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1
Energy
Energy is produced when organic molecules undergo oxidation. Energy is either released as heat or is trapped in high—energy bonds for subsequent use for the metabolic processes in animals.
Energy content in feedstuffs can be expressed as calories (cal), kilocalories (kcal), or megacalories (Mcal) of gross energy (GE), digestible energy (DE), metabolizable energy (ME), or net energy (NE). Energy can also be expressed as joules (J), kilojoules (kJ), or megajoules (MJ) (1 Mcal = 4.184 MJ; 1 MJ = 0.239 Mcal; 1 MJ = 239 kcal). The terms used in this publication to describe energy requirements and energy content of feeds are similar to those defined and extensively discussed in Nutritional Energetics of Domestic Animals and Glossary of Energy Terms (National Research Council, 1981). Whittemore and Morgan (1990), Chwalibog (1991), Ewan (1991), Noblet and Henry (1991), and Hoffmann (1994) have published reviews of energy utilization by swine.
Determination of the energy values of feedstuffs for swine is a difficult and tedious task. Originally, energy values were estimated from studies with chicks or were calculated from Total Digestible Nutrients (TDN) (National Research Council, 1971). Since the original direct determinations of energy in feedstuffs for pigs by Diggs et al. (1959, 1965) and Tollett (1961), the database has grown. A summary of energy values of feedstuffs from around the world has been compiled by Ewan (1996). Still, where data are not available by direct means from pig studies, energy concentrations can only be estimated from chemical composition of the feedstuff. Prediction equations that have been used for estimating energy concentrations in feeds are given in the subsequent sections. In all of these equations, the energy and nutrient concentrations are expressed on a dry matter basis.
Classification Of Energy
Gross Energy
Gross energy is the energy liberated when a substance is combusted in a bomb calorimeter. The GE concentration of a feed ingredient is dependent on the proportions of carbohydrate, fat, and protein present in the ingredient. Water and minerals contribute no energy; carbohydrates provide 3.7 (glucose) to 4.2 (starch) kcal/g, protein provides 5.6 kcal/g, and fat provides 9.4 kcal/g. If the composition of a feed is known, GE can be predicted fairly accurately. The following relationship was reported by Ewan (1989) for predicting GE (kcal/kg) from ether extract (EE), crude protein (CP), and ash.
Digestible Energy
Dietary GE intake minus the GE of the excreted feces is DE. Apparent indigestible energy is a major variable in the evaluation of feed ingredients. Farrell (1978), Agricultural Research Council (1981), and Morgan and Whittemore (1982) suggest that DE is preferable in describing the energy requirements of swine and the energy content of swine feeds, because DE is easily and precisely determined and is, in principle, additive. In addition, DE values are available for most of the commonly used feeds. However, in the conventional scheme of energy utilization, DE is apparent, not true, because fecal metabolic energy is not considered.
Chemical composition of feed ingredients is a major determinant of DE, with positive effects of ether extract and negative effects of fiber and ash. The following equations have been reported for predicting DE (kcal/kg) from chemical composition:
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in which SCHO is soluble carbohydrate calculated as 100 - (% CP + % EE + % Ash + % NDF), ADF is acid detergent fiber, NDF is neutral detergent fiber, and CF is crude fiber.
Digestibility of dietary energy increases slightly with increased body weight (Noblet and Shi, 1993) because of increased degradation of undigested carbohydrate in the large intestine. Noblet and Shi (1993) proposed that for finishing pigs and particularly sows fed at restricted feed intakes, DE concentrations (kcal/kg) should be corrected by one of the following relationships.
Metabolizable Energy
The DE minus the GE of gaseous and urinary losses is metabolizable energy (ME). The loss of energy as gas produced in the digestive tract of swine is usually between 0.1 and 3.0 percent of DE (Noblet et al., 1989b; Shi and Noblet, 1993). These amounts are generally ignored because they are small and not easily measured. For most practical swine diets used in North America, ME is 94 to 97 percent of DE, with an average of 96 percent (Farrell, 1979; Agricultural Research Council, 1981).
