, severe emaciation, weakness, anemia, decreased estrous activity, and decreased milk and wool production (Ammerman, 1981; Underwood, 1981). For mature sheep grazing grossly cobalt-deficient pastures, the amount of cobalt necessary to ensure optimum growth is 0.08 mg/d when supplementary cobalt is administered orally 3 times per week. For young, rapidly growing lambs the requirement is greater and during the first few months is probably as much as 0.2 mg/d (Lee and Marston, 1969). With sheep confined to pens and fed a cobalt-deficient diet, 0.07 mg cobalt per day is required for maintenance of normal growth rate; however, for maintenance of maximum vitamin B12 status, based on serum and liver vitamin B12 concentrations, a supplement of between 0.5 and 1.0 mg Co/d is necessary (Marston, 1970). Although levels of vitamin B12 in the contents of the rumen and in the blood and liver are indicators of the cobalt status of sheep, the vitamin B12 content of the feces is an indicator that can be used advantageously. Jones and Anthony (1970) developed an equation for estimating oral intake of cobalt on the basis of concentration of vitamin B12 in the feces: where Y represents the oral intake of cobalt expressed as mg/kg in the dry feedstuff and X represents the concentration of vitamin B12 in the feces expressed as µg of vitamin B12 per gram of dry feces. In the study no signs of cobalt deficiency were observed in lambs fed a diet containing 0.09 mg Co/kg diet DM for a 7-month period. The corresponding level of vitamin B12 for this level of cobalt in the diet was 2.13 µg/g of dry feces. In the 1975 edition of this report, the recommended amount of cobalt was 0.1 mg/kg diet DM. The same value is proposed here for all classes of sheep; however, young, rapidly growing lambs may have a slightly higher requirement, as suggested by Lee and Marston (1969). Areas deficient in cobalt have been reported in the United States and Canada. The most severely deficient areas in the United States include portions of New England and the lower Atlantic Coastal Plain. Moderately deficient areas include New England, northern New York, northern Michigan, and parts of the Central Plains (Ammerman, 1981). Research has demonstrated that cobalt should be ingested frequently (MacPherson, 1983). This can be accomplished by adding cobalt to salt at a rate of 2.5 g Co/100 kg salt using either cobalt chloride or cobalt sulfate. Other effective methods are the addition of cobalt to the soil (Griffiths et al., 1970; Burridge et al., 1983) or the administration of cobalt pellets (MacPherson, 1983) or a soluble glass containing cobalt that dissolves slowly in the reticulum (Telfer et al., 1984). Sheep have been fed 350 mg Co/100 kg of live weight for short periods of time without ill effects. Levels of approximately 450 mg/100 kg of live weight have been suggested as toxic (Becker and Smith, 1951). The National Research Council (1980) suggests 10 mg Co/kg diet DM as a maximum tolerable level for ruminants. Manganese Manganese (Mn) deficiency in animals results in impaired growth, skeletal abnormalities and ataxia of the newborn, and depressed or disturbed reproductive function (Hidiroglou, 1979a; Underwood, 1981). The minimum dietary manganese requirements for sheep are not exactly known; however, it appears that the requirement for growth is less than for optimal reproductive performance. Requirements may also be increased by high intakes of calcium and iron (Underwood, 1981). Bone changes similar to those seen in other manganese-deficient animals were observed when early-weaned lambs received a purified diet containing less than 1 ppm of manganese over a 5-month period (Lassiter and Morton, 1968). When a diet containing 8 ppm of manganese was fed to 2-year-old ewes for a 5-month period prior to breeding and throughout gestation, more services per conception (2.5 versus 1.5) were required than for ewes fed a diet containing 60 ppm manganese (Hidiroglou et al., 1978). Levels of manganese in wool appear to be sensitive to changes in the manganese status of lambs (Lassiter and Morton, 1968). The growth of female goats fed 20 ppm of manganese for the first year of life and 6 ppm during the following year was not affected, but the onset of estrus was delayed and more inseminations were required per conception (Anke and Groppel, 1970). No goats aborted in the control group (100 ppm), but 23 percent of those on the low-manganese diet aborted. The low-manganese diet also resulted in a 20-percent reduction in birth weights, the birth of more male than female kids, and the death of more female than male kids. Bone structure was not affected. In mature goats the manganese content of the hair was a better indicator of manganese status than the manganese content of any other part of the body. Although the exact requirements of sheep for manganese are not known, 20 mg/kg, on a DM basis, should be adequate for most production stages. With a well-balanced diet, it appears that 1,000 mg/kg of dietary manganese is the maximum tolerable level for sheep (NRC, 1980). Zinc Zinc (Zn) deficiency in sheep is characterized by a decrease in appetite and a reduction in the rate of growth.
