Efficiency of Individual Amino Acids for Wool Growth

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As with protein, utilization efficiency of individual amino acids for wool growth is defined as amino acids incorporated into wool, with or without weight gain-related amino acid

Table 17.1. Amino acid concentrations (g kg 1 protein) of microbial protein in the rumen, the whole-body of sheep (excluding wool) and wool protein.

Wool protein (c-d)

Table 17.1. Amino acid concentrations (g kg 1 protein) of microbial protein in the rumen, the whole-body of sheep (excluding wool) and wool protein.

Wool protein (c-d)

Microbial

Whole-body

Whole

Low-sulphur

High-sulphur

High-tyrosine

protein (a'b)

protein (c)

wool

proteins

proteins

proteins

His

16-21

24

8-13

6

7-13

0

lie

54-62

36

27-32

38

17-26

0

Leu

74-83

73

67-79

102

14-34

35-64

Lys

81-115

67

27-35

41

6-9

0

Met

16-25

18

4.4-6.3

6

0

0

Phe

49-57

39

25-36

20

5-16

27-96

Thr

52-66

49

54-66

48

102-111

0-34

Val

53-65

43

46-57

64

43-53

0-47

Ala

34-62

80

32-52

77

20-29

0-31

Arg

46-53

73

62-91

78

62-69

35-47

Asp

112-129

86

55-66

96

6-23

19-22

Cys

20-26

13

86-131

60

221-229

64-102

Glu

127-141

132

111-142

169

79

0

Gly

49-65

96

46-86

52

42-62

265-388

Pro

34-40

63

53-75

33

126-128

17-67

Ser

41-47

42

83-108

81

127-132

124-126

Tyr

44-51

31

38-63

27

18-21

180-208

aStorm and 0rskov (1983). bMartin etal. (1996).

cMacRae etal. (1993). Calculated from the amino acid profiles of the individual tissues and their protein contents by the authors. dReis (1979).

Table 17.2. Estimated net efficiency of digested crude protein (DCP) for wool/fibre protein growth.

Species Efficiency Refe ren [email protected]

Merino sheep 0.20-0.25 Standing Committee on Agriculture, 1990

Angora goats 0.39 Sahlu et al. (1999)a

Angora rabbits 0.43 Liu et al. (1992)

aAngora goats: DCP (g day 1) = 26.5 + 0.25 LWG (g day 1) + 2.58 Clean fibre growth (g day 1).

deposition, relative to the absorption of amino acids from the digestive tract. However, a significant practical difference is that protein efficiency is a single term whereas efficiency of amino acids becomes 20, corresponding to 20 different amino acids, or for wool growth 18 efficiencies for 18 amino acids in wool protein (Reis, 1979). Zebrowska et al. (1987) measured the efficiencies in growing Merino lambs using comparative slaughtering techniques and the highest efficiency was obtained when the animals were fed a diet containing 140 g CP kg-1 compared to another two diets containing 110 and 170 g CP kg-1. The values of the gross efficiency for weight gain plus wool growth for nine indispensable amino acids were: His 0.33, lie 0.21, Leu 0.27, Lys 0.33, Met 0.21, Phe 0.23, Thr 0.32, Trp 0.22 and Val 0.26. The arithmetic mean was 0.26. The mean efficiency for nine dispensable amino acids (Ala, Arg, Asp, Cys, Glu, Gly, Pro, Ser and Tyr) was 0.36 and the efficiency for Cys was 0.55. Hogan et al. (1979) estimated absorption of amino acids in mature Merinos and, by assuming that all Met was converted into Cys in the body, calculated an efficiency for Cys 0.45-0.63 at the highest wool growth rate.

There are two difficulties related to using this efficiency system. The first is in defining the 'maintenance requirement' for individual amino acids, that is, the determination of the amino acid composition for the maintenance protein requirement. This is usually derived from the endogenous loss of nitrogen in urine and in the digestive tract. The average amino acid composition of the whole body may be used, as in a simulation of amino acid utilization in pigs (Black et al., 1986), but the reliability of this method has not been experimentally confirmed. The second prob lem is the interconversion of some amino acids and synthesis de novo making quantification of the efficiency very complex. For example, the high efficiency of Cys, was calculated from Cys retention and absorption. The amount of Cys converted from Met was not included in the efficiency calculation, so the efficiency is overestimated.

Because of the problems described above, a new approach to the measurement of amino acid requirements for human beings has been proposed (Young and Borgonha, 2000). In this system the oxidation of amino acids in response to varying levels of amino acid intakes was measured directly using isotope-labelling techniques and the requirement was defined from the minimum intake to balance the daily rate of the irreversible oxidation of the amino acid. The oxidation, from any of the catabolic pathways, accounts for the net loss of amino acids from the metabolic pool. This net loss can be measured directly and has to be matched by dietary supply. Using this system the requirements for humans of Leu, lie, Lys, Met, Cys, Phe, Tyr, Thr and Trp have been measured (Young, 1998; Raguso et al., 1999; Young and Borgonha, 2000).

The requirements of amino acids for animals must include amino acids used for maintenance-related protein synthesis, the outputs of products (increment of body protein mass, milk, or wool) and the oxidative (or catabolic) loss. Liu and Masters (2000) proposed a conceptual model to quantitate the requirements and utilization efficiencies of Met and Cys for wool growth in the growing Merino sheep. In this model the oxidation rate was defined as the ratio of the total oxidation to the flux per kg live weight to account for variations in the live weights from different experiments. Oxidation rate was found to be closely associ ated with the flux per unit of live weight. The total oxidative loss was then quantified, using published data, from the whole-body flux multiplied by the oxidation rate. Both the flux rate and oxidation were based on measurements using isotopic techniques for Met (Egan et ah, 1984) and for Cys (Lee et al., 1995; McNabb et al., 1993; Sun et al., 1994). In the model the factors used for the calculation of the requirement and the utilization efficiency included Met and Cys compositions in both the body and wool, rates of weight gain and wool growth, a fractional degradation rate of the whole-body protein, oxidation rates and a conversion rate of Met into Cys. A sensitivity analysis of the model indicated that the oxidation rate was the major factor in the determination of the requirement and efficiency (Liu and Masters, 2000). This suggests that differences in oxidation rates of individual amino acids could indicate their relative order of limitation to production. For this purpose published data on oxidation (catabo-lism) of some amino acids, as measured using isotope-labelling techniques in sheep, were collected, the oxidation rate was estimated using a linear regression analysis and is shown in Table 17.3.

Among the indispensable amino acids, Met had the highest oxidation rate, followed by Lys. The size of the free Met pool in the body is much smaller than that of Lys (0.8 vs. 3.0 mmol kg-1 day-1 in Suffolk cross wethers; Lobley et al., 1996b). A small pool coupled with a high oxidation rate for Met indicates that there is less Met available for protein synthesis in the body, and therefore Met supply could be more limiting, compared with Lys, for protein utilization efficiency in sheep. A very high oxidation rate of Cys in combination with high demand for wool protein synthesis would indicate that Cys supply will be the limiting factor to wool production.

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