Chapter 1: Principals of energy and protein nutrition

The potential of feed resources for pigs in the tropics is superior to that of the temperate zone. However, paradoxically, there has been much less effective research in the tropics on locally available, non-conventional feed resources and their nutritional value as animal feedstuffs (Ly, 1993). Undoubtedly, pig production has improved through a growing under-standing of the mechanisms governing feed utilization and by the practical application of this new knowledge (Black et al., 1986). Feed efficiency is now widely recognized as the principal factor in profitable animal production; thus, Phillips (1984) noted that the efficiency of utilization of ingested feed is affected by digestion, while Ball et al. (1986) called attention to the role of appetite on nutrient uptake, rate of digestion and rate of absorption. Often valuable information resulting from studies in temperate countries requires proper interpretation before it can be applied in the tropics. And, although information concerning nutritional physiology has not always had a direct impact on animal performance, Rerat (1978) maintained that it was essential to any feeding strategy, while Braude (1979) pointed out that integrating information from studies of digestion and on-site feeding trials would result in improved performance and lowered costs.

Sugar cane is a good example of how the study of digestion in pigs has been used to develop strategies for tropical swine production. Sugar cane molasses (Figueroa and Ly, 1990), or sugar cane juice (Preston and Murgueitio, 1992), is now recommended as the sole source of energy for feeding pigs in tropical America. Trials have included the characterization of the digestive efficiency, as well as the metabolic aspects of these non-conventional alternative feedstuffs (Ly, 1990c,d).

DIGESTIVE PHYSIOLOGY

The gastrointestinal tract of the pig is relatively simple (Ly, 1979a). It has three main compartments: the stomach, the small intestine and the large intestine (Figure 1.1). The stomach acts as a reservoir in which the mixed feed is subjected to the action of proteolytic enzymes in an acid medium, prior to being evacuated into the small intestine (Laplace ,1982). The first section of the small intestine, the duodenum, governs the gastric-emptying of digesta and serves to create an equilibrium between the rate of passage and the rate of nutrient absorption. Digestion, a complicated hydrolytic process, involves the movement of digesta along the small intestine. It must be sufficiently slow to allow for the action of the bile and the hydrolytic enzymes secreted by the pancreas, and for the absorption of the most important nutrients, such as amino acids, fatty acids and glucose (Cunningham et al., 1963; Darcy, 1982).

The small intestine of the adult pig is approximately 18 meters long. For this reason, the elapsed time between the ingestion of feed and nutrient absorption, approximately four hours, is relatively short compared to the time required for the passage of digesta along the entire gastrointestinal tract, some 24 hours (Ly, 1979b; Laplace, 1981). Digesta, as it leaves the small intestine, is a mixture of undigested feed residues, intestinal secretions and dequamated cellular particles that arise from the mucosa of the small intestine which is in a constant state of renewal. This material (digesta) enters into the relatively complex large intestine through the ileocaecal valve.

In the large intestine, the digesta is modified by flora indigenous to the gastrointestinal tract. This fermentative process "digests" 10 to 20% of the feed from mouth to rectum (Rerat, 1978; Kesting, 1985) and determines the entire transit time of the digesta through the gastrointestinal tract (Laplace, 1981; Kesting, 1985). In the caecum and in the proximal colon, the digesta is still liquid. As it advances to the anus to be voided as faeces, it loses water and acquires a sticky, solid consistency (Hecker and Grovum, 1975).

The digestive capacity of the pig increases with age. Neonates and lactating piglets depend on a well-developed gastric ability to effectively clot milk (Moughan et al., 1992). During the first few weeks of life, both the small intestine and the exocrine pancreas grow and develop. This prepares the piglet for weaning. The large intestine matures slower; this explains why the pig tends to digest fibrous feeds better in direct relation to its age (Mason, 1979; Laplace, 1981; Kesting, 1985).

Figure 1.1. Gastrointestinal tract of the pig.

Each section of the gastrointestinal tract harbours a mixed microbial population that lives in equilibrium with the host animal (Cranwell, 1968; Rerat, 1978; Ratcliffe, 1985). Whenever this equilibrium is disrupted, a digestive upset can occur. Fermentation of undigested feed residue by gut microflora is a normal process and can partially determine the utilization of energy contained in the fibrous fractions of the diet. Pond (1989) observed that pigs fed a high fibre diet have heavier large intestines than those fed low fibre diets. In fact, in a review of this subject, it was pointed out that up to 30% of the energy requirements of growing/finishing pigs might be provided by short-chain, fatty acids arising from the degradation of fibre by microbes (Varel, 1987, cited by Pond, 1989).

