convert calories to joules, i.e. 1 Mcal = 4.184 MJ, 1 MJ = 0.239 Mcal and 1 MJ = 239 kcal. Therefore the tables of feedstuff composition in this publication show DE and ME values expressed as MJ or kJ as well as kcal per kg.
Some countries in Europe such as Denmark and The Netherlands (CVB, 2000) use feeding systems based on NE. These systems are more precise than systems based on DE or ME but they need the requirements to be based on NE also, the data for which are not yet widely available.
Net energy is defined as ME minus the heat increment, which is the heat produced (and thus energy used) during digestion of feed, metabolism of nutrients and excretion of waste. The energy left after these losses is the energy actually used for maintenance and for production (growth, gestation, lactation). This means that the NE system is the only one that describes the energy that is actually used by the pig. NE is, therefore, the most accurate and unbiased way to date of characterizing the energy content of feeds. However, NE is much more difficult to determine and more complex than DE or ME, which may be a reason why it is not as widely used as it should be. Currently, only France, The Netherlands and Germany have developed NE systems to describe dietary NE contents, although research into NE has been conducted in several countries.
Digestible energy requirements are applied most precisely in the case of animals which are fed rationed amounts of feed, e.g. gestating sows. The DE requirement of the gestating sow varies with body weight, weight gain, environmental temperature and other factors. Assuming that the sow gains 25 kg in body weight during pregnancy and gains 20 kg in the form of piglets plus uterine and udder tissue, the maintenance need can be calculated as being similar to that of a growing-finishing pig and the need for gain can be calculated as 1.29 Mcal DE/kg. Thus the total requirement is 5.8–6.8 Mcal DE/day for a 120–160 kg animal at the start of pregnancy, equivalent to 1.8–2.0 kg daily of a maize/soy diet. The powerful anabolic effect of pregnancy can be seen in Table 3.1.
In Table 3.1 all animals received the same total amount of feed, yet the bred animals were able to produce a litter of pigs while achieving the same live weight after farrowing as the unbred animals. These data
Table 3.1. Growth and farrowing data for gilts bred or not bred at puberty and given restricted amounts of feed (from Friendet al., 1982).
Bred Unbred
Age at breeding (days) 163 166
Weight at breeding (kg) 98 98
Weight at 85 days (kg) 116 108
Weight at 100 days (kg) 122 113
Weight after farrowing (kg) 115 118
Piglets born (n) 9.1 –
Piglets born alive (n) 8.5 –
demonstrate, in a rather spectacular way, the increased efficiency of the pregnant animal and explain why gestating pigs need to be fed restricted amounts of feed to prevent them becoming too fat. This increased efficiency can be explained by hormonal changes in metabolism. For the lactation stage a milk yield of 5–7.5 kg/day may be assumed, also that the maintenance need is the same as for as for the pregnant sow. Thus the total need is 14.5–20.5 Mcal DE/day for sows of 145–185 kg post-farrowing, equivalent to 4.4–6.1 kg daily of a maize/soy diet.
In the case of other classes of pigs which are fed ad libitum, the DE requirements are met by regulating the DE content of the dietary mixture.
The main sources of energy are starch and fat. Very young weaned pigs need some simple sugars. Fibre is not well utilized and increased fibre in the diet may result in reduced digestibility of energy and protein. Although fibre cannot be digested in pigs it may be broken down to some extent by microbial action in the lower gut, providing 5–28% of the DE requirement for maintenance in the form of volatile fatty acids. In addition to providing energy, fats supply some essential fatty acids (EFA). According to the NRC (1998), linoleic acid is the only required EFA.
The requirement tables published in the NRC (1998) report assumead libitum feeding (feeding to appetite) in most cases. Where limit (restricted) feeding is used the requirements are set out as total amounts of nutrients to be consumed daily. Because of the need in most instances to try and obtain maximum intake of feed and in other instances to find ways of controlling feed intake, research has been conducted on the factors that control voluntary feed intake. Among the factors known to be important are diet composition: for instance, pigs will eat more of a low-protein diet than of an unbalanced protein diet; fat inclusion in the diet delays stomach emptying; intake is related inversely to energy concentration in the diet; a high environmental tempera- ture reduces appetite; and a low environmental temperature increases intake but the higher intake may be used mainly to maintain body temperature and not enhance growth. Also, it is known that castrates eat more than boars or gilts and that some breeds such as Durocs and crossbreeds eat more. In the case of piglets and growing pigs the digestibility and palatability of the diet, as well as the balance of dietary AA, have to be considered in ensuring a desirable feed intake. With finishing pigs nearing market weight it may be desirable to reduce the energy level of the diet to ensure good carcass grades. Bulky feeds are sometimes used with gestating sows to prevent them becoming too fat.