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Page 20 Other signs are brief periods of excessive salivation, parakeratosis, wool loss, reduced testicular development (or testicular atrophy), defective spermatogenesis, and delayed wound healing. In addition, all phases of the reproductive process in females from estrus to parturition and lactation may be adversely affected (Smith et al., 1962; Hidiroglou, 1979b; Miller, 1979b; Underwood, 1981). Ott et al. (1965) found that a diet containing 18 mg Zn/kg diet DM did not support maximal live weight gains of lambs fed a purified diet. Mills et al. (1967) estimated that a dietary zinc level of 7.7 mg/kg diet DM satisfied the growth requirements of lambs but did not maintain the plasma zinc levels within the normal range. Data presented by Underwood and Somers (1969) indicate that a diet containing 2.4 mg Zn/kg DM is grossly inadequate for growth and metabolic requirements of ram lambs. A similar diet supplying 17.4 mg Zn/kg DM was adequate for body growth and for the maintenance of normal appetite, although this level was not adequate to permit normal testicular development and spermatogenesis. Histological and other evidence suggest that dietary zinc at a level of 32.4 mg/kg DM is adequate for maximal testicular development and function (Underwood and Somers, 1969). Pond (1983) concluded that a zinc level of 19 to 26 mg/kg DM was adequate for growth of lambs. Based on these reports it appears that the zinc requirements of ram lambs for testicular growth and development and for spermatogenesis are greater than the requirements for body growth. Zinc requirements for pregnancy and lactation have not been established. The few studies that have been conducted indicate that the lactating ewe is clearly susceptible to zinc deficiency, but whether zinc is necessary for normal parturition in sheep (as it is in rats) requires further study (Apgar and Travis, 1979; Masters and Moir, 1983). Under Australian field conditions, Egan (1972) obtained an increased conception rate when grazing ewes were given supplemental zinc. The zinc content of the forage varied between 17 and 28 mg/kg DM. The suggested minimum requirements are 20 mg Zn/kg DM for growth and 33 mg Zn/kg DM for maintenance of normal reproductive function in males and for pregnancy and lactation in females. Diets high in calcium (1.2 to 1.8 percent calcium) have been reported to adversely affect zinc utilization (Mills and Dalgarno, 1967). Although there appears to be a wide margin of safety between requirements for zinc and amounts that are toxic, zinc toxicity has been described for growing lambs (Ott et al., 1966; Davies et al., 1977) and for pregnant sheep (Campbell and Mills, 1979). One gram of zinc per kilogram of diet caused reduced consumption of feed and reduced gain in lambs (Ott et al., 1966), and 0.75g Zn/kg diet induced severe copper deficiency in pregnant ewes and caused a high incidence of abortions and stillbirths (Campbell and Mills, 1979). Selenium In the northwestern, northeastern, and southeastern parts of the United States, there are extensive areas where the selenium (Se) content of crops is below 0.1 ppm (Figure 5), which is the level considered adequate for preventing deficiency in sheep. Thus, selenium-responsive diseases are most likely to occur in these regions. In an area extending roughly from the Mississippi River to the Rocky Mountains, the selenium content of crops is predominantly in the nutritionally adequate but nontoxic range of selenium concentration. Parts of South Dakota, Wyoming, and Utah produce forage that causes selenium toxicity in farm animals (Kubota et al., 1967; Muth, 1970; NRC, 1983). The most commonly noticed lesion in sheep resulting from an inadequate supply of selenium is degeneration of the cardiac and skeletal musculature (white muscle disease), but unthrifitiness, early embryonic death, and periodontal disease are also signs of a possible selenium deficiency (McDonald, 1968; Muth, 1970; Underwood, 1981). Lamb production is