The apparent digestibility of nutrients in a diet measures overall gastrointestinal function. As with other animals, in the pig this is influenced by factors such as age or body weight (Oude et al., 1986). Since digestibility is closely correlated with performance, the concepts of digestible energy and digestible protein are commonly used to describe the nutritional value of feeds for swine (Dierick, 1991).

Digestive and metabolic efficiency of sugar cane

The soluble fraction of sugar cane can provide different energy sources for pigs: juice, molasses or raw and refined sugar (see Chapter 3). However in all cases, the carbohydrate is sucrose and the products of its hydrolysis: glucose and fructose. Sugar cane molasses, in addition to sucrose, and depending on its type, contains different amounts of important minerals and non-identified organic matter (NIOM).

There are a number of constraints to using sugar cane products for pigs; one is that they have lower gross energy compared to cereals (Christon and Le Dividich, 1978). The data in Table 1.1 suggest that the gross energy in sugar cane products is about 80% that of maize. The digestibility of sugar cane molasses appears to be directly proportional, either to its sucrose content, or to the ratio of carbohydrates to NIOM (Ly 1990c). When starch is substituted by sucrose in the diet, the metabolizable energy/digestible energy (ME/DE) ratio is lowered by about four per cent (Ly, 1987b; Cuarón, 1992). Other constraints to the use of sugar cane products are a reduction in the digestion of energy due to the presence of NIOM and its influence at the intermediary level of metabolism. These ideas will be fully developed in the following sections.

Table 1.1. Caloric value of sources of energy: sugar cane derivatives vs. maize.

Source of energy Gross energy, kj/g DM Source
Syrup-off 15.9 Swine Research Institute, Minag, Cuba (1989)*
High-test molasses 15.0
A molasses 14.9
B molasses 14.7
C molasses 13.5
Refined sugar 16.4 Ly et al. (1984)*
Maize 18.4 NRC (1988)

* Unpublished data

The energy efficiency of sugar cane-based feeds

Only one enzyme, sucrase, is needed to hydrolyze sucrose to glucose and fructose, which in turn are absorbed through the wall of the small intestine (Figure 1.2). This suggests that the digestion of carbohydrate in sugar cane-derived feeds is simple, compared to that of starch in cereals or roots.

Figure 1.2. Digestion of sugars in the small intestine of pig.

Source: Ly, 1971.

The amount of energy produced from digestion in the small intestine of sucrose, or fructose and glucose, is the same, 15.66 kilojoules per gram (Ly, 1990c, d). Digestion is essentially a hydrolytic process and, since the heat of combustion from the final stage of digestion of starch and sucrose is identical (Table 1.2), it would appear that neither has an advantage over the other. However, the digestion of certain organic compounds which accompany sucrose in some sugar cane derived feeds, such as molasses, merits a more detailed inspection.

Table 1.2 . Energy efficiency of the intestinal digestion of different sources of sucrose* compared to starch** from cereals or roots.

Dietary source of sucrose Initial stage Final stage
Substrates Heat of combustion (kj/g DM) Products Heat of combustion (kj/g DM)
Raw cane sugar, sugar cane juice, C molasses sucrose l6.58 glucose

fructose

15.66

15.66

High-test molasses glucose

fructose

15.66

15.66

glucose

fructose

15.66

15.66

fructose 15.66
Fructose syrup fructose 15.66 fructose 15.66
Cereals and roots amylose amylopectin 17.42 glucose 15.66

Source: *Ly (1990c); ** Ly (1990d)

Digestion and adsorption of fructose from sugar cane

Feeds obtained from the soluble part of sugar cane contain varying amounts of fructose, either linked to glucose to form the sucrose molecule, or in a free state. This free fructose is not completely absorbed when it is ingested (Ly, 1992). Small amounts can escape from the small intestine and are presumably fermented in the caecum and colon. However, when the sucrose molecule is split in the small intestine into glucose and fructose, both monosaccharides are completely absorbed. It is for this reason that it is better to feed pigs sucrose-containing feeds and not sucrose hydrolysis products.

A certain quantity of fructose is not metabolized and escapes in the urine (Ly and Macías, 1979; Ly et al., 1985, 1989). Following the ingestion of fructose-containing feeds, an atypical urinary excretion of lactic acid occurs and the pH of the urine decreases, dramatically. The ensuing metabolic acidosis may depress feed intake and interfere with bone metabolism. It has been suggested (Ly, 1990a, b) that perhaps this could be avoided through the manipulation of the acid-base balance of the diet.

