Farm Animal Metabolism and Nutrition

of ileal amino acid digestibility values
among different samples of the same protein
supplement. Differences in dietary contents
of crude protein and amino acids are partly
responsible for the large variation in ileal
amino acid digestibility values within the
same feedstuff, especially for the limiting
amino acids. For example, the relatively
large variation that was reported for the
digestibility of methionine among samples
of soybean meal may result, in part, from the
fact that methionine is the limiting amino
acid in soybean meal. A relatively small
change in the methionine content in the diet
may therefore cause a relatively large change
in its digestibility value. Nevertheless,
differences in analytical techniques for
measuring methionine may also have
contributed to this variation.
With respect to peas, there was a rela-
tively large variation in the apparent ileal
digestibility values of methionine, cysteine
and tryptophan compared with the other
amino acids. The sulphur-containing amino

acids and tryptophan are the first and

second limiting amino acids, respectively

in peas. Therefore, small differences in

dietary contents of these amino acids may

have been responsible for a large propor-

tion of the variation in digestibility values.

The effect of methods for determination

The determination of apparent ileal

digestibility values of amino acids in

cereal grains is usually carried out by the

direct method, partly due to the fact that

most cereal grains are very palatable.

However, as discussed previously, the ileal

digestibility values of amino acids in

cereal grains determined with the direct

method are influenced by their amino acid

content in the assay diet. Therefore, it is

questionable whether the direct method is

valid for determining ileal amino acid

digestibility values in cereal grains (Sauer

et al., 1989).

Methods for Measuring Ileal Amino Acid Digestibility 289

Table 13.2.The initial plateau ileal digestibility values of crude protein and amino acids and the threshold
levels of dietary crude protein and amino acids (Fan et al., 1994).

Items Initial plateaua Threshold levelb Corresponding protein levelsc

Crude protein 80.9 171 154
Amino acids
Indispensable
Arginine 90.6 11.5 154
Histidine 87.2 4.3 161
Isoleucine 86.2 8.3 164
Leucine 85.6 13.3 163
Lysine 86.0 9.4 147
Methionine 89.0 2.4 137
Phenylalanine 87.9 8.2 147
Threonine 77.0 6.3 155
Valine 83.7 8.4 165
Dispensable
Alanine 80.2 6.9 154
Aspartic acid 83.5 17.1 140
Cysteine 78.5 2.4 143
Glutamic acid 86.2 23.1 110
Glycine 71.2 6.7 160
Serine 82.6 8.7 157
Tyrosine 87.6 6.2 163

aLower end points of 95% confidence intervals at the estimated plateau ileal digestibility values.

bg kg
(^1) (dry matter basis).
cThe dietary levels of crude protein (g kg
(^1) , on dry matter basis) corresponding to the threshold levels of
each amino acid.