seriously affected; the major manifestations of deficiency in lambs are reduced growth and white muscle disease, which affects lambs 0 to 8 weeks of age (Pope, 1971). Selenium-responsive infertility has been described in Australia (Godwin et al., 1970; Piper et al., 1980) and New Zealand (Hartley, 1963) but not elsewhere (Pope, 1971; Phillippo, 1983). Supplementation with 0.1 mg Se/kg DM (as sodium selenite) of the diet of ewes during gestation through weaning consistently provided essentially complete protection against white muscle disease in their lambs (Schubert et al., 1961). Feeding ewes a natural diet containing 0.07 mg Se/kg DM or the addition of 0.1 mg Se/kg DM to a low-selenium diet (‹ 0.02 mg Se/kg DM) prevented white muscle disease in their lambs (Oldfield et al., 1963). Oh et al. (1976) concluded that the selenium requirement of reproducing ewes and their lambs fed a practical diet was 0.12 mg/kg DM. This conclusion was based on the dietary selenium level required to reach a plateau in tissue glutathione peroxidase levels. In contrast, Moksnes and Norheim (1983) found that tissue glutathione peroxidase activity plateaued above a level of 0.23 mg Se/kg diet. Glutathione peroxidase was the first selenoenzyme to be identified in animal tissues. The level of this enzyme in tissue and red blood cells can be considered a more sensitive indicator of dietary adequacy for lambs than tissue selenium content (Oh et al., 1976; Paynter et al., 1979). An extensive review of New Zealand data indicated selenium-responsive unthriftiness in grazing lambs occurred
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Page 21 Figure 5 The regional distribution of forages and grain, containing low, variable, or adequate levels of selenium, in the USA and Canada. where the selenium content of spring pastures was ‹ 0.02 mg/kg DM. Pastures containing › 0.03 mg/kg DM were apparently adequate, whereas intermediate levels were probably marginally deficient. An occasional positive response was obtained, however, with pastures having selenium levels in the range of 0.09 to 0.10 mg/kg DM (Grant and Sheppard, 1983). Whanger et al. (1978) have proposed that the selenium requirements for sheep be raised to at least 0.2 mg/kg diet DM when legume forages are fed. Schubert et al. (1961) and Pope et al. (1979) found an antagonistic effect of dietary sulfur on selenium absorption and retention. Thomson and Lawson (1970) reported an interaction between selenium and copper in sheep. Adding selenium to the diet improved the copper status of sheep on deficient or marginally adequate copper diets. A number of methods can be used to prevent white muscle disease in lambs caused by a selenium deficiency. The Food and Drug Administration has approved the following uses of selenium for ewes and ewes with lambs up to 8 weeks of age: (1) selenium can be added to a complete feed at a level not to exceed 0.1 mg/kg diet; (2) selenium can be added to a feed supplement at a level that, when consumed with the base feed, will not exceed an intake of 0.23 mg Se per sheep per day; and (3) up to 30 mg Se/kg diet can be added to a salt-mineral mixture for free-choice feeding at a rate not to exceed an intake of 0.23 mg Se per sheep per day (Federal Register, Vol. 43. pp. 11700-11701, 1978). These uses of selenium have been shown to be safe and effective (Paulson et al., 1968; Rotruck et al., 1969, Ullrey et al., 1977, 1978). One may also inject a commercial pharmacological product containing selenium and vitamin E (see the subsection on Vitamin E on pp. 24 for levels). Other experimental methods of supplying selenium to sheep include an oral drench (Whanger et al., 1978; Paynter et al., 1979; Piper et al., 1980; MacPherson, 1983), subcutaneous or intramuscular injection (Kuttler et al., 1961; Whanger et al., 1978), application of selenium to the soil (Allaway et al., 1966; Watkinson, 1983), introduction of a heavy selenium pellet (composed of finely divided metallic iron and elemental selenium in a