The nutritional influence of NIOM in cane molasses

The notion of the possible influence of NIOM on the performance of pigs was first discussed by researchers in Cuba. Velázquez and Preston (1970) suggested that "impurities" present in integral molasses might have caused the excessive water content of the faeces, 78% compared to only 59% in pigs fed high-test molasses. Later, Ly et al. (1985) reported that this NIOM fraction, if absorbed, was excreted in the urine (Table 1.3). This fraction accounts for the low ME/DE ratio usually found when sugar cane molasses is fed to pigs as the only source of energy (Pérez et al., 1988).

Table 1.3. Urinary energy losses in pigs fed cane A molasses or cassava starch.

A molasses plus: Cassava starch
alone torula yeast soya bean meal plus torula yeast
(ME/DE) x100 92.3 94.8 93.9 97.0
Urinary energy partition, %:
N compounds 14.6 52.8 52.3 67.3
Fructose 19.9 10.8 7.5 0.0
Non-identified compounds 65.5 36.4 40.2 32.7
Non-metabolizable energy, % DE:
N compounds 0.8 2.8 3.1 2.0
Fructose 1.2 0.6 0.4 0.0
Non-identified compounds 3.7 l.8 2.6 1.0
Total 5.7 5.2 6.1 3.0

Ly et al. (1985)

Figueroa and Macías (1988) isolated a fraction from different types of Cuban molasses based on a method developed for C molasses (McLaren, 1950). They fed rats up to 20% of this non-sugar fraction (NIOM) and found a marked decrease in fecal dry matter. Several additional studies in pigs have shown that either total or ileal digestibility of NIOM from different types of molasses is relatively low (Table 1.4).

Table 1.4. Ileal and total digestibility of the non-identified organic matter of sugar cane molasses in the pig (%).

Carbohydrates: Ileal Total Source
Sucrose 98.3 - Macías et al. (1981)
Fructose 90.5 -
Glucose 98.2 -
Cassava starch 98.0 - Figueroa et al. (1986)
A molasses 96.0 -
High-test molasses 95.0 - Figueroa et al. (1988)
C molasses 83.0 - Díaz et al. (1990)
Non-identified organic matter:
A molasses 14.0 79.0 Ly et al. (1987a)
High-test molasses 13.6 80.0
Final molasses 51 70.0

ENERGY AND PROTEIN IMBALANCE

There is a direct relationship between energy and protein in the diet of pigs. Protein imbalance is related to the amino acid composition of the ration, particulary the limiting amino acids.

Protein imbalance

Perhaps one of the more interesting experiments demonstrating the questionable effects of a high level of dietary protein on the performance of growing/finishing pigs is that of Sugahara et al. (1970). These authors demonstrated that increasing the level of protein in a maize/soya bean meal diet from 16 to 32% crude protein, compared to feeding only soya bean meal of 48% protein, resulted in decreased appetite and a lowered daily weight gain but had no effect on feed conversion (Table 1.5). In addition, there was an increase in the percentage of lean cuts which was in direct relation to the amount of soya bean meal in the diet. Perhaps, the most important conclusion is that pig performance does not necessarily improve by increasing protein in the diet.

Table 1.5. The effect of high levels of protein in the diet on the performance of growing pigs.

Protein in the diet, %
16* - 12 32 48
AD feed intake, kg/d 2.41 2.22 1.92
ADG, g 700 640 550
AD feed conversion 3.45 3.48 3.50

Source: Sugahara et al. (1970); * 16% during first stage

Many studies have examined protein imbalance on low protein feeds to identify the minimum amount of essential amino acids. Low protein diets pose a greater risk of amino acid imbalance and incorrect amino acid supplementation produces poor pig performance (Table 1.6). Russell et al. (1986) demonstrated that the addition of methionine to a tryptophane deficient diet depressed feed intake and growth rates. Henry (1988), pointed out that the imbalance between the first and the second limiting amino acid is very common on low protein diets incorrectly supplemented with synthetic amino acids.

Table 1.6. The effect of low levels of protein in a diet supplemented with amino acids on the performance of growing pigs.