With lactating sows a high-energy feed may assist in maintaining adequate milk production when the appetite is low. Also the correct calcium and phosphorus contents of the diet have to be used otherwise intake can suffer. It is also known that too much feed in pregnancy leads to reduced intake during lactation.
Protein
The term protein usually refers to crude protein (CP; measured as nitrogen content × 6.25) in requirement tables. Protein is required in the diet as a source of AA, which can be regarded as the building blocks for the formation of muscle tissue, milk, etc. There are 22 different AA in the pig’s body, ten of which are essential (EAA; arginine, methionine, histidine, phenylalanine, isoleucine, threonine, leucine, tryptophan, lysine and valine), i.e. cannot be manufactured by the body and must be derived from the diet. Cystine and tyrosine can be regarded as semi-essential in that they can supply part of the sulphur-containing and aromatic AA requirements, respectively. The other ten are non-essential (NEAA) and can be made by the body. The most important EAA is lysine, which is usually the first-limiting AA and has to be at the correct level in the diet. The level of the first-limiting AA in the diet normally determines the use that can be made of the other EAA. If the limiting AA is present at only 50% of requirement, then the efficiency of use of the other AA will be limited to 50%. This concept explains why a deficiency of an individual AA is not accompanied by specific deficiency signs: a deficiency of any EAA results in a generalized protein deficiency. The primary sign is usually a reduction in feed intake that is accompanied by increased feed wastage, impaired growth and general unthriftiness. Excess AA are not stored in the body but are excreted in the urine as nitrogen compounds.
In most pig diets, a portion of each AA that is present is not biologically available to the animal. This is because most proteins are not fully digested and the AA are not fully absorbed. The AA in some proteins such as milk products are almost fully bioavailable, whereas those in other proteins such as certain plant seeds are less bioavailable. It is therefore more accurate to express AA requirements in terms of bioavailable AA.
For optimal performance the diet must provide adequate amounts of EAA, adequate energy and adequate amounts of other essential nutrients.
All of these must be of high bioavailability. Some variation in total dietary protein level can be tolerated, provided the correct levels of AA are maintained. Gilts and boars, which are leaner and eat less, need higher levels of EAA and CP in the diet than castrates which are fatter and eat more. Maximal carcass leanness requires a greater intake of EAA than does maximal growth rate.
The CP requirement values outlined by the NRC (1998) assume a maize/soy diet and have been adjusted for the average bioavailabilities of EAA in maize/soy diets. A separate table on bioavailabilities is now included in the NRC publication, allowing the dietary target values to be adjusted when diets based on other feedstuffs are formulated.
Bioavailability is measured as the proportion of dietary EAA that has disappeared from the gut when digesta reach the ileum. The NRC recommends formulation on the basis of bioavailable EAA if the ileal digestibility of lysine, methionine, threonine or tryptophan is less than 70%
or more than 90%.
The bioavailability of AA in a wide range of feedstuffs has been measured by researchers. The primary method has been to measure the proportion of a dietary AA that has disappeared from the gut when digesta reach the terminal ileum, using surgically altered pigs. Interpretation of the data is however somewhat complicated. The values determined by this method are more correctly termed ‘ileal digestibilities’ rather than bioavailabilities because AA are sometimes absorbed in a form that cannot be fully used in metabolism. Furthermore, unless a correction is made for endogenous AA losses, the values are ‘apparent’ rather than ‘true’.
Minimum endogenous losses are accounted for in the NRC (1998) estimates of requirements; thus both requirements and ingredient contents are expressed in terms of ‘true’ (or standardized) ileal digestible AA.
The estimates of requirement are based on the assumption that the pattern of dietary bioavailable EAA should remain relatively constant during all growth stages. This pattern has been called ideal protein.
The CP need is minimized as the dietary EAA pattern approaches that of ideal protein. The nearer the EAA composition of the diet is to ideal protein, the more efficiently the diet is utilized and the lower the level of nitrogen excretion. Energy is also used most efficiently at this point; thus both protein and energy utilization are maximized.
The ideal proportions of AA, based on digestible AA and lysine as the first-limiting AA, are shown in Table 3.2.