A correction is sometimes made to ME concentrations for nitrogen gained or lost from the body (MEn, Morgan et al., 1975). ME is corrected to nitrogen equilibrium because the energy that is deposited as retained protein cannot be totally recovered by the animal if the amino acids are degraded for energy. This correction to nitrogen equilibrium may be valid for mature animals but is not valid for growing pigs that retain considerable amounts of nitrogen. Therefore, the correction probably is not necessary (Farrell, 1979) or should be made to a constant positive nitrogen retention. The correction factor that is used has been obtained by expressing the GE of urine per gram of urinary nitrogen. For swine, Diggs et al. (1959) used a correction factor of 6.77, Morgan et al. (1975) used 9.17, and Wu and Ewan (1979) used 7.83 kcal of ME/g of nitrogen to correct for each gram of nitrogen above or below nitrogen equilibrium. This correction is added to the determined ME for pigs in negative nitrogen balance and subtracted when animals are in positive nitrogen balance.
If protein is of poor quality or in excess, ME decreases because the amino acids not used for protein synthesis are catabolized and used as a source of energy, and the nitrogen is excreted as urea. Therefore, as the nitrogen content of the urine increases, the energy losses in the urine increase and the ME of the diet decreases.
Estimates of ME (kcal/kg) may be calculated from DE (kcal/kg) and CP using one of the following relationships.
The ME of diets fed to finishing pigs or to sows fed at restricted intakes increases because digestibility is improved. Noblet and Shi (1993) proposed that ME concentrations (kcal/kg) determined with growing pigs (<60 kg) should be adjusted by one of the following relationships for finishing pigs and sows.
Net Energy
Net energy (NE) is the difference between ME and heat increment (HI). The HI is the amount of heat released because of the energy costs of the digestive and metabolic processes. The energy of the HI is not used for productive processes but can be used to maintain body temperature in cold environments. Net energy, therefore, is the energy that the animal uses for maintenance (NEm) and production (NEp). The energy used for maintenance (NEm) is also dissipated as heat, so that total heat production is the sum of HI and NEm. Evaluation of NE requires the measurement of energy balance or heat production. If energy is required to maintain body temperature or excess activity, NEp is reduced. Although difficult to measure, NE is the best indication of the energy available to an animal for maintenance and production (Noblet et al., 1994).
For pigs fed conventional diets and kept at thermoneutral temperatures, the ratio of NE to ME ranged from 0.66
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to 0.75 (Thorbek, 1975; Noblet et al., 1994). Ewan (1976), Phillips and Ewan (1977), and Pals and Ewan (1978) reported the efficiency of ME utilization for energy gain and maintenance (NE) in growing pigs to vary from 27 percent for wheat middlings, to 69 percent for corn, to 75 percent for soybean oil. Noblet et al. (1994) reported efficiencies of energy utilization of 90, 82, 80, 72, and 60 percent for rapeseed oil, cornstarch, sucrose, and mixtures of protein and fiber sources, respectively, for pigs ranging in weight from 45 to 150 kg. Some of the reported relationships between NE (kcal/kg) and chemical composition are as follows:
in which St is starch.
Heat Production
Measurement of total heat production includes the energy associated with HI, the energy required for maintenance, and energy expended in response to changes in the environment. The major environmental factors that influence heat production are temperature and physical activity.
Temperature
Cold thermogenesis influences energy requirements when the ambient temperature (T, °C) is below the critical temperature (Tc, °C). The critical temperature is the point below which an animal must increase heat production to maintain body temperature. Below Tc, the pig must increase its rate of metabolic heat production to maintain homeothermy (National Research Council, 1981). Factors that alter the rate of energy exchange between the animal and its environment will alter Tc (National Research Council, 1981). The energy cost of cold thermogenesis can be described by the following equation:
where MEHc is energy cost of cold thermogenesis, BW is animal weight in kg, and Tc and T are expressed in °C (Agricultural Research Council, 1981). Verstegen et al. (1982) estimated that during their growth period, from 25 to 60 kg, pigs needed an additional 25 g of feed/day (80 kcal of ME/day) to compensate for each 1°C below Tc. During the finishing period, from 60 to 100 kg, pigs required an additional 39 g of feed/day (125 kcal of ME/day) for each 1°C below Tc.
For each 1°C below the lower critical temperature (18 to 20°C), there is an increase in heat production of approximately 3.7 to 4.5 kcal of ME/kg of body weight raised to the 0.75 power (BW0.75) (Noblet et al., 1985; Close and Poorman, 1993). The lower critical temperature is reduced by group housing, by use of bedding, and by decreased ventilation rate. For 180-kg sows in normal condition individually housed on concrete, the increase in energy required to maintain body temperature is about 4 percent of maintenance requirement per °C below the lower critical temperature (Verstegen et al., 1987).
