Mosenthin 1 * and M. Rademacher 2

Jaroslav Heger*

**R. Mosenthin 1 * and M. Rademacher 2**

1Institute of Animal Nutrition, Hohenheim University, Stuttgart, Germany; ²Feed Additives Division, Degussa AG, Applied Technology, Hanau, Germany

Introduction

It is generally accepted that the protein requirement of the pig is primarily a requirement for indispensable amino acids (Table 1.1). The amount of these amino acids in the diet and their relative proportions determine the deposition of protein in the pig.

However, feedstuffs used in diets for pigs vary not only in their amino acid composition but also in their digestibility. Depending on the type and source of feedstuff variable amounts of amino acids disappear from the gastrointestinal tract to be utilized by the pig for maintenance and tissue accretion. As a result, digestibility of amino acids between feed components varies considerably. For example, amino acids originating from casein are almost completely released during digestion whereas for certain cereal by-products more than half of the amino acids may remain undigested and unabsorbed from the animal’s gastrointestinal tract. Consequently, accurate data on the digestibility of amino acids in feedstuffs are needed to meet the pig’s daily requirement for indispensable amino acids more precisely from both a physiological and an economic point of view.

Protein supplements usually represent 20% of the diet but make up approximately 35% of the cost of the diet. One way to lower

the cost of protein is to reduce the concentra- tion of protein in the diets or to reduce the quantity of amino acids possibly provided in excess of the actual requirement, i.e. to reduce the margin of safety. However, this cannot be done efficiently without affecting optimum performance unless the quantity of available amino acids in the diet and the quantity required by the pig are known.

Animal growth and digestibility assays are the two major evaluation systems for assess- ing the bioavailability of amino acids in feed ingredients for pigs. At this point it is important to distinguish between the concepts of digestible and available amino acids. By defini- tion, apparent amino acid digestibility is calcu- lated as the percentage of amino acid intake that does not appear in digesta or faeces.

Using the term ‘apparent’ implies that no correction for endogenous amino acid losses has been made. On the other hand, bioavailability of amino acids is defined as the proportion of amino acids in the diet not associated with compounds which may interfere with digestion, absorption or utilization for maintenance or tissue accretion (ARC, 1981). For example, certain amino acids such as lysine, threonine, methionine and tryptophan are susceptible to the effect of heat treatment.

As a result, they may be partly damaged and absorbed in a form which renders these

2nd edition (ed. J.P.F. D’Mello) 169

*E-mail address: [email protected] Amino Acids - Chap 10 12/3/03 12:25 pm Page 169

complexes unavailable for protein metabolism by the pig. In this case, ileal amino acid digestibility values usually overestimate amino acid availability values (Wiseman et al., 1991;

Batterham, 1992). Batterham et al. (1979) introduced the animal growth assay, also referred to as the slope-ratio technique, for measuring the availability of amino acids for maintenance and tissue accretion. This growth assay provides a combined estimate of digestibility and postabsorptive utilization of amino acids at the tissue level.

However, many other factors including amino acid balance, dietary protein level, energy level, chronology of appearance of absorbed amino acids at the tissue level as well as genotype and physiological stage of the animal may have an impact on protein metabolism and therefore may affect the results (e.g.

Adeola, 1996). Furthermore, the animal growth assay is rather expensive, time con- suming and provides an estimate of the availability of only one amino acid per assay (e.g.

Austic, 1983; Sibbald, 1987). On the other hand, Batterham (1992) concluded that partic- ularly for cereals ileal digestibility values were the most appropriate as these account for losses in digestibility. Moreover, Laplace et al.

(1989) have shown that apparent ileal digestibility coefficients accurately describe the extent of amino acid absorption, at least for a range of commonly used feed ingredients which have not been subjected to high temper- ature treatment during feed manufacturing.

Finally, according to Williams (1995) the use of ileal digestibility values represents a compro- mise between total amino acid values in feed ingredients and amino acid availability that will improve the accuracy of diet formulation.

The main focus of this review is directed to the development of different concepts of amino acid digestibilities and the use of true digestible amino acids in diet formulation for pigs.

Expression of Apparent Ileal and Faecal Amino Acid Digestibilities

The digestion of amino acids in the large intestine is the result of microbial activity.

