Tryptophan Isoleucine Valine and Arginine

A semipurified diet (Table 13.2) based on AA-fortified maize gluten meal (CGM) was developed in our laboratory (Peter et al., 2000) for use in studying the requirements for several AA. True digestibility of AA was determined in the high-protein CGM sample used (Table 13.3), and the same sample of CGM was used for all AA bioassays. Using 10.7 g kg-1 digestible lysine as a requirement reference point (Han and Baker, 1991, 1993), the CGM basal diet was fortified with essential AA so that all essential AA other than the one being studied would meet or slightly exceed the ideal AA ratios of Baker (1997) shown in Table 13.1. Levels of leucine and aromatic AA (phenylalanine + tyrosine), however, exceeded their ideal levels because of the surfeit levels of these AA in CGM. L-Glutamate

Table 13.2. Composition (as-fed basis) of complete (fortified) diet used to determine amino acid requirements of chicks fed a maize gluten meal semi-purified diet3 6.

Ingredient g kg 1

Maize starch 275.35

Dextrose 282.00

Maize gluten meal (CGM)0 183.40

Soybean oil 40.00

Mineral mixd 53.70

NaHCOg 15.00

Vitamin mixd 2.00

Choline chloride 2.00

L-Glutamate 109.40

L-Lysine-HCI (FG)e 11.93

L-Tryptophan (FG)e 1.34

L-Arginine 8.08

L-Cystine 1.87

L-Threonine (FG)e 3.91

L-lsoleucine 2.77

L-Valine 3.68

L-Histidine-HCI-HgO 1.38

DL-Methionine 1.69

Bacitracin MD premix' 0.50

DL-a-tocopheryl acetate (50 mg kg-1) +

aThe diet contained 14.2 MJ kg-1 ME and 120 g CP kg 1 furnished by maize gluten meal (CGM) and was fortified to 225 g CP kg 1 using glutamic acid and essential amino acids (Phe + Tyr, and Leu from CGM exceeded NRC (1994) requirements). Individual amino acids whose requirements were determined were adjusted downward to facilitate graded dose levels.

bThe digestible AA profile met or exceeded the Illinois Ideal ratios for poultry (Baker and Han, 1994; Baker, 1997) during 0-3 weeks of age (Lys, 100; Met, 36; Cys, 36; Arg, 105; Val, 77; Thr, 67; Trp, 16; lie, 67; His, 35; Phe + Tyr, 105; Leu, 109). The CGM was analysed to contain 653.1 g CP kg 1. It was obtained from ADM Corp., Decatur, Illinois.

dPeter etal. (2000). eFG, feed grade.

'Contributed 27.5 mg kg 1 bacitracin methylene disalicylate.

was then added as a source of non-specific amino nitrogen to achieve a final dietary crude protein level (nitrogen x 6.25) of 225 g kg-1. Also, as the level of an essential AA was varied within (and between) individual bioassays, L-glutamate was varied so that all diets within and between bioassays would be isoni-

Table 13.3. Essential amino acid composition and true digestibility values of maize gluten meal.

Composition

True digestible

Amino acid

(g kg-1)3

Digestibility (%)b

level (g kg 1)

Lysine

11.1

79.7

8,85

Arginine

20.8

94.9

19.74

Threonine

20.9

90.4

18.89

Valine

28.7

96.8

27.78

Isoleucine

24.9

98.6

24.55

Histidine

12.8

88.0

11.26

Cystine

11.4

73.4

8.37

Methionine

15.0

93.2

13.98

Phenylalanine

41.1

95.8

39.37

Tyrosine

28.0

95.5

26.74

Leucine

105.6

98.0

103.49

Tryptophan

3.40

97.0°

3.30

aAve rage of duplicate determinations; data expressed as a percentage of as-fed maize gluten meal (880 g kg 1 dry matter). bMeans of five caecectomized cockerels.

°Not determined; value obtained from Rhodimet™ Nutrition Guide (Rhône-Poulenc, 1993).

aAve rage of duplicate determinations; data expressed as a percentage of as-fed maize gluten meal (880 g kg 1 dry matter). bMeans of five caecectomized cockerels.