Between the upper and lower critical temperatures, a zone of thermoneutrality exists where heat production is relatively stable. Environmental temperatures above the critical temperature will reduce feed intake (Ewan, 1976). The National Research Council (1987) suggested that DE intake is reduced by 1.7 percent for each 1°C that the effective ambient temperature of the pig exceeds the upper critical temperature. Here, effective ambient temperature is the temperature the animal experiences.
Activity
Physical activity also influences heat production. Petley and Bayley (1988) measured the heat production of pigs running on a treadmill and reported that heat production of the exercised pigs was 20 percent greater than that of control animals. Close and Poorman (1993) calculated that the additional expenditure of energy by growing pigs for walking was 1.67 kcal of ME/kg of BW for each kilometer. Noblet et al. (1993) measured the increase in heat production associated with standing by sows as 6.5 kcal of ME/kg of BW0.75 for each 100 minutes. This figure was similar to reports by Hörnicke (1970) of 7.2, by McDonald et al. (1988) of 7.1, by Susenbeth and Menke (1991) of 6.1, and by Cronin et al. (1986) of 7.6 kcal/kg of BW0.75 for each 100 minutes. Noblet et al. (1993) also determined that the energy cost of consuming feed was 24 to 35 kcal of ME/kg of feed consumed.
Energy Requirements
Maintenance
The ME requirement for maintenance (MEm) includes the needs of all body functions and moderate activity. These
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requirements are usually expressed on a metabolic body weight basis, which is defined as body weight raised to the 0.75 power (BW0.75). Other exponents have been suggested as more appropriate: 0.67 (Heusner, 1982); 0.60 (Noblet et al., 1989b); 0.42 (Noblet et al., 1994). Estimates of the MEm requirement/kg of BW0.75 vary from 92 to 160 kcal/day, with most values falling between 100 and 125 kcal/day. The mean estimate for MEm is 106 kcal of ME/kg of BW0.75/day (Whittemore, 1976; Böhme et al., 1980; Wenk et al., 1980; Agricultural Research Council, 1981; Noblet and Le Dividich, 1982; Campbell and Dunkin, 1983; Close and Stanier, 1984; McNutt and Ewan, 1984; Gadeken et al., 1985; Noblet et al., 1985), which is equivalent to 110 kcal of DE/kg of BW0.75. However, Whittemore (1983) suggested that MEm can be more accurately described as:
where Pt is the whole body protein mass in kg.
Robles and Ewan (1982) reported daily NE requirements for maintenance (NEm) as 71 kcal/kg of BW0.75; Just (1982c) reported NEm as 78 kcal/kg of BW0.75; and Noblet et al. (1994) reported this figure as 86 kcal/kg of BW0.42.
During gestation, 60 to 80 percent of the total energy requirement is used for maintenance. The National Research Council (1988) concluded from the available literature that the daily requirement for maintenance of pregnant sows was 106 kcal of ME or 110 kcal of DE/kg of BW0.75 /day. Noblet et al. (1990), on the basis of recent estimates, concluded that the daily requirement was 105 kcal of ME/kg of BW0.75 for primiparous and multiparous sows. Beyer et al. (1994) reached a similar conclusion from the literature (103 kcal of ME/kg of BW0.75/day) for primiparous sows but reported data to indicate an increase from 93 kcal in the first parity to 104 kcal in the second parity and to 113 kcal of ME/kg of BW0.75 in the fourth parity. Whittemore and Yang (1989) reported the daily requirement as 115 kcal of ME/kg of BW0.75 from observations over four parities during gestation, lactation, and the interval from weaning to conception. Based on the literature, there seems little justification for altering the value used for growing pigs of 106 kcal of ME/kg of BW0.75 (or 110 kcal of DE/kg of BW0.75) for the daily maintenance requirement. Whittemore and Morgan (1990) suggested that the maintenance requirement was proportional to body protein mass (Pt) by the following relationship.
The daily maintenance energy requirement for the lactating sow is presumably also 106 kcal of ME/kg of BW0.75 (or 110 kcal of DE/kg of BW0.75) (National Research Council, 1988), which is the same as that for the gestating sow. But some recent reports have suggested that the requirement of the lactating sow may be 5 to 10 percent higher than that of the gestating sow; the higher figure probably reflects the heat production associated with the synthesis of milk (Noblet and Etienne, 1986, 1987; Burlacu et al., 1986).