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Page 22 proportion of 20 to 1) into the reticulum (Kuchel and Buckley, 1969; Handreck and Godwin, 1970; Whanger et al., 1978; Paynter, 1979; MacPherson, 1983), and introduction of a soluble glass containing selenium to the reticulum (Telfer et al., 1984). Chronic selenium toxicity occurs when sheep consume over a prolonged period of time seleniferous plants containing more than 3 ppm of selenium. Signs include loss of wool, soreness and sloughing of hooves, and marked reduction in reproductive performance (NRC, 1980; Underwood, 1981; Howell, 1983). Toxicity of forage depends somewhat on its protein and sulfur content. The extent to which plants take up selenium varies greatly. Some species of plants grown on seleniferous soils contain as much as 1,000 ppm of selenium, whereas other species grown on the same soils contain only 10 to 25 ppm. The most practical way to prevent livestock losses from selenium poisoning is to manage grazing so that animals alternate between selenium-bearing and other areas. Selenium is a cumulative poison, but mild chronic signs can be overcome readily. The mineral is eliminated rapidly from the body of an affected animal when it is fed selenium-low forage. Small amounts of arsanilic acids are effective in reducing the toxicity of selenium. Fluorine Fluorine (F) exerts a cumulative toxic effect. Signs may not be observed until the second or third year of intake of high levels of fluoride. Affected animals usually exhibit anorexia; the normal ivory color of their bones gradually changes to chalky white; bones thicken because of periosteal hyperostosis; and the teeth, especially the incisors, may become pitted and eroded to such an extent that the nerves are exposed (Underwood, 1977; NRC, 1980). Fluorine rarely occurs free in nature but combines chemically to form fluorides. In some parts of the world fluoride occurs in the water supply in amounts that may be high enough to have deleterious effects. Another danger lies in the use of rock phosphate that contains fluorine in amounts sufficient to be toxic (O'Hara et al., 1982). Proper defluorination procedures are necessary to make rock phosphate safe for animal supplementation. Forage growing near manufacturing units processing minerals containing fluorides may be highly contaminated with fluoride. Finishing lambs can tolerate up to 150 ppm of fluorine in the diet on a DM basis (Harris et al., 1963). Acute toxicity can occur at 200 ppm. Data are not available on lifetime tolerance levels for sheep; however, breeding sheep should not be fed diets containing more than 60 ppm fluorine on a DM basis (NRC, 1980). Vitamins Vitamin A In the previous revision of Nutrient Requirements of Sheep (NRC, 1975), 17 IU/kg of body weight for vitamin A alcohol (retinol) or 25 µg/kg of body weight for ß-carotene were the values used to calculate the vitamin A and ß-carotene requirements. An IU is defined as 0.300 µg of retinol or 0.550 µg of vitamin A (retinyl) palmitate. Requirements for late pregnancy, lactation, and early-weaned lambs were calculated by multiplying these values by 5; those for replacement lambs and yearlings were obtained by multiplying by 2.5; and those for finishing lambs and for ewes during maintenance and the first 15 weeks of gestation were calculated by multiplying by 1.5. These values were based on the amounts of retinol or carotene required to prevent night blindness in sheep and the amounts required for storage and reproduction (Guilbert et al., 1937, 1940). Recent studies with growing calves have effectively demonstrated that elevated pressure in the cerebrospinal fluid (CSF) is a more sensitive indicator of vitamin A status than is night blindness. For example, Eaton (1969) reported that the minimum ß-carotene or retinol requirement of calves (µg/kg live weight per day) based on prevention of night blindness was 24 to 35 µg for carotene and 5.1 to 6.4 µg for retinol (17 to 21 IU), whereas for prevention of elevated CSF pressures, the values are 66 to 73 µg for ß-carotene and 14.1 µg