Cereal grains, such as maize, barley, wheat and sorghum, are the main ingredients of pig diets and usually provide 30–60% of the total AA requirements. Other sources of protein such as soybean meal and canola meal must be provided to ensure adequate amounts and a proper balance of EAA. The protein levels necessary to provide adequate intakes of EAA will depend on the feedstuffs used. Feedstuffs that contain ‘high-quality’
proteins (i.e. with an AA pattern similar to the pig’s needs) or mixtures of feedstuffs in which the AA pattern of one complements the pattern in another will meet the EAA requirements at lower dietary protein levels than Table 3.2. Ideal dietary amino acid pattern for pigs, relative to lysine at 100 (true digestible basis) (from Baker, 1996; Close and Cole, 2000).
Amino acid
Growing-finishing pigs (weight kg)
Lactating sows
Pregnant sows
5–20 20–50 50–100
Lysine 100 100 100 100 100
Isoleucine 60 60 60 60 70
Leucine 100 100 100 112 100
Methionine 30 30 30 30 30
Methionine + cystine 60 65 70 50 55
Phenylalanine + tyrosine 95 95 95 110 100
Threonine 65 67 70 60 70
Tryptophan 18 19 20 18 20
Valine 68 68 68 70 78
Histidine 32 32 32 35 33
feedstuffs with a less desirable AA pattern. This is important if one of the goals is to minimize nitrogen excretion.
The profile of AA in a feedstuff is a main determinant of its value as a protein source. If the profile indicates a relatively high content of lysine and is close to that of ideal protein (as in milk, fish or meat), it is considered a high-quality protein. Correct formulation of the diet ensures that the dietary AA (preferably on a bioavailable basis) are as close to ideal protein as possible and with minimal excesses of EAA.
The AA requirements of growing-finishing pigs, expressed in terms of dietary concentration, increase as the energy density of the diet increases.
Research data (Chibaet al., 1991a,b) indicate that, at higher or lower energy densities than those found in standard grain/soybean meal diets, AA requirements (expressed as a percentage of the diet) may need to be adjusted upwards or downwards, respectively. Therefore formulation should also ensure that the diet has the correct amount of energy in the diet to promote lean tissue growth. Too much energy in relation to lysine will lead to a fat carcass. Too little energy in relation to lysine will result in reduced muscle growth and protein wastage. Some nutritionists formulate pig diets using the ratio of DE to lysine to optimize pig performance. This ratio has not yet been well developed for general usage since the optimal DE:lysine ratio is not an absolute value but is affected by factors such as genotype, sex, health status and environmental conditions.
Estimated AA requirements are shown in the tables at the end of this chapter, based on the concept of ideal protein (NRC, 1998).
Minerals
Under natural conditions it is likely that pigs can obtain part of their mineral requirements by eating pasture and rooting in the soil. However, these sources cannot be guaranteed to provide all of the requirements consistently. Therefore pig diets must be supplemented with minerals.
Minerals required in large amounts are known as macro minerals. These include calcium, phosphorus, sulphur, sodium, chloride, potassium and magnesium. Minerals required in small amounts are called micro or trace minerals. These include iron, zinc, copper, manganese, iodine and selenium.
Minerals perform important functions in the animal body and are essential for proper growth, reproduction and lactation. In addition to being constituents of bone and teeth they take part in other essential processes. A lack of minerals in the diet can result in deficiency signs, including reduced or low feed intake, reduced rate of growth, soft or brittle bones, beading of the ribs, stiffness or malformed joints, posterior paralysis, goitre, unthriftiness, hairless piglets, breeding and reproductive problems, poor milk production and death.
Pigs need at least 14 mineral elements and it is possible that other minerals may also be essential in the body. Of the essential mineral elements, ten are likely to be deficient in pig diets. These are calcium,
phosphorus, sodium, chloride, cobalt, iron, copper, zinc, iodine and selenium, but cobalt is not needed if the diet contains sufficient vitamin B12. Deficiencies of the other four essential mineral elements are not common and the feeds used probably contain them in sufficient quantities. There are some indications that magnesium supplementation may be beneficial under certain situations.
Required minerals can be categorized as shown in Table 3.3.