Noblet et al. (1989a) reported no difference in maintenance requirement among growing boars, barrows, and gilts (112 kcal of ME/kg of BW0.75 ). Kemp (1989) reported the maintenance requirement for mature boars as 99 kcal of ME/kg of BW0.75. McCracken et al. (1991) reported measurement of maintenance requirements of mature boars of 126 kcal of ME/kg of BW0.75. Although the limited data available may suggest a higher maintenance requirement for boars, the estimate suggested for growing pigs and sows is preferred (106 kcal of ME/kg of BW0.75 or 110 kcal of DE/kg of BW0.75).
Growth
Estimates for the energy costs of protein retention (MEDr) range from 6.8 to 14.0 Mcal of ME/kg, with a mean of 10.6 Mcal of ME/kg (Tess et al., 1984). Literature estimates of the energy costs of fat deposition (MEf) range from 9.5 to 16.3 Mcal of ME/kg, with a mean of 12.5 Mcal of ME/kg (Tess et al., 1984). Although the mean energy costs/kg of protein or fat deposited are approximately equal (Wenk et al., 1980), 1 kg of lean muscle tissue is only 20 to 23 percent protein, whereas I kg of adipose tissue is 80 to 95 percent fat. Therefore, the energy cost for muscle tissue production is considerably less than that for fat tissue deposition.
Pregnancy
The feed and energy requirements of the pregnant sow will vary with body weight, target body weight gain during pregnancy, and other management and environmental parameters. The Agricultural Research Council (1981), Cole (1982), Seerley and Ewan (1983), and Aherne and Kirkwood (1985) reviewed the effects of energy intake during gestation on sow weight gain and reproductive performance. Aherne and Kirkwood (1985) and Williams et al. (1985) suggested that sows should be fed and managed so that they gain 25 kg of maternal tissues throughout pregnancy for at least the first three or four parities. The weight of the placenta and other products of conception should be approximately 20 kg, for a total of 45 kg of gestational weight gain of the sow (Verstegen et al., 1987; Noblet et al., 1990).
In general, an increase in the energy intake of the pregnant sow above 6.0 Mcal of ME/day will increase maternal weight gain but will not significantly affect litter size at parturition (Elsley, 1973; Agricultural Research Council, 1981). Whittemore et al. (1984) reported that gestation feed intakes between 1.7 and 2.3 kg/day of sows maintained for five parities had no significant effect on the total number of pigs born. Sows receiving the lowest level of feed did
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have a higher overall culling rate, however. The majority of experiments on this topic have demonstrated that pig birth weights progressively increase when sow feed or energy intake increases during pregnancy. However, a birth weight increase with a maternal feed intake of more than 6.0 Mcal of ME/day is seldom significant (Libal and Wahlstrom, 1977; Henry and Etienne, 1978; Agricultural Research Council, 1981).
Increasing feed intake during early gestation does not affect the number of pigs born (den Hartog and van Kempen, 1980; Toplis et al., 1983). High levels of feed intake (> 2.5 kg/day) during the first three days after mating reduced embryo survival in gilts by about 5 percent in one study (Aherne and Williams, 1992) and by 15 percent in another (Dyck et al., 1980), but the reduction in survival does not consistently result in reduced litter size. Elsley et al. (1971) and Cromwell et al. (1980, 1989) demonstrated that the pattern of feed intake during pregnancy was less important in influencing sow performance than the total amount of feed given to the sows. Increasing feed intake in late gestation may increase the average birth weight of pigs (Hillyer and Phillips, 1980; Cromwell et al., 1982). Cromwell et al. (1989) also reported that by increasing feed intake 1.36 kg during the last 23 days of pregnancy, pig weight increased at birth by 40 g and at 21 days of age by 170 g. Weldon et al. (1991) reported that increased energy intake (5.76 to 10.5 Mcal of ME/day) of gilts from day 75 to 105 of pregnancy reduced mammary cell numbers and suggested that milk production may be reduced.
Pregnant sows offered feed ad libitum will consume more energy during gestation than required for maintenance and growth of the conceptus tissue, thus resulting in an increase in deposition of body fat and protein. As energy intake and weight gain during pregnancy increase, energy intake during lactation decreases and weight loss during lactation increases (Salmon-Legagneur and Rérat, 1962; Baker et al., 1969; Brooks and Smith, 1980; O'Grady, 1980; Cole, 1982; Williams et al., 1985; Weldon et al., 1994). Therefore, it is desirable to limit energy intake during pregnancy to control weight gain. The daily energy requirements for pregnancy include the costs of maintenance, energy required for the deposition of protein and fat in the maternal tissue, and energy requirements of the conceptus.