for retinol (47 IU). Increased CSF pressure has also been observed in sheep deficient in vitamin A (Eveleth et al., 1949; May, 1982). Based on increased CSF pressure the minimum requirement for growing-finishing lambs appears to be between 8 and 16 µg of retinol/kg live weight per day (May, 1982). This is supported by the work of Faruque and Walker (1970), who reported that 14 µg retinyl palmitate or 69 µg ß-carotene/kg live weight per day permitted the establishment of a small liver reserve of retinol in young lambs. In the absence of more definitive information, the minimum carotene and vitamin A requirements of sheep are assumed to be 69 µg of ß-carotene/kg live weight per day or 47 IU of vitamin A/kg live weight per day. These values were used as the basis for establishing requirements for vitamin A for all categories (Tables 1 and 2) except ewes in late gestation and during lactation, in which cases the requirements are 125 µg/kg live weight per day for ß-carotene and 85 IU/kg live weight per day for vitamin A. During the first 6 to 8 weeks of lactation, the requirements for ewes suckling twins were further increased to 147 µg/kg live weight per day for ß-carotene and 100 IU/kg live weight per day for vitamin A. Ewe milk contains about 1,500 IU vitamin A per liter and
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Page 23 conceivably these additional amounts should be added per day to compensate for what is produced in milk. Plant products do not contain preformed vitamin A, and sheep meet their vitamin A needs mainly from carotenoid precursors in the diet (Moore, 1957). Vitamin A compounds and carotenoids exist in many forms, each with different biological activity. The all-trans forms exhibit the highest biological activity. The international standards for vitamin A activity as related to vitamin A and b-carotene are as follows: 1 IU of vitamin A = 1 USP unit = vitamin A activity of 0.300 µg of crystalline all-trans retinol, which is equal to 0.344 µg of all-trans retinyl acetate or 0.550 µg of all-trans retinyl palmitate (Anonymous, 1963). All-trans b-carotene is the reference standard for provitamin A. It is the major carotenoid pigment in most plant feeds. Although the vitamin A equivalence used for b-carotene in this publication is 681 IU of vitamin A/mg of b-carotene, this value probably only applies to all-trans b-carotene fed at a level to meet the minimum requirement. The biological potency of b-carotene, relative to preformed vitamin A, is not a single standard value but is dependent on a number of factors, such as the level of supplementation, the previous nutritional history of the animal, and the response criteria used to determine the relative potencies (Myers et al., 1959; Faruque and Walker, 1970). Other factors that have been reported to influence the biological availability of carotene in natural feedstuffs are the mixture of carotenoid isomers present, the digestibility of the diet, the presence of antioxidants, and the protein and fat contents of the diet (Ullrey, 1972). Both vitamin A and b-carotene are subject to loss by oxidation. Stabilized vitamin A, which is resistant to oxidation, may be added to diets of low-carotene content. The vitamin A value of carotene from artificially dehydrated alfalfa meal ranged from 254 to 520 IU/mg in a study with growing lambs (Myers et al., 1959). The vitamin A activity of carotenes in corn silage fed to lambs was 436 IU/mg (Martin et al., 1968). Sun-cured hay is usually lower in carotene than dehydrated hay. With the exception of yellow corn, grains are poor sources of vitamin A activity. Vitamin A is fat soluble and is stored in the body. Approximately 200 days are required to deplete entirely the vitamin A stores in the livers of ewe lambs previously pastured on green feed. Because of this storage, animals that graze on green forage during the normal growing season perform normally on low carotene diets for periods of 4 to 6 months. In situations where sheep are grazing forage low in carotene for extended