°Not determined; value obtained from Rhodimet™ Nutrition Guide (Rhône-Poulenc, 1993).

trogenous. All bioassays were done using male New Hampshire x Columbian chicks during a 12-14 day growth period representing the second and third weeks of life. Previous work in our laboratory had established that lysine requirements (g kg-1 of diet) as well as protein gain as a proportion of total weight gain were similar in this strain compared with commercial broiler strains (Han and Baker, 1991, 1993). For each of the requirement bioassays, four pens of four male chicks were fed each of six AA dose levels, and chicks were fed in battery pens located in an environmentally controlled building with constant lighting. Other details of the bioassay procedures have been described by Baker et al. (2002).

Data in Table 13.4 show that (i) the completely fortified CGM diet produced weight gain and feed efficiency levels that were not different (P > 0.10) from those obtained with a methionine-fortified maize-soybean meal positive-control diet, and (ii) the CGM diet could be made markedly deficient in at least eight essential AA. Thus, the CGM diet was deemed a very useful bioassay tool for dose-response studies with several essential AA.

Weight gain and gain/feed responded quadratically (P<0.01) in all requirement bioassays. Both weight gain and feed effi ciency pen means data were subjected to one-slope broken-line and quadratic curve-fitting procedures, with dietary AA concentration serving as the independent variable (Robbins et al., 1979; Draper and Smith, 1981). We considered the breakpoint of fitted broken lines the best basis for predicting ideal AA ratios, and Mack et al. (1999) came to the same conclusion. The fits of our broken-line responses were excellent in all cases, and quadratic fits were also very good (Baker et al., 2002). Inflection points of best-fit broken lines predict minimal requirement values, and this is viewed as desirable for calculating AA ratios. Another advantage of the fitted broken-line approach is that the inflection point is established objectively rather than subjectively. Quadratic fits do not produce objective breakpoints, and therefore one must subjectively select some percentage of the upper asymptote (e.g. 90%) as an estimate of the requirement.

It is possible, however, to take advantage of both broken-line and quadratic response curves to arrive at realistic objective estimates of AA requirements. If one superimposes the best-fit quadratic response curve on the best-fit broken-line response curve, the quadratic curve will intersect the broken line in at least three places: once (or twice)

Table 13.4. Limiting amino acids in maize gluten meal for young chicks as determined by amino acid deletion313.

12-day weight

Gain/feed

Diet

gain (g)

(g kg-1)

1. Maize gluten meal complete diet0

268rs

687r

2. As 1 - Lys

6Z

64z

3. As 1 - Trp

32V

238V

4. As 1 -Arg

49*

296x

5. As 1 - Thr

82w

4G8W

6. As 1 - Val

1G4V

468v

7. As 1 - He

156u

550"

8. As 1 - His

171u

601s'1

9. As 1 - Met - Cys

194'

574t,u

10. As 1 - Met

209'

620s

11. As 1 - Gly

256s

685r

12. Maize-soybean meal positive control

284r

683r

Pooled sem

6

14

aData represent means of triplicate groups of five New Hampshire X Columbian male chicks during the period 8-20 days posthatching; average initial weight was 89 g (Peter et al., 2000).

bAII diets were formulated to be isonitrogenous by varying the l-GIu level. °Maize gluten meal diet fortified with essential amino acids and l-GIu to 225 g CP kg-1 (see Table 13.2).

dMaize-soybean meal positive control diet, supplemented with 2 g kg 1 dl-Met, contained 230 g CP kg-1 and 13.4 MJ kg-1 ME.

rst u vwxyzWrthin a column, means lacking common superscript letters differ (P<

aData represent means of triplicate groups of five New Hampshire X Columbian male chicks during the period 8-20 days posthatching; average initial weight was 89 g (Peter et al., 2000).

bAII diets were formulated to be isonitrogenous by varying the l-GIu level. °Maize gluten meal diet fortified with essential amino acids and l-GIu to 225 g CP kg-1 (see Table 13.2).

dMaize-soybean meal positive control diet, supplemented with 2 g kg 1 dl-Met, contained 230 g CP kg-1 and 13.4 MJ kg-1 ME.

rst u vwxyzWrthin a column, means lacking common superscript letters differ (P<

on the ascending and twice on the plateau (zero slope) portion of the broken line. The first intercept x (AA level) value where the quadratic curve intersects the plateau of the broken line can be calculated, and we feel that this value is a realistic and objective estimate of the requirement for a population of animals. This procedure is illustrated in the discussion that follows.

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