Weight gain during pregnancy is a sum of maternal protein and fat deposition and the gain of the products of conception. Beyer et al. (1994) reported from a comparative slaughter experiment that the total weight gain of the uterus, uterine fluids, products of conception, and mammary tissue was 22.8 kg for sows fed three levels of energy during the first, second, or fourth parity. Assuming a litter size of 10 pigs, this equates to 2.28 kg per pig. The weight gain of protein was 2.46 kg and of fat was 0.46 kg. Total energy gain was 19.94 Mcal. Total maternal weight gain was dependent on the amount of energy consumed. They found that there was an obligatory deposition of about 20 Mcal of NE due to pregnancy, or 174 kcal of NE/day. Assuming an efficiency of utilization of ME for NE of 0.486 (Noblet and Etienne, 1987), the energy requirement for pregnancy would be 358 kcal of ME/day. Additional energy above the maintenance and pregnancy requirement would be used for maternal gain, presumably with the same efficiency as for growth.
Lactation
The long-term reproductive efficiency of the sow is best served by minimizing weight loss during lactation (Dourmad et al., 1994). Such a strategy requires only minimal restoration of weight in the next pregnancy. The daily energy requirements during lactation include a requirement for maintenance (MEm) and a requirement for milk production. The energy requirement for milk production can be estimated from the growth rate of the suckling pig and the number of pigs in the litter (Noblet and Etienne, 1989):
in which milk energy is in kcal of GE/day, ADG is the growth rate of the suckling pig averaged over the lactation period (g/day), and pigs is the number of pigs in the litter. Assuming that the efficiency of conversion of dietary energy to milk energy is 0.72 (Noblet and Etienne, 1987), the relationship is as described below.
If dietary energy intake is not adequate to meet the demands of maintenance and milk production, tissue will be mobilized to provide the necessary nutrients for milk production. Noblet and Etienne (1987) concluded that the efficiency of conversion of tissue energy to milk energy is 0.88; this figure suggests that the major source of energy used is fat.
Developing Boars and Gilts
Developing boars and gilts should be given ad libitum access to diets until selected as breeding animals at about 100 kg BW to allow evaluation of the potential growth rate and lean gain. After the animals are selected for the breeding herd, energy intake should be restricted to achieve the desired weight at the time the animals are used for breeding (Wahlstrom, 1991).
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Sexually Active Boars
The energy requirement of the working boar is the sum of the energy required for maintenance, mating activity, semen production, and growth. Kemp (1989) reported that the heat production associated with the collection of semen when mounting a dummy sow was 4.3 kcal of DE/kg of BW0.75. Close and Roberts (1993) estimated the energy required for semen production from the average energy content of each ejaculation (62 kcal of DE) and an estimate of the efficiency of energy utilization (0.60). The energy required was 103 kcal of DE per ejaculation.
Energy Sources
Sugars and Starch
Satisfactory survival and growth rates of pigs fed diets containing high levels of different sugars suggest that glucose and lactose are the sugars most effectively utilized by pigs less than 7 days old (Kidder and Manners, 1978; Sambrook, 1979). Pigs less than 7 days old fed diets containing fructose or sucrose develop severe diarrhea, weight loss, and high mortality (Becker and Terrill, 1954; Aherne et al., 1969). After pigs reach 7 to 10 days of age, they can utilize fructose and sucrose.
Starch is the main carbohydrate and energy source in most diets fed to pigs. However, pigs less than 2 to 3 weeks old fed diets containing large amounts of starch do not grow as well as pigs fed diets in which glucose, lactose, or sucrose is the carbohydrate source. The poor growth was attributed to insufficient pancreatic amylase and intestinal disaccharidases (Cunningham, 1959; Sewell and Maxwell, 1966). After pigs are 2 or 3 weeks old, their digestive enzyme systems can digest cereal starch more efficiently. Pigs can then be fed starchor cereal-based diets (Becker and Terrill, 1954; Cunningham, 1959; Sewell and Maxwell, 1966).