periods, however, vitamin A deficiency can be prevented by intramuscular injection with a commercially available vitamin A preparation or by the addition of preformed vitamin A to the diet as part of a salt mixture or as a pasture supplement. Vitamin A is involved in a number of physiological functions in animals. It is essential for the stimulation of growth, the proper development of skeletal tissues, normal reproduction, vision, and the maintenance of normal epithelial tissue (Moore, 1957; Weber, 1983). Consequently, vitamin A deficiency results in clinical deficiency signs such as growth retardation; bone malformation; degeneration of the reproductive organs; night blindness; increased CSF pressure; and keratinization of the respiratory, alimentary, reproductive, urinary, and ocular epithelia (Moore, 1957; Weber, 1983). Also, a deficiency can result in the production of lambs that are weak, malformed, or dead at birth. Retained placenta also is encountered with ewes deficient in vitamin A. Available high-potency vitamin A preparations and the common practice of vitamin A fortification of sheep diets necessitates caution because acute and chronic vitamin A toxicities have been reported for several animal species. For example, growing calves fed daily retinol intakes in excess of 2,200 µg/kg live weight (150 times the requirement) for 12 weeks exhibited changes in serum constituents and bone composition (Hazzard et al., 1964). Vitamin D Vitamin D activity is measured in international units (1 IU = 1 USP unit = antirachitic activity of 0.025 µg crystalline D3) (Windholz et al., 1983). The vitamin D requirement for all classifications of sheep except early-weaned lambs is 555 IU/100 kg live weight per day; for early-weaned lambs, it is 666 IU/100 kg live weight per day. These are the same values used in the 1975 publication on sheep (NRC, 1975) and are based on the research of Andrews and Cunningham (1945). These values are only slightly higher than those proposed by ARC (1980) for all classes of sheep (520 IU/100 kg live weight). Sheep use vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) equally well (Church and Pond, 1974). Recent research indicates that cholecalciferol is converted to active forms in the liver and kidney and acts in metabolism by affecting calcium absorption, deposition, and mobilization from bone (DeLuca, 1974; 1976). Vitamin D is fat soluble and stored in the body and therefore is less important in mature animals, except in the case of pregnancy, when demands are greater. Congenital malformations in the newborn may result from extreme vitamin D deficiencies. Vitamin D is required in addition to calcium and phosphorus for preventing rickets in young lambs and osteomalacia in older sheep, but newborn lambs are provided with enough vitamin D from their dams to prevent early rickets if their dams have adequate storage (Church and Pond, 1974). Animals exposed to sunlight generally obtain sufficient vitamin D through ultraviolet irradiation. Animals with
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Page 24 white skin or short wool receive more vitamin D activity through irradiation than animals with black skin or long wool. Sheep on pasture seldom need additional vitamin D, but under some conditions rickets has been observed (Fitch, 1943; Crowley, 1961; Nisbet et al., 1966). The question of adequacy arises if the weather is cloudy for long periods (Crowley, 1961) or if sheep are maintained indoors (Hidiroglou et al., 1979). Under these conditions, it is especially important that attention be given to the vitamin D content of diets of fast-growing lambs. Sun-cured hays are good sources of vitamin D. Dehydrated hays, green feeds, seeds, and by-products of seeds are poor sources. Vitamin D is subject to loss by oxidation. Although it oxidizes more slowly than vitamin A, its stability is poor when it is mixed with minerals (and especially poor when it is mixed with calcium carbonate). Use of high-potency vitamin D preparations in animal feeds requires