Calcium and phosphorus
Calcium and phosphorus make up over 70% of the mineral content of the pig body, mainly combined with each other. Approximately 80% of the phosphorus and 99% of the calcium in the body are present in the bones and teeth. These figures indicate the importance of calcium and phosphorus in the diet and the role they play in giving rigidity and strength to the skeletal structure. An inadequate supply of either one in the diet will limit the utilization of the other. These two minerals are discussed together because there is a close relationship between them.
A deficiency of calcium is more likely than a deficiency of phosphorus.
Cereal grains, which constitute most of the pig diet, are quite low in calcium. The phosphorus content of cereal grains is higher, although about one-half or more is in the form of phytin which is relatively unavailable to the pig. As a result the requirements are now set out in terms of available phosphorus. Generally the calcium present in cereal grains and most feedstuffs is of higher availability than that of phosphorus. Legumes and pasture provide some calcium.
Dietary calcium and phosphorus requirements are considered to be higher for maximum bone development than for gain and feed efficiency.
Higher dietary mineral levels should also be considered for pregnant sows which are on a restricted daily feed intake of 1.8 kg. Calcium and phosphorus requirements of sows are higher with longer lactation periods and as the number of litters increases.
Table 3.3. Minerals required by pigs.
Macro minerals Trace minerals
Calcium (Ca)a Cobalt
Chlorine (Cl)a,c Copper (Cu)b
Magnesium (Mg) Iodine (I)c
Phosphorus (P)a Iron (Fe)b
Potassium (K) Manganese (Mn)b
Sodium (Na)a Selenium (Se)b
Sulphur (S) Zinc (Zn)b
aIncluded as dietary ingredients.
bIncluded in a premix.
cNormally included as iodized salt (NaCl).
Adequate calcium and phosphorus nutrition for all classes of pigs is dependent upon: (i) an adequate supply of each element in an available form in the diet; (ii) a suitable ratio of available calcium and phosphorus in the diet; and (iii) the presence of adequate vitamin D (NRC, 1998). A wide calcium : phosphorus ratio lowers phosphorus absorption, resulting in reduced growth and bone calcification, especially if the diet is marginal in phosphorus. The ratio is less critical if the diet contains excess phosphorus.
A suggested ratio of total calcium to total phosphorus for grain/soybean meal diets is between 1 : 1 and 1.25 : 1. When based on available phosphorus, the ratio is between 2 : 1 and 3 : 1. An adequate amount of vitamin D is also necessary for proper metabolism of calcium and phosphorus, but a very high level of vitamin D can mobilize excessive amounts of calcium and phosphorus from bones. The dietary calcium and phosphorus requirements, expressed as a concentration in the diet, may be slightly higher for gilts than for castrates. The calcium and phosphorus requirements of the developing boar are greater than those of the castrate and gilt (NRC, 1998). The calcium and phosphorus requirements of sows and gilts are influenced by how long the sow suckles the piglets, a factor of importance to organic producers. Weaning the litter at 5–8 weeks of age results in a greater drain on the sow than weaning at 3–4 weeks. Larger and heavier litters also place more stress on calcium and phosphorus needs.
During pregnancy, the requirements for calcium and phosphorus increase in proportion to the need for fetal growth and reach a maximum in late gestation. Generally, the requirements for calcium and phosphorus are based on a feeding level of 1.8–2.0 kg feed/day during gestation and 5–6 kg feed/day during lactation. If sows are fed less than 1.8 kg feed/day during gestation, the diet should be formulated to contain sufficient concentrations of calcium and phosphorus to meet the daily requirements. The voluntary feed intake of lactating sows may be reduced by high environmental temperature. In this event the lactation diet should be formulated to meet the daily needs for calcium and phosphorus. Adequate calcium and phosphorus intakes are more critical in first-parity sows than in mature sows.
In cereal grains, grain by-products and oilseed meals about 60–75% of the phosphorus is organically bound in the form of phytate, which is poorly available to the pig (NRC, 1998). The biological availability of phosphorus in cereal grains is variable, ranging from less than 15% in maize to approximately 50% in wheat. The greater availability of phosphorus in wheat and wheat by-products is due to the presence of a phytase enzyme in the grain. The phosphorus in oilseed meals also has low bioavailability. In contrast, the phosphorus in protein sources of animal origin is largely inorganic (meaning in this context not containing carbon; organic compounds are those containing carbon), and most animal protein sources (including milk and blood by-products) have high phosphorus bioavailability (Cromwell, 1992). The phosphorus in dehydrated lucerne meal is highly available. Steam pelleting has been shown to improve the bioavailability of phytate phosphorus in some studies but not in others. The