Nonstarch Polysaccharides
Crude fiber determination is an imprecise analytical procedure. Cellulose, hemicellulose, and lignin in crude fiber are 50 to 80 percent, 20 percent, and 10 to 50 percent, respectively, for typical feedstuffs (Van Soest and McQueen, 1973). In view of the diverse composition of fiber, methods have been developed to quantify fiber based on solubility. Neutral detergent fiber (NDF) is an estimate of the total plant cell wall, which consists primarily of cellulose, hemicellulose, and lignin (Goering and Van Soest, 1970). Acid detergent fiber (ADF) is an estimate of cellulose and lignin. The difference between NDF and ADF is the estimated hemicellulose content of a feed sample (Goering and Van Soest, 1970).
The addition of fiber (crude fiber, NDF, ADF) to swine diets decreases the DE concentration of the diet (King and Taverner, 1975; DeGoey and Ewan, 1975; Kennelly et al., 1978; Kennelly and Aherne, 1980b). Increased feed intake generally results as the pig attempts to maintain DE intake (Baird et al., 1975; Agricultural Research Council, 1981; Low, 1985). When dietary crude fiber exceeds 10 to 15 percent of the diet, however, feed intake may be depressed because of excessive bulk or reduced palatability (Braude, 1967). Low-energy (high-fiber) diets will support growth rates equal to those of pigs fed higher-energy diets during periods of low environmental temperatures, but diets of this type usually depress the growth rate during periods of high temperatures (Coffey et al., 1982; Stahly, 1984).
Utilization of fiber by nonruminants has been shown to vary considerably, depending on the fiber source (Bell, 1960; Nehring and Uhlemann, 1972; Laplace and Lebas, 1981), degree of lignification (Forbes and Hamilton, 1952), level of inclusion (Farrell and Johnson, 1970; Just, 1979), and extent of processing (Saunders et al., 1969; McNab, 1975). Fiber utilization is also influenced by the physical and chemical composition of the total diet (Schneider and Lucas, 1950; Myer et al., 1975), level of feeding (Cunningham et al., 1962), age and weight of the animal (Zivkovic and Bowland, 1970), adaptation to the fiber source (Pollman et al., 1979), and individual variation among pigs (Keys et al., 1970; Farrell, 1973; King and Taverner, 1975). When these factors are considered, it is not surprising that the digestibility of fiber has been shown to vary between 0 and 97 percent (Rérat, 1978) and that the literature contains conflicting reports about the effects of fiber on the digestibility of nutrients. Just (1982a) reported that an increase in 1 percent of dietary crude fiber depressed digestibility of gross energy by approximately 3.5 percent.
Fibrous components of the diet are poorly digested in the small intestine and provide substrates for microbial fermentation in the large intestine. The principal end products of microbial fermentation in the large intestine are volatile fatty acids (VFA). The caloric contribution of VFA to swine has been estimated at values ranging from about 5 to 28 percent of the maintenance energy requirement, depending on the level and frequency of feeding and the fiber level of the diet (Friend et al., 1964; Farrell and Johnson, 1970; Imoto and Namioka, 1978; Kim et al., 1978; Kass et al., 1980; Kennelly et al., 1981). Energy derived from fermentation in the large intestine is utilized with lower efficiency than energy digested in the small intestine (52 versus 76 percent [Noblet et al., 1994]; 57 versus 74 percent [Hoffmann et al., 1990]).
There is disagreement concerning the influence of fiber on protein digestibility. Several reports suggest that when the source of fiber does not contribute significant amounts of protein to the diet, then an increase in the level of fiber
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does not affect protein digestibility significantly (Gouwens, 1966; Friend, 1970; Eggum, 1973; Kennelly and Aherne, 1980a). Other researchers have observed, however, that an increase in the dietary level of fiber decreases protein digestibility (Pond et al., 1962; Cole et al., 1967; Kass et al., 1980; Just et al., 1983; Frank et al., 1983; Noblet and Perez, 1993).