caution. Excess vitamin D causes abnormal deposition of calcium in soft tissues and brittle bones subject to deformation and fractures (Church and Pond, 1974). Nevertheless, the amounts of vitamin D necessary to produce signs of toxicity are many times greater than the amounts required for nutritional purposes. Vitamin E Vitamin E is essential for all sheep, but unlike vitamin A, it does not appear to be stored in the body in appreciable concentrations (Rammell, 1983). On a practical basis, vitamin E fortification of the diet is more critical for young lambs than for older sheep. Recent estimates of the vitamin E requirements of ruminants vary from 10 to 60 mg/kg diet DM (NRC, 1975; ARC, 1980; NRC, 1984). This is not unexpected, since there are few studies specifically designed to determine requirements and actual requirements depend on the levels of selenium, polyunsaturated fatty acids, and (possibly) sulfur in the diet, as well as on the physiological status of the sheep and measurements used to assess deficiency (Muth et al., 1961; Hintz and Hogue, 1964; Rammell, 1983). Rousseau et al. (1957) reported no signs of vitamin E deficiency in lambs fed 51.3 mg of d-a-tocopheryl acetate per kilogram DM. Ewan et al. (1968) found that 11.0 mg/kg live weight of dl-a-tocopherol added weekly to lamb diets containing 0.1 to 1.0 ppm selenium prevented deaths due to white muscle disease (WMD) and maintained serum enzymes within the normal range. For a 10-kg lamb consuming 0.6 kg of feed per day (DM basis), this is equivalent to a dietary tocopherol concentration of 26.2 mg/kg DM. Dietary supplementation with 20 IU of vitamin E/kg of feed was successful in preventing nutritional muscular dystrophy (NMD) in rapidly growing (› 300 g/d) early-weaned lambs fed a dystrophogenic diet (Sharman, 1973). Data summarized by ARC (1980) indicated that the minimum requirements for vitamin E in the diet of growing or pregnant sheep were between 10.0 and 15.0 mg/kg DM. If dietary selenium levels are below 0.05 ppm, however, even 15 to 30 mg of vitamin E/kg DM may prove inadequate. For young beef calves (NRC, 1984), 15 to 60 mg of dl-a-tocopheryl acetate per kilogram DM is suggested. In the absence of more definitive information on the vitamin E requirements of sheep, the following levels are recommended: lambs under 20 kg live weight should receive 20 IU/kg DM and lambs over 20 kg live weight and pregnant ewes should receive 15 IU/kg DM. (The IU is defined as 1 mg of dl-a-tocopheryl acetate; 1 mg dl-a-tocopherol has the biopotency of 1.5 IU of vitamin E activity.) The above recommendations assume that dietary selenium levels are › 0.05 ppm. Vitamin E is now recognized as an important biological antioxidant. It functions in the body's intracellular defense against the adverse effects of reactive oxygen and free radicals (Rammell, 1983) and, as such, plays an important role in maintaining the integrity of biological cell membranes. Its mode of action is not well defined, but it is closely associated with selenium in metabolism. Some signs of vitamin E deficiency, such as WMD or NMD, may respond to either selenium or vitamin E or may require both (Hopkins et al., 1964; Ewan et al., 1968). Vitamin E and selenium also appear to have an additive effect on reducing serum levels of glutamic-oxalacetic transaminase (GOT), increasing survival time, and decreasing the level of urinary creatine excretion in deficient lambs less than 8 weeks old (Ewan et al., 1968). The signs of WMD in nursing lambs are stiffness (especially in the rear quarters), tucked-up rear flanks, and arched back. On necropsy, the disease is shown as white striations in cardiac muscles and is characterized by bilateral lesions in skeletal muscles. Serum levels of the enzymes glutamic-oxalacetic transaminase and lactic dehydrogenase are elevated, indicating muscle damage. Blood levels of the selenium-containing enzyme glutathione peroxidase are reduced. Affected lambs often die of pneumonia, starvation, or heart failure (Suttle and Linklater, 1983). Vitamin E