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, pancreatic 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 nonspecific endogenous amino acid losses, expressed as g kg-1 dry matter is constant at different dietary amino acid levels. The transformation of apparent ileal amino acid

A,A, lossss

Sources Amino Acids
Dietary AA content Fig. 10.2. Sources of amino acid (AA) losses in ileal digesta.

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

AA intake - (AA excretion

AA intake 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 calculation 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

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

Huisman et al. (1992)

Sève et al. (1994)

Field peas

Soy protein isolate

Item

(g 100 g 1 nitrogen)

(g 100 g 1 amino 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

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 nonspecific protein and amino acid recoveries in the literature. Surprisingly, even when the same methodological approach was used for

Amino Acid Function
Fig. 10.3. Expression of apparent and true ileal amino acid digestibilities as a function of amino acid (AA) intake.

Table 10.4. Observed and calculated apparent and true ileal digestibilities for selected amino acids in barley, canola meal and a mixture of barley and canola meal. (Adapted from Nyachoti eta!., 1997b.)

Item

Barley

Canola meal

Mixture of barley and canola meal Observed Calculated3 Difference13

Apparent ileal digestibility (%) Lys Thr He Val

True ileal digestibility0 /0> Lys Thr He Val

67.6

87 97

62.6

69.5

63.6

  1. 7 78.6 72.1
  2. 8 96.6 100.2 96.4

70.6

65.7

93.1

  1. 6 4.8 8.0 6.4
  2. 0 -0.9 2.8 3.3

Calculated from observed digestibilities in the pure ingredients and their contents in the mixture of barley and canola meal.

bCa!cu!ated as observed minus calculated values, determined with the homoarginine method.

Table 10.5. Comparison of non-specific endogenous protein and amino acid recoveries in ileal digesta of growing pigs (g kg 1 dry matter intake).

Item

Thr Trp laboratory.

Lab3 A

AmiPig (2000) Lab B

Lab C

Rademacher et at. (1999)

estimating non-specific protein and amino acid recoveries, these estimates exhibited a large variation between laboratories (AmiPig, 2000). Boisen and Moughan (1996) concluded from a literature review that differences in methods, including analytical procedures to estimate these losses, and between animal variation are major factors contributing to the considerable variation between estimates for non-specific protein and amino acid recoveries. For example, more conventional approaches are based on the feeding of protein-free diets, the use of the regression analysis method or the feeding of diets containing protein sources (e.g. casein) with an assumed 100%

ileal protein digestibility. Other methods include the peptide alimentation ultrafiltration method, also referred to as the enzy-matically hydrolysed casein (EHC) method, the homoarginine method and calculation methods based on the difference between the in vitro and in vivo digestibility of protein and amino acids. Comprehensive descriptions and evaluations of these methods were provided by Tamminga et al. (1995), Boisen and Moughan (1996) and Nyachoti et al. (1997a). According to Nyachoti et al. (1997a) estimates of endogenous protein and amino acid recoveries in ileal digesta are not only affected by animal and dietary factors but also differ for various methods. For example, Boisen and Moughan (1996) reported that the non-specific endogenous protein recoveries varied between 10 and 15 g kg-1 dry matter intake when protein-free diets were fed. However, under more physiologically normal conditions (i.e. when protein-containing diets were given), the non-specific recoveries accounted for about 20 g kg-1 dry matter intake. Nyachoti et al. (1997a) concluded that estimates of endogenous protein and amino acid recoveries obtained with the regression method as well as by feeding protein-free diets should be referred to as the minimum values that are not related to the protein and amino acid content of the diet.

Different estimates for the correction of non-specific protein and amino acid losses are currently used in feed tables in which true ileal protein and amino acid digestibilities are summarized (Jondreville et al., 1995; CVB, 1998; NRC, 1998; Rademacher et al., 1999; AmiPig, 2000). For example, in AmiPig (2000), promoted by AFZ, Ajinomoto Eurolysine, Aventis Animal Nutrition, Institut National de la Recherche Agronomique (INRA) and Institut Technique des Céréales et des Fourrages (ITCF), the calculations for the correction of non-specific endogenous ileal protein and amino acid recoveries are based on data that were obtained by feeding protein-free diets to growing pigs. On the other hand, Rademacher et al. (1999) transformed values of apparent ileal protein and amino acid digestibility into values of true digestibility by using existing literature data on endogenous recoveries of protein and amino acids in ileal digesta. These authors selected 33 experiments from the literature that were based on different experimental approaches. These included conventional methods such as feeding protein-free diets without (n=16) or with parenteral infusion of amino acids (n=l) (e.g. De Lange et al., 1989a,b), the regression method (n=3) (e.g. Fan et al., 1995) and the feeding of highly digestible protein sources such as wheat gluten or casein (n=ll) (e.g. Chung and Baker, 1992). In addition, the corrections for non-specific protein and amino acid recoveries in ileal digesta were based on the EHC method (n=2) (e.g. Butts et al., 1993). It was claimed that the diets in the experiments selected by Rademacher et al. (1999) contained no specific anti-nutritional factors and not more than 8% cellulose or purified neutral detergent fibre (NDF). The data from these experiments were pooled and mean values for non-specific losses of protein and amino acid recoveries were calculated. As discussed by De Lange and Fuller (2000), extreme care should be taken in combining true digestibility values from different feed tables since differences in the methods used to estimate the non-specific endogenous protein and amino acid losses may affect the relationship between dietary amino acid levels and corrected amino acid digestibilities across studies and within ingredient.

The feeding of protein-free diets as proposed by AmiPig (2000) gives generally lower estimates of non-specific endogenous protein and amino acid recoveries as compared to estimates by Rademacher et al. (1999) which were based on different experimental approaches. The data presented in Table 10.5 reveal that these differences were considerably higher for threonine which is present in relatively large concentrations in endogenous protein (Holmes et al., 1974; De Lange et al., 1989a; Mosenthin et al., 1994).

As can be expected, the method used for correction of apparent digestibility values has little effect on true digestibility estimates in high-protein feedstuffs with a relatively high apparent digestibility of amino acids and protein such as soybean meal (Table 10.6). Differences in true digestibility values in relation to the method used for estimating nonspecific protein and amino acid recoveries are more pronounced in feed ingredients with lower apparent protein and amino acid digestibilities, in particular with respect to threonine and tryptophan.

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