Lipids
The term "lipid" includes both fats and oils. Originally, linoleic and arachidonic acids were both identified as essential fatty acids (EFA) that must be supplied in the diet (Cunnane, 1984). Now it is recognized that these fatty acids are members of N–6 series of EFA and that arachidonic acid can be derived in vivo from linoleic acid. It is difficult to produce overt signs of an EFA deficiency in pigs. Enser (1984) has reported normal growth in pigs from weaning to slaughter weight when they are fed diets containing only 0.1 percent linoleic acid. The Agricultural Research Council (1981) suggested the EFA requirements are 3.0 percent of dietary DE for pigs up to 30 kg and 1.5 percent of dietary DE from 30 to 90 kg. These are equivalent to about 1.2 and 0.6 percent of the diet. Christensen (1985) reported that for maximum performance and efficiency of feed utilization, pigs weaned at 5 weeks of age and raised to 100 kg BW require a dietary linoleic acid of 0.2 percent of GE, or about 0.1 percent of the diet. This level of linoleic acid is usually present in diets based on commonly used cereal grains and protein supplements. In addition to EFA of the N–6 series, pigs probably require EFA of the N–3 series. However, practical diets also contain adequate amounts of these EFA. Therefore, the main concern is the use of lipids as an energy source. Energy concentrations of selected fats are presented in Chapter 11 (Table 11-10).
The value of adding fat to the diets of weanling pigs is uncertain. Pettigrew and Moser (1991) summarized data involving 92 comparisons of fat additions for pigs from 5 to 20 kg. In this weight range, addition of fat reduced growth rate and feed intake while it improved gain-to-feed ratio. The response of growth rate was small (0.01 kg) and variable, with similar numbers of positive (37) and negative (38) responses. Inconsistent responses to added fat may be a result of a number of factors, including the age of the pig at the start of the experiment, the amount of fat added, the type of fat, and the method by which the fat was added. Pettigrew and Moser (1991) reported responses for studies in which a constant protein-to-energy ratio was maintained and found no response in growth rate, a reduction in feed intake, and an improvement in gain-to-feed ratio when fat was added.
These data suggest that there is an optimal protein-to-energy ratio for young pigs. Consequently, nutrient requirements often are expressed as the amount per Mcal of DE (Agricultural Research Council, 1981). Such an expression assumes that the optimal nutrient-to-energy ratio for maintenance is the same as for a high level of production. However, this assumption is probably not fully correct because the relative maintenance and gain requirements for specific nutrients probably differ from those for energy. Hence, the ratio will change, usually decreasing as the rate of production or body weight increases. The concept of a fixed optimal protein-to-energy ratio is not supported by the results of several experiments; in these, fat added to diets containing high levels of protein and other nutrients depressed the rate and efficiency of gain (Crampton and Ness, 1954; Smith and Lucas, 1956; Peo et al., 1957; Crampton et al., 1960). Clawson et al. (1962) found little correlation between rate or efficiency of gain and the protein-to-energy ratios. Tribble et al. (1979) and Lewis et al. (1980) reported that the addition of fat to the diet did not influence the lysine requirement of starter pigs fed sorghum- or corn-based diets. Cuaron et al. (1981) reported that protein-to-energy ratios within the range of 53 to 71 g of protein/Mcal of DE did not significantly influence the performance of starter pigs.
For growing-finishing swine (20 to 100 kg), the summary by Pettigrew and Moser (1991) indicated consistent improvement in growth rate, reduction in feed intake, improvement in gain-to-feed ratio, but an increase in back-fat thickness in response to addition of fat to swine diets. Chiba et al. (1991) reported that a ratio of 3.0 g of lysine (or 49 g of balanced protein) per Mcal of DE was necessary to maximize the beneficial effects of fat addition to diets. The digestibility of the dietary fat, quantity of ME and fat consumed, and environmental temperature in which pigs are housed influence the nutritional value of fat as an energy source for pigs (Stahly, 1984). In general, the substitution of fat for carbohydrate calories in a diet for pigs maintained in a thermoneutral environment increases growth rate and decreases the ME required per unit of body weight gain. But for pigs housed in a warm environment, voluntary ME intake increases by 0.2 to 0.6 percent for each additional 1 percent of fat added to the diet. This increase is because the heat increment of fat is less than that of carbohydrate (Stahly, 1984).
The age of the pig, chain length of the fatty acids in the fat, free fatty acid concentration, and unsaturated-to-saturated (U:S) fatty acid ratio influence the apparent digestibility of fat (Stahly, 1984). Dietary fat digestibility is low in the weaned pig and improves as the pig grows. The apparent digestibility of short- or medium-chain fatty acids (14 carbons or less) is high (80 to 95 percent), regardless of the dietary ratio of U:S fatty acids (Stahly, 1984). Powles et al. (1995) summarized a series of studies and reported a curvilinear increase in the digestibility of fat as the ratio of U:S fatty acids increased from 1 to 4. They
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also reported a linear decrease in digestibility as free fatty acid concentrations increased from 100 to 800 g/kg of fat. Apparent fat digestibility decreases by 1.3 to 1.5 percent for each additional 1 percent of crude fiber in the diet (Just, 1982a,b,c).
