Farm Animal Metabolism and Nutrition

lower end points of 95% confidence
intervals of the plateau digestibility values
are defined to be the initial plateau
digestibility values. The dietary crude
protein and amino acid contents, corres-
ponding to the initial plateau digestibility
values, are referred to as the dietary
threshold levels. The initial plateau ileal
digestibility values and the corresponding
threshold levels of crude protein and
amino acids are presented in Table 13.2.
As illustrated in Fig. 13.4 for crude
protein, methionine, threonine and
tyrosine, the apparent ileal digestibility
values of crude protein and amino acids
did not reach their initial plateau values
simultaneously at the same dietary crude
protein content. This shows that the
dietary amino acid content affects apparent
ileal amino acid digestibility values,
irrespective of the dietary crude protein
content. Therefore, to obtain the plateau
values, the level of inclusion of a feedstuff

in the assay diet should be such that the

amino acid contents in the assay diet are

equal to or exceed the corresponding

threshold levels. This consideration is

especially important for the determination

of the digestibility values of the limiting

amino acids (Fan et al., 1994).

On the other hand, although the

dietary threshold levels of crude protein

and amino acids are not warranted to be

constant under other experimental condi-

tions, these values remain valuable for

reference. Dietary factors that influence

amino acid recoveries at the distal ileum,

including source of fibre and level (e.g.

Sauer et al., 1977a,b; Taverner et al., 1981)

and anti-nutritional factors (e.g. Begbie and

Pusztai, 1989), can alter the ratio of

exogenous to endogenous amino acids,

thereby affecting the threshold levels.

Differences in dietary amino acid

levels are responsible for the variation in

ileal amino acid digestibility values within

Methods for Measuring Ileal Amino Acid Digestibility 287

Table 13.1.The quadratic with plateau relationship between apparent ileal crude protein and amino acid
digestibility values and dietary contents (Fan et al., 1994).

Items Regression equationsa–c Plateau valuesd R^2

Crude protein Y= 41.6 + 0.38X
0.0009X^2 83.1 ± 2.2 0.99
Amino acids
Indispensable
Arginine Y= 68.0 + 3.16X
0.103X^2 92.1 ± 1.5 0.99
Histidine Y= 67.8 + 7.04X
0.590X^2 88.8 ± 1.6 0.99
Isoleucine Y= 64.7 + 4.18X
0.190X^2 87.6 ± 1.3 0.99
Leucine Y= 65.7 + 2.42X
0.069X^2 86.9 ± 1.3 0.99
Lysine Y= 66.5 + 3.28X
0.129X^2 87.4 ± 1.4 0.99
Methionine Y= 67.1 + 14.99X
2.433X^2 90.1 ± 1.1 0.99
Phenylalanine Y= 71.8 + 3.06X
0.133X^2 89.4 ± 1.5 0.99
Threonine Y= 30.0 + 12.39X
0.786X^2 78.9 ± 1.9 0.99
Valine Y= 58.5 + 4.87X
0.223X^2 85.2 ± 1.5 0.99
Dispensable
Alanine Y= 47.8 + 7.59X
0.422X^2 82.0 ± 1.8 0.99
Aspartic acid Y= 61.2 + 2.12X
0.048X^2 84.8 ± 1.3 0.99
Cysteine Y= 40.9 + 25.34X
4.073X^2 80.3 ± 1.8 0.99
Glutamic acid Y= 71.1 + 1.04X
0.017X^2 87.3 ± 1.2 0.99
Glycine Y= 2.0 + 16.46X
0.915X^2 76.0 ± 1.8 0.99
Serine Y= 46.3 + 6.95X
0.320X^2 84.2 ± 1.6 0.99
Tyrosine Y = 65.2 + 5.71X
0.345X^2 88.7 ± 1.1 0.99

aY= apparent ileal crude protein or amino acid digestibility values (%).

bX= crude protein or amino acid content (g kg
(^1) , on dry matter basis).
cThe intercept, linear and quadratic slopes of the equations are significant (P< 0.01, n= 36).
dThe plateau digestibility values of apparent ileal crude protein and amino acids up to which the quadratic
regression equations are valid and their 95% confidence intervals.