Establishment of the Ideal Dietary Amino Acid Profile for Pigs

Basically, it would be expected that sow's milk, which can be considered to be optimized for suckling piglets during evolution, would also provide the weaned piglets with an ideal amino acid profile. This assumption is supported by the fact that the profile is very constant in sow's milk and apparently not influenced by the dietary composition (Boisen, 1997).

Furthermore, this composition is very close to the amino acid composition in the body as well as in the deposited protein during growth (Table 9.2). Because the composition of deposited protein, during growth up to 100 kg live weight is relatively constant and, furthermore, the requirements for protein deposition account for a dominating portion of the total amino acid requirements, it would also be expected that the ideal dietary amino acid profile is relatively constant during this period. The residual amino acid requirements are related to maintenance requirement, which appear to be dominated by the ileal endogenous protein loss during the digestion processes (Fuller, 1991, 1994; Boisen and Moughan, 1996a). Compared with sow's milk ileal endogenous protein losses are low in most essential amino acids except threonine, cystine and tryptophan (Table 9.2). The concentration of cystine is furthermore very high in hair (Table 9.2) which is lost continuously during growth. Tryptophan and tyrosine are precursors for important hormones, i.e. serotonin, tyroxine and adrenaline. After weaning, the requirements for these amino acids may increase due to extra losses of endogenous protein when dietary fibre and antinutritional factors in the feed are increased and a generally increased hormone production. On the other hand, the higher concentration of histidine in deposited protein than in sow's milk

Table 9.2. Amino acid composition (g 160 g 1 N) of sow's milk compared with the composition in whole body and deposited protein during growth, ileal endogenous protein loss and hair, respectively.

Essential and semi- Endogenous

Table 9.2. Amino acid composition (g 160 g 1 N) of sow's milk compared with the composition in whole body and deposited protein during growth, ileal endogenous protein loss and hair, respectively.

Essential and semi- Endogenous

essential amino acids

Sow's milk3

Whole bodyb

Deposited0

proteind

Hair0

Lysine

71

66

69

30

33

Threonine

39

39

38

45

59

Methionine

18

19

19

10

4

Cystine

13

11

10

16

134

Tryptophan

12

8

n.d.e

12

n.d.

Isoleucine

41

35

40

25

35

Leucine

81

72

77

40

77

Valine

54

48

51

35

60

Histidine

25

29

32

15

11

Phenylalanine

39

39

37

30

23

Tyrosine

42

27

28

20

9

aMean of 32 samples (Boi sen, 1997). bDetermined at 20 kg live weight (Fuller, 1994). ┬░From 20 to 90 kg live weight (Jorgensen et al., 1988). dMean of 36 determinations (Boisen and Moughan, 1996a). 8n.d., not determined.

may be explained by its ability to be stored in carnosine when there is a dietary surplus (Fuller etai, 1989).

The amino acid profile (relative to lysine) of sow's milk is compared with proposals for ideal protein from the literature in Table 9.3. Characteristically, amino acids in the primary limiting group are generally represented at a higher ratio, whereas those in the secondary limiting group are often represented at a lower ratio in the proposals than found in sow's milk. As described in the introduction, all amino acids in the first limiting group have been intensively studied. Nevertheless, there is still some disagreement about the relative requirements for these amino acids, in particular for the sulphur amino acids, which vary by up to 30% between the different proposals.

The amino acids in the secondary limiting group are much less investigated. However, there is no obvious reason for a lower ratio of these amino acids in the ideal profile than found in sow's milk.

It is generally believed that, due to the increasing contribution of maintenance requirements of protein, which is relatively high in threonine, sulphur amino acids and tryptophan, the ideal dietary amino acid profile changes during growth (e.g. Rademacher et al., 1999). However, for fast-growing pigs this may be of little practical importance before 100 kg live weight (Boisen, 1997). A major portion of the extra maintenance amino acid requirements may derive from the feed-specific extra endogenous ileal protein losses which is compensated for when using values of standardized ileal digestible amino acids as discussed above. Thus, the optimal threonine:lysine ratio was not found to increase significantly up to 100 kg live weight in a recent study (Pedersen et al., 2003). On the other hand, the requirements of amino acids relative to energy appear to be generally reduced with almost 50% from the extrapolated value at zero to 100 kg live weight (for lysine and threonine from 10.5 and 6.8, respectively to 5.25 and 3.4, respectively) as shown in Fig. 9.2.

In conclusion, sow's milk can be considered as a valuable guide for establishment of ideal dietary amino acid profile for growing pigs after weaning and until slaughter at 100 kg live weight in fast growing pigs.

Table 9.3. Proposals for ideal dietary amino acid profile (relative to lysine) for growing pigs compared with the profile of sow's milk.

Aa

B

C

D

E

F

G

Primary limiting amino acids

Lysine

100

100

100

100

100

100

100

Threonine

55

60

75

65

66

60

64

Methionine

25

26

27

31

-

-

26

Met + Cys

44

50

59

60

50

55

52

Tryptophan

17

15

19

18

18

18

17

Secondary limiting amino acids

Isoleucine

58

55

61

60

50

54

57

Leucine

114

100

110

100

100

102

114

Valine

76

70

75

68

70

68

74

Histidine

35

33

32

32

33

32

35

Phenylalanine

55

49

59

51

-

-

57

Phe + Tyr

114

96

122

95

100

93

114

Arginine

62

-

-

42

-

48

-

aA: Sow's milk (Table 9.2); B: ARC (1981); C: Fuller etal. (1989); D: Chung and Baker (1992); E: Cole and van Lunen (1994); F: NRC (1998); G: Boisen etal. (2000).

Fig. 9.2. Requirements for lysine and threonine (g per FUp (feed units for pigs)) relative to dietary energy of fast growing pigs until 100 kg live weight.

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