Evidence suggests that the addition of fat to the diets of sows during late gestation or lactation increases the milk yield, fat content of colostrum and milk, and survival of pigs from birth to weaning, especially for lightweight pigs (Moser and Lewis, 1980; Coffey et al., 1982; Seerley, 1984; Pettigrew and Moser, 1991). Improvements in survival of pigs from birth to weaning were dependent on the total amount of fat the sow consumed before farrowing (< 1,000 g) and the birth-to-weaning survival of the control groups (> 80 percent). Fat supplementation can also reduce sow weight loss during lactation and decrease the interval from weaning to mating (Moser and Lewis, 1980; Pettigrew, 1981; Cox et al., 1983; Seerley, 1984; Moser et al., 1985; Shurson et al., 1986; Pettigrew and Moser, 1991).
Voluntary Feed Intake
The control of feed intake is influenced by a number of factors in the following groups:
Physiological factors, including genetics, neural and hormonal mechanisms, and sensory factors, including olfaction and taste (Baldwin, 1985; Fowler, 1985; National Research Council, 1987);
Environmental factors, including environmental temperature, humidity, air movement, feeder design and location, number of pigs per pen, and available space per pig (National Research Council, 1987); and
Dietary factors, including deficiencies or excesses of nutrients, energy density, antibiotics, flavors, feed processing, and availability and quantity of water (Agricultural Research Council, 1981; Fowler, 1985; National Research Council, 1987).
The factors that affect feed intake have been extensively reviewed in Predicting Feed Intake of Major Food-Producing Animals (National Research Council, 1987). These values are for pigs allowed ad libitum access to a balanced corn–soybean meal diet. If the feed intake is restricted, as it sometimes is for gilts and boars used for breeding, the daily nutrient (but not energy) intakes must be maintained at least at the levels suggested for market pigs. To accomplish this, the nutrient-to-energy ratio of the diet must be increased. Voluntary energy intake formulas for various classes of swine are presented below.
Suckling Pigs
According to the National Research Council (1987), the DE intake of creep feed by the suckling pig can be expressed by the following relationship:
where day is age of the pig. The consumption of dry feed is not predicted until pigs are 13.5 days old.
Weanling Pigs
Based on a review of the literature, the National Research Council (1987) concluded that feed intake increases linearly during the postweaning period except for the first 24 hours after weaning, when little or no feed is consumed. Estimates of this rate of increase in feed intake range from 17 to 23 g/day for corn—soybean meal diets containing 3,200 kcal of DE/kg of feed. These data could be described by the following equation:
which describes the relationship of BW to the DE intake of the 5- to 15-kg pig.
Growing-Finishing Pigs
When growing-finishing pigs weighing 15 to 110 kg are allowed to consume feed ad libitum daily, the energy content of the diet generally controls the amount consumed (Agricultural Research Council, 1981; Cole, 1984; Chiba et al., 1991). Pigs will compensate for decreases or increases in the energy density of the diet by increasing or decreasing their feed intake (Owen and Ridgeman, 1967, 1968; Cole et al., 1968). Within limits, this compensation normalizes energy intake. However, voluntary feed intake varies considerably from day to day and among individual pigs (Frank et al., 1983). For pigs allowed ad libitum access to feed, energy intake is generally about 3 to 4 times the maintenance energy requirement.
The National Research Council (1987) described feed intake for pigs that weigh from 15 to 110 kg by an asymptotic relationship to body weight (Figure 1-1).
This equation is similar to a relationship reported by the Agricultural Research Council (1981).
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FIGURE 1-1
Digestible energy intake by growing-finishing pigs as an asymptotic function of body weight. Based on research conducted before 1983 and involving 8,072 observations of 1,390 pens of pigs fed nutritionally adequate corn–soybean meal diets (National Research Council, 1987).
Sows
Because feed intake is restricted during gestation, predictions of DE intake are not appropriate. For lactating sows, however, voluntary energy intake responds quadratically, as indicated by the following relationship:
where days is day postfarrowing (National Research Council, 1987). O'Grady et al. (1985) summarized feed intake during lactation from 3,559 sows and observed that feed intake increased with parity, number of pigs nursed, and lactation length but decreased with increased gestation weight gain.
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Representative terms from entire chapter:
growing pigs
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