The disappearance of amino acids from this

part of the digestive tract would not be neces- sarily a problem if disappearance represented absorption of amino acids. However, several studies have clearly demonstrated that protein digestion through microbial activity does not contribute to maintenance or tissue accretion since the absorbed end products of microbial fermentation are ultimately excreted in the urine (Zebrowska, 1973;

Wünsche et al., 1982; Mosenthin et al., 1992). In conclusion, due to microbial metabolism of nitrogenous material both from exogenous (dietary) and endogenous sources, only a relatively small proportion of the amino acid excretion in faeces is directly related to the amino acids recovered at the distal ileum. According to Low (1982), amino acids of dietary origin appear to account for less than 10% of the total amino acids at the faecal level, the main part consisting of microbial and endogenous sources.

Depending on diet composition, between 50 and 90% of total nitrogen in faeces can be attributed to bacterial nitrogen assimilation (Poppe et al., 1983; Kreuzer et al., 1989;

Sauer et al., 1991).

There is general agreement that in most cases apparent ileal digestibility values of most indispensable amino acids are lower than corresponding digestibilities determined at the faecal level. For example, cystine, threonine and tryptophan usually disappear to a considerable extent in the large intestine (Zebrowska et al., 1978; Sauer et al., 1982; Mosenthin et al., 1994). On the other hand, microbial net synthesis for methionine and sometimes for lysine has been reported in some studies resulting in lower faecal than ileal digestibility values (Sauer et al., 1982; Tanksley and Knabe, 1982; Sauer et al., 1991). Thus, depending on the amino acid and on the feedstuff, digestibility values obtained by the faecal analysis method overestimate (which is usually the case) or underestimate those obtained by the ileal analysis method. Therefore, it is now recognized that the ileal analysis method should be considered as an improvement over the faecal analysis method which was originally developed by Kuiken and Lyman (1948) for rats and which thereafter has been used extensively in studies with pigs (Dammers, 1964; Eggum, 1973).

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A comparison of apparent faecal and ileal amino acid digestibilities in raw and heated soy flakes illustrates the inadequacy of the faecal analysis method for measuring amino acid digestibility values (Table 10.1).

The difference between faecal and ileal digestibility values of amino acids in raw soy flakes averaged 30% whereas the corresponding difference in heated soy flakes was approximately 7%. Although faecal digestibilities indicated that heated soy flakes were a better nutrient source than raw soy flakes, the magnitude of this difference was substantially underestimated (Vandergrift et al., 1983).

It is obvious that apparent ileal amino acid digestibilities are a more sensitive approach to describe the nutritive value of feedstuffs than faecal digestibilities. The poorer the protein quality of feed, the more important ileal digestibility values are compared to faecal digestibility values. Convincing evidence that ileal rather than faecal digestibility values should be used in practical diet formulation for growing pigs was provided by Dierick et al. (1988). In this study the performance of pigs was related to digestibility mea- surements. There was a higher correlation between average daily gain and ileal rather than faecal protein digestibility (r= 0.76 vs. r

= 0.34). In the same order, for feed efficiency (kg feed kg¹ carcass gain) the correlation coefficients were 0.87 and 0.65, respectively. In agreement with these findings, apparent ileal lysine digestibility coefficients

were found to accurately indicate the amount of dietary lysine available for growth (Moughan and Smith, 1985; Schulz and Böhme, 1994; Rademacher et al., 1995).

These results provide sufficient evidence that nitrogen absorbed in the large intestine does not contribute significantly to protein synthesis in growing pigs.

There is, however, a need to focus on some potential drawbacks in the interpreta- tion and the use of apparent ileal amino acid digestibilities in diet formulation for pigs. It has to be emphasized that the impact of microbial fermentation in the small intestine on protein digestion and amino acid absorption is probably underestimated (Bergner et al., 1986; Torrallardona et al., 1994; De Lange and Fuller, 2000). It can be assumed that in particular threonine that accounts for approximately 25% of mucus protein (Lien et al., 1997) is the preferential source of endogenous nitrogen for microbes attached to the intestinal surface. Furthermore, it has to be stressed that apparent ileal digestibility estimates are not corrected for endogenous nitrogen and amino acid losses that contribute at different levels to the total flow of nitrogen and amino acids at the distal ileum. As a result, apparent ileal protein and amino acid digestibility values may vary considerably, depending on the relative contribution of endogenous nitrogen and amino acids to non- digested dietary (exogenous) sources of nitrogen and amino acids. Approaches for adjust- Amino Acids in Diet Formulation for Pigs 171

Table 10.1. Apparent ileal and faecal amino acid digestibility values of the indispensable amino acids in raw and heated soy flakes. (From Vandergrift et al.,1983.)