Feed intake (kg/day) ADG (g) Feed conversion (kg/kg gain)
Protein, 12% (negative control) 1.56 557 2.78
Protein, 12% + threonine 0.10 + tryptophan 0.04 1.54 626 2.44
Protein, 12% + methionine 0.10 + tryptophan 0.10 1.46 560 2.56
Protein, 12% + threonine 0.10 + tryptophan 0.04 + methionine 0.10 1.60 659 2.38
Protein, 16% (positive control) 1.58 654 2.38

Source: Russell et al. (1986)

Amino acid imbalance

The amount of amino acids consumed by the pig is determined by feed intake and the digestible energy in the diet. These factors, in turn, directly influence voluntary intake in growing pigs fed ad libitum (Chiba et al. 1991a,b). Therefore, the level of amino acids should be related to the digestible energy in the diet. Energy and amino acid imbalance results in improper growth, poorer feed efficiency and in an inadequate rate and efficiency of protein and fat deposition.

The concept of the "ideal" protein for pigs is relatively new (Cole, 1978); it should supply the exact proportion of required essential amino acids. The biological value of an ideal protein should be 1.0; this can be achieved if young animals are fed that protein together with an appropriate amount of non-protein energy, minerals and vitamins (Fuller and Chamberlain, 1983). The data in Table 1.7 illustrate how the crude protein content of the diet may be reduced from 17.6 to 14.5% by adding synthetic lysine, thus improving the balance of essential amino acids, in their order of importance.

Table 1.7. The reduction of dietary crude protein through the addition of synthetic lysine.

Balance in the ideal protein (Cole, 1978, 1990) Balance in the crude protein (Taylor et al., 1979)
17.6% 14.5% plus lysine
Lysine 100 100 100
Methionine plus cystine 50 66 56
Threonine 60 78 63
Tryptophan 18 21 18
Isoleucine 50 75 59
Leucine 100 144 118
Histidine 33 43 35
Phenylalanine plus tyrosine 100 158 134
Valine 70 95 79

An "ideal" protein for pigs means that the proportion of essential amino acids should remain constant but the amount may vary depending on factors such as weight, sex and breed. There is reason to believe that climate should be included among these factors, as feed intake tends to be lower in tropical environments (Le Dividich and Rinaldo, 1988). Further studies about ideal proteins suggest slight changes in connection with threonine, methionine and tryptophan (Table 1.8).

Table 1.8. The proportion of essential amino acids in the ideal protein for the growing pig.

Cole (1978) Fuller et al. (1979) Yen et al. (1986a) Wang and Fuller (1989)
Lysine 100 100 100 100
Methionine plus cystine 50 53 50 63
Threonine 60 56 57 72
Tryptophan 18 12 20 19
Isoleucine 50 44 55 60
Leucine 100 84 100 110
Phenylalanine plus tyrosine 100 96 100 120
Histidine 40 32 35 -
Valine 70 63 70 75

Another approach to "ideal" protein is related to the availability of amino acids. ARC (1981) defined available amino acids as the proportions of dietary amino acids which are digested, absorbed and utilized to sustain life and/or the growth of new tissue. Amino acid availability increases the accuracy of the proportions of essential amino acids in the ideal protein. Lysine was used by Batterham (1979) as the reference amino acid to establish an order of merit for eight protein sources (Table 1.9).

Table 1.9.. Availability of lysine in eight protein sources according to growth or carcass evaluation trials in pigs (%).

Growth assay with pigs Carpenter available lysine
Blood meal 102 97
Fish meal 89 90
Skim milk powder 85 79
Soya bean meal 84 77
Rapeseed meal 68 71
Meat and bone meal 49 71
Cottonseed meal 43 65
Sunflower meal 40 88

Source: Batterham (1992)

It should be emphasized that amino acid availability and ileal digestibility are not the same. Batterham (1992) demonstrated that not all the lysine absorbed up to the distal ileum is completely utilized, probably due to some structural change in the absorbed amino acid molecule. Presently, ileal digestibility, otherwise known as absorbability of amino acids, has been generally accepted to define or to categorize an ideal protein level for growth. An example is given in Table 1.10.

Table 1.10 The proportion of absorbable essential amino acids in the ideal protein for the growing/finishing pig

Growing (25-55 kg) Finishing (50-90 kg)
Boars Gilts Castrated males Boars Gilts Castrated males
Lysine 100 100 100 100 100 100
Methionine 36 37 37 38 39 40
Methionine + cystine 51 52 52 55 58 60
Threonine 63 64 64 69 67 69
Tryptophan 18 19 19 20 21 22
Isoleucine 71 73 73 74 76 78
Leucine 128 130 130 136 140 145
Phenylalanine 83 86 86 91 95 99
Histidine 42 43 43 45 46 48
Valine 89 90 90 94 97 100

Source: Yen et al. (1986a,b)

Amino acid/energy ratio

Since the amino acid and energy ratio must be held constant, the supply of amino acids should be adjusted to the amount of energy supplied in the diet (Henry, 1988). Given this notion, it becomes necessary to clarify aspects of pig nutrition in the tropics, such as including locally available fibrous feeds which often contain appreciable amounts of protein, as well as the interaction between the amino acid/energy ratio and the climate.

