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 ai, 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 curvilinear 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 definition, 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-

Table 10.2. Apparent ileal digestibilities (%) of crude protein and indispensable amino acids in cereal grains, protein supplements and legume seeds. (Adapted from Mosenthin etal., 1997.)

N x 6.25 Lys Met Thr

Ingredients na DCb Range sdc 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

rotein 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

sgume 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.

Initial plateau value (86.0)

co Q

Initial plateau value (86.0)

co Q

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 eta/., 1994.)

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 eta/., 1994.)

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.

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