Digestibility (%)

Faecal Ileal Difference (%)^a

Amino acid Raw Heated Raw Heated Raw Heated

His 80 90 48 82 32 8

Ile 68 84 43 78 25 6

Leu 68 87 37 80 31 7

Lys 72 87 44 85 28 2

Met 61 83 47 82 14 1

Thr 65 83 32 72 33 11

Trp 75 87 25 72 50 15

Val 64 85 35 78 29 7

aDifference between faecal and ileal digestibility values.

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ments of endogenous nitrogen and amino acid recoveries that allow for the determination of true ileal protein and amino acid digestibilities are discussed here.

Expression of True Ileal Amino Acid Digestibilities

Sources of variation in apparent ileal digestibility values

Several studies have shown that apparent ileal protein and amino acid digestibilities increased curvilinearly with the level of protein and/or amino acids in the assay diet (e.g.

Furuya and Kaji, 1989; Li et al., 1993; Fan et al., 1994). This observation gave rise to some concern about possible underestimation of amino acid digestibility values from low- protein feedstuffs such as cereal grains through measurement and expression of apparent digestibility.

However, it comes somewhat as a surprise to note considerable variation in apparent ileal amino acid digestibility values among different samples of the same feedstuff rather than between feedstuffs (Sauer et al., 1990). For example, as summarized by Mosenthin et al.

(1997) for protein and the indispensable amino acids, the differences were relatively large for protein, lysine, methionine and threonine within wheat and barley, ranging from 71 to 86%, 62 to 84%, 79 to 92%, and 51 to 78%, respectively, in wheat, and from 45 to 80%, 38 to 79%, 67 to 88% and 44 to 76%, respectively, in barley (Table 10.2). However, as shown in Table 10.2, there were relatively small differences in the average apparent ileal protein and amino acid digestibility values between different cereal grains compared to differences within the same cereal grain. For instance, the digestibility values ranged from 70 to 81%, 66 to 73%, 78 to 85%, and 64 to 72% for protein, lysine, methionine and threonine, respectively, between wheat and barley. Similar to cereal grains there were in most cases relatively small differences in the average apparent ileal protein and amino acid digestibility values between different protein supplements and legume seeds in comparison to differences in digestibility coefficients within the same feedstuff (Table 10.2).

In conclusion, the rather larger within than between variation in different feedstuffs indicates that methodological rather than other factors such as inherent factors (e.g.

fibre level, antinutritional compounds, fertilizer application) may be responsible for a large proportion of this variation (Sauer and Ozimek, 1986; Gatel, 1992; Sauer et al., 2000). Therefore, a major part of the variation in apparent ileal digestibility values of protein and amino acids within the same feedstuff may be simply a reflection of experimental error, and this variation may misrepresent the real variation among samples of the same feedstuff. In fact, Fan et al. (1994) identified differences in the protein and amino acid content of the assay diets as the largest single contributor to the variation of apparent ileal protein and amino acid digestibility values within the same feedstuff. Feeding maize- starch-based diets with graded levels of crude protein from soybean meal (4, 8, 12, 16, 20 and 24%) to growing pigs resulted in curvilin- ear effects of protein and individual amino acids on apparent ileal digestibility values. The analysis of the digestibility values according to a segmented quadratic with plateau model resulted in quadratic relationships between the apparent ileal amino acid digestibility values and the amino acid content in the assay diet as illustrated in Fig. 10.1 for lysine. Initially, the apparent ileal digestibility values increased sharply; thereafter the increases became smaller and reached their individual plateau values after which there were no further increases which means that the digestibility coefficients became independent of the dietary amino acid levels. In this model lower endpoints of 95% confidence intervals of the plateau digestibility values were defined to be the initial plateau digestibility values. By defini- tion, the dietary protein and amino acid contents, corresponding to the initial plateau digestibility values, were referred to as the dietary threshold levels.

Sauer et al. (2000) concluded from a comprehensive literature review that the total contents of crude protein and amino acids in assay diets containing cereal grains were usually far below these threshold levels. As a result, small differences in dietary contents of crude protein and amino acids below the cor-

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Amino Acids in Diet Formulation for Pigs173 Table 10.2. Apparent ileal digestibilities (%) of crude protein and indispensable amino acids in cereal grains, protein supplements and legume seeds. (Adapted

from Mosenthin et al., 1997.)

N 6.25 Lys Met Thr

Ingredients n^a DC^b Range SD^c DC Range SD DC Range SD DC Range SD

Cereal grains

Wheat 22 81 71–86 4.2 73 62–84 6.5 85 79–92 3.5 72 51–78 6.7

Barley 20 70 45–80 8.2 [19]^d 66 38–79 10.1 78 67–88 6.0 [18] 64 44–76 8.6

Maize 8 70 49–82 11.7 [7] 68 50–82 5.9 85 79–92 4.3 65 53–79 9.6

Triticale 6 78 76–82 3.0 [4] 72 62–81 6.7 82 77–85 3.1 62 46–74 11.1

Protein supplements

Soybean meal 30 80 72–89 3.7 84 76–91 3.4 86 77–97 4.3 [27] 75 68–83 3.9

Canola meal 14 69 64–73 2.5 73 69–81 3.1 82 76–93 4.7 [11] 67 60–74 3.7

Cottonseed meal 12 75 67–86 5.7 66 42–87 11.5 77 65–87 8.6 [11] 66 55–79 7.7

Meat and bone meal 16 67 57–82 6.8 70 56–85 8.5 77 66–85 6.6 [7] 64 50–81 9.2

Fishmeal 7 76 72–82 3.9 83 77–89 5.0 88 82–94 5.2 [4] 78 73–84 3.6

Legume seeds

Peas 9 73 69–76 2.7 81 73–84 3.6 73 68–76 2.5 65 60–74 4.1

Faba beans 6 74 69–77 2.8 80 77–87 3.6 67 61–77 7.1 [4] 73 57–84 11.5

aNumber of observations.

bDigestibility coefficient, mean values and range of values.

cStandard deviation.

dValues in parentheses following protein and methionine digestibility values indicate the number of samples in which the protein and methionine digestibility was determined.

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responding threshold levels will result in relatively large variations in the digestibility coefficients of amino acids, as dietary amino acid levels quadratically affect ileal amino acid digestibilities. Those amino acids present at low levels in cereal grains (lysine, threonine and tryptophan) and/or amino acids of which the ileal endogenous recovery is relatively high (e.g. threonine) will be especially affected.

In conclusion this variation of apparent digestibility in relation to the level of dietary protein and amino acids is consistent with the hypothesis of a non-specific endogenous loss proportional to dry matter intake rather than proportional to the level of protein and/or amino acid intake.

Transformation of apparent into true digestibility values

If one accepts that the determination of amino acid digestibility values should be based on the ileal analysis method, these digestibility coefficients should be consistent

with two main specifications. First, they must allow feed ingredients to be accurately compared, thus being independent of experimental and dietary conditions. Secondly, they must include any variation of the endogenous fraction related to the feedstuff itself, which is one of its attributes and must be considered in diet formulation. These specifications hold true for estimates of true ileal protein and amino acid digestibility.

At this point it is important to distinguish between specific and non-specific endogenous protein and amino acid losses that originate from various sources such as saliva, pancre- atic and bile secretions, sloughed off epithelial cells and from mucus (Souffrant, 1991). As illustrated in Fig. 10.2, the non-specific recovery – also referred to as basal recovery or minimum gut loss – is related to the dry matter intake only but independent of dietary and experimental conditions. The level of non- specific endogenous amino acid losses, expressed as g kg¹dry matter is constant at different dietary amino acid levels. The transformation of apparent ileal amino acid

174 R. Mosenthin and M. Rademacher

Initial plateau value (86.0)

Threshold level (8.5) 90

Lysine digestibility (%)

2 4 6 8 10 12 14 16

Dietary lysine level (g kg^–1)

Fig. 10.1. The quadratic with plateau relationship between the apparent ileal lysine digestibility and the level of lysine in the diet. (Fan et al., 1994.)

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digestibility values into values of true digestibility by correction for non-specific amino acid losses is described by the equation:

AA intake (AA excretion non-specific AA) TID (%) =

AA intake 100

where TID = true ileal digestibility and AA = amino acid.

Apparent digestibility values that have been transformed into true digestibility values according to this equation were originally referred to as ‘standardized ileal digestibility values’ (Mariscal-Landin, 1992).

In contrast to the non-specific protein and amino acid recoveries the specific recovery – also referred to as extrarecovery – is

variable and related to the presence of inherent factors in the feedstuff such as fibre, lectins, tannins and protease inhibitors.

Corrections of apparent ileal digestibility values for both specific and non-specific protein and amino acid losses would allow for the cal- culation of the so-called ‘real’ ileal protein and amino acid digestibility coefficients (Low, 1982). The data presented in Table 10.3 clearly indicate that the specific rather than the non-specific nitrogen and amino acid losses compensate for the differences in apparent digestibility between field peas and soy protein isolate. The difference in specific endogenous losses between these feedstuffs can be attributed to variable amounts of inherent factors such as protease inhibitors. There Amino Acids in Diet Formulation for Pigs 175

- AA losses

Dietary AA content Protein free

Dietary loss

Specific endogenous loss Non-specific endogenous loss

Fig. 10.2. Sources of amino acid (AA) losses in ileal digesta.

Table 10.3. Comparison of apparent, true and real nitrogen and amino acid digestibilities in field peas and soy protein isolate. (Adapted from Huisman et al.,1992; Sève et al.,1994.)

Huisman et al.(1992) Sève et al.(1994) Field peas Soy protein isolate

Item (g 100 g¹nitrogen) (g 100 g¹amino acids)

Apparent digestibility 74.1 89.3

Non-specific endogenous loss 7.8 3.5

True digestibility 81.9 92.8

Specific endogenous loss 11.0 4.5

Real digestibility 92.9 97.3

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is great interest in the evaluation of real digestibilities from a scientific point of view (e.g. De Lange et al., 1990; Mosenthin et al., 1993). However, the use of these values in practical diet formulation for pigs is limited since any variation of the endogenous fraction related to the feedstuff itself as one of its specific attributes is completely eliminated from the digestibility values.

True ileal protein and amino acid digestibility has the advantage over both apparent and real digestibility in that it represents a fundamental property of the individual feedstuff. In other words, true digestibility values include any variation of the endogenous fraction related to the feedstuff itself. Figure 10.3 shows that true digestibility values are not affected by the level of amino acid intake or amino acid content of the assay diet, whereas the corresponding apparent digestibility values increase exponentially with higher levels of intake because the non-specific amino acid recoveries, as percentage of total recovery, decrease proportionally.

There is growing evidence that non-specific endogenous amino acid losses are likely to interfere with additivity of apparent amino acid digestibilities in mixtures of feed ingredients (Imbeah et al., 1988; Fan et al., 1995;

Nyachoti et al., 1997a,b). For example, Nyachoti et al. (1997b) concluded from the results of their study that there may be a lack

of additivity in apparent ileal amino acid digestibilities when low-protein feedstuffs such as barley are combined with high-protein feed ingredients such as canola meal (Table 10.4). The correction of apparent ileal amino acid digestibilities for non-specific amino acid losses that are assumed not to be affected by differences in diet composition, will eliminate these effects. The resulting true amino acid digestibilities are more likely to be additive than the corresponding apparent digestibility values (Mariscal-Landin, 1992;

Jondreville et al., 1995; Boisen and Moughan, 1996; NRC, 1998; Rademacher et al., 1999). Additivity of amino acid digestibility values in the diet formulation for pigs by least-cost formulation programmes is essential since these programmes use individual digestibility coefficients for each feedstuff to fulfil the amino acid specifications. True digestibility values allow feed ingredients to be accurately compared and contribute to the precision of diet formulation.

The key issue for the quantification of true ileal protein and amino acid digestibilities is the quantification of the non-specific protein and amino acid recoveries in ileal digesta.

The data presented in Table 10.5 reveal considerable variation between estimates of non- specific protein and amino acid recoveries in the literature. Surprisingly, even when the same methodological approach was used for

176 R. Mosenthin and M. Rademacher

Digestibility (%)

TID 1 = TID 2

Dalam dokumen Amino Acids in Animal Nutrition (Halaman 181-188)