It appears that fibre depresses overall, but not ileal, amino acid digestibility; therefore, the fibrous plant cell wall apparently does not influence the availability of dietary amino acids (Sauer et al., 1991). Although it has been claimed that the ratio of available lysine/MJ of digestible energy is not linear during the entire life cycle (Carr et al., 1977; Whittemore and Fawcett, 1976), more recent studies have suggested, that for growing pigs up to 50 kg, the interdependence between energy intake and the rate of protein deposition is linear (Chiba et al., 1991a, b). A more detailed prediction of the essential amino acid to digestible energy ratio for the male pig can be seen in Table 1.11.

The studies carried out by Stahly and Cromwell (1987) and Le Dividich and Rinaldo (1988) have shown that if the protein:energy ratio is increased, similar performance traits might be obtained in a warm or temperate climate. This is attributable to the low energy requirement for pigs in warm climates compared to higher needs in temperate climates and the need to maintain a proper ratio of protein and essential amino acids.

Table 1.11. Available (predicted) essential amino acids to energy ratio (g/MJ DE) in a non-castrated, fast-growing genotype male pig.

Liveweight (kg)
20 50 90
Phenylalanine plus tyrosine 0.67 0.54 0.45
Phenylalanine 0.36 0.29 0.24
Methionine plus cystine 0.24 0.28 0.23
Methionine 0.23 0.19 0.16
Valine 0.48 0.39 0.32
Isoleucine 0.44 0.36 0.30
Leucine 0.69 0.56 0.47
Threonine 0.39 0.32 0.27
Histidine 0.21 0.17 0.14
Lysine 0.77 0.62 0.52
Tryptophane 0.12 0.09 0.08

Source: Black et al. (1986)

ADJUSTED NUTRITIONAL REQUIREMENTS

Nutritional requirements for pigs reared in the tropics have yet to be adequately established. High energy diets are not necessary to maintain body temperatures and a depression in growth rate is a well known consequence of a reduction in voluntary feed intake due to heat (Stahly et al., 1979). These same authors have observed that, in heat-stressed pigs, these factors can be partially alleviated by altering the heat increment of the diet, i.e., by lowering the dietary protein level, thereby reducing the essential amino acids to the required minimum, as has been done with poultry (Waldroup et al., 1976).

To sustain efficient pig feeding systems, some tropical countries have centred nutrient requirements on high energy, locally available crops. Research with genetically-improved pigs, fed non-conventional feeds such as sugar cane juice and raw oil or fibrous residues from the African oil palm, has shown that there are clear economic advantages when the protein supplement for growing/finishing pigs is lowered to 500 g/day to provide approximately 200 g of crude protein daily (see Table 3.5). At first, these results appear to contradict the current nutrient requirements for protein in pigs (NRC, 1988; ARC, 1981). However, they are probably a consequence of the use of soya bean meal (see Chapter 2) as the major source of protein, or to the absence of poor amino acid-profile cereals such as maize (Speedy et al., 1991).

In general, the basic non-conventional energy feeds like sugar cane juice, molasses, oil palm products, tubers and roots require supplementary vitamins and minerals. Perhaps one practical solution is to combine the macro elements with the restricted protein supplement (Ocampo et al., 1990a,b; Ngoan, 1994; Ngoan and Sarria, 1994) which, if supplied as soya bean meal and fed at 500 g/day, would equal approximately 20% of total daily dry matter intake. The vitamin premix should not be included until feeding time due to the potential negative effect of extended storage in the tropics of concentrated amounts of vitamins and minerals together. Perhaps, this could be solved by more frequent preparation of the supplement.

A growing/finishing pig from 25 to 90 kg, fed according to a "tropical" as opposed to "temperate" (cereal) feeding system, requires 2.5 to 2.8 kg/day of dry matter which, in general, will consist of from 2 to 2.3 kg of an energy source plus 500 g of a protein supplement. Since the supplement is one-fifth of the total daily dry matter intake, the minerals and vitamins should be included in the supplement at five times the required concentration.

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