blood plasma levels of 0.3 mg a-tocopherol/dl are considered marginal in cattle, and similar values may apply to sheep (Adams, 1982). Wheat germ meal, dehydrated alfalfa, some green feeds, and vegetable fat are good sources of vitamin E. Grains and grass hays are fair to good sources, but variations in levels are considerable. Protein-rich feeds such as fish and meat meal and solvent-extracted soybean and cottonseed meals are relatively poor sources. The level of vitamin E in ensiled forages is questionable (Bunnell et al., 1968; Kivimae and Carpena, 1973). Reports by Bunnell et al. (1968) and Adams (1982) suggest that a-tocopherol levels in feedstuffs may be lower than previously reported. Furthermore, the extreme
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Page 25 variations in a-tocopherol levels in the same kind of feeds as affected by stage of harvest, storage (oxidation may reduce levels 50 percent in 1 month), length of time between cutting and dehydrating, grinding of grains, stresses (such as adding minerals or fat in mixed feeds), and pelleting detract measurably from the reliability of book values for a-tocopherol content of rations. For example, the a-tocopherol content of 12 samples of 17 and 20 percent dehydrated alfalfa ranged from 28 to 141 mg/kg. Adams (1982) reported a range in plasma a-tocopherol values of 0.01 to 2.2 mg/dl among feedlot cattle. Of 286 plasma samples, 60 percent were below 0.3 mg a-to-copherol/dl, a level generally considered borderline between adequate and deficient. Based on average a-tocopherol contents of feedstuffs (Bunnell et al., 1968) generally used in lamb growing-finishing rations (corn, soybean meal, and alfalfa hay), the typical ration may contain less than 15 mg a-tocopherol/kg, which could result in inadequate intake of vitamin E. In addition, preintestinal destruction of vitamin E increases from 8 to 42 percent of an orally administered dose as the corn content of the diet increases from 20 to 80 percent (Alderson et al., 1971). Many sheep rations, heretofore believed adequate in vitamin E, may be inadequate, explaining the sporadic outbreaks of WMD in areas considered adequate in selenium. Values for the vitamin E requirements of sheep are presented in Tables 1, 2, and 9. The values presented in Table 1 were calculated from values per kilogram of dry feed consumed, given in Table 2. Table 9 presents daily vitamin E requirements for lambs and the suggested amounts of a-tocopheryl acetate to add to rations to provide 100 percent of these requirements. Vitamin B Complex The B vitamins are not required in the diet of sheep with functioning rumens, because the microorganisms synthesize these vitamins in adequate amounts. Lambs fed a niacin-deficient diet for 8 months have developed normally (Winegar et al., 1940). Mature sheep fed a diet low in thiamin, riboflavin, pyridoxine, and pantothenic acid have synthesized these vitamins in their rumens (McElroy and Goss, 1940 a,b; 1941 a,b). Cobalt is necessary for the synthesis of vitamin B12 (cyanocobalamin) in the rumen (see the section on Cobalt, p. 18). There is no evidence that supplementation with the vitamin B complex affects the performance of ewes during breeding and pregnancy (Miller et al., 1942). Before their rumens are developed, young lambs (up to 2 months of age), if early weaned, have a dietary need for B vitamins. A thiamin-responsive disease condition has been reported in feedlot lambs fed diets with high levels of grain and little roughage (see the section on Polioencephalomalacia, p. 27) (Barlow, 1983). Vitamins K1 and K2 Vitamins K1 (phylloquinone) and K2 (menaquinone) are fat soluble, and one or the other is necessary in the blood-clotting mechanism. Green leafy materials of any kind, fresh or dry, are good sources of vitamin K1 (Church and Pond, 1974). Vitamin K2 is normally synthesized in large amounts in the rumen, and no need for dietary supplementation has been established (McElroy and Goss, 1940a; Matschiner, 1970).
Representative terms from entire chapter: