Farm Animal Metabolism and Nutrition

ileal amino acid digestibility values among
samples of the same feedstuff (in name)
(Sauer et al., 1990). Some factors responsible
for differences between feedstuffs have
been discussed previously (e.g. Sauer and
Ozimek, 1986; Knabe et al., 1989). The
objective of this chapter was narrowed
down to discuss the major factors that
influence the variation within rather than
between feedstuffs, with emphasis on
methodology for determining ileal amino
acid digestibility values.

Determination of Amino

Acid Digestibility

Several in vivo assays for determining
amino acid digestibility in feedstuffs for
pigs have been developed during the last
two decades. Considerable efforts have
been made to compare the validity of the
different amino acid digestibility assays.
Evaluation of different digestibility assays
has been addressed by Sauer and Ozimek
(1986) and Sibbald (1987). In the first part
of this chapter, the principles of different
digestibility assays will be introduced.
Faecal versus ileal amino acid digestibility
assays are discussed thereafter. Apparent
ileal versus true ileal amino acid digesti-
bility assay are also discussed.

Principles of digestibility assays

Apparent ileal and faecal amino acid digesti-
bility values are determined from studies in
which a diet is fed and the corresponding
ileal digesta or faeces are collected. Samples
of the diet and ileal digesta or faeces are
analysed for the contents of amino acids and
other items. There are two techniques avail-
able, namely: the total collection and the
indicator technique. With the total collec-
tion technique, the intake of the diet and the
output of ileal digesta or faeces have to be
measured accurately and the digestibility
values are calculated according to Equation
13.1.

DA= ((AI
AD)/AI)  100 (13.1)

where DAis the apparent ileal or faecal

digestibility values of amino acids in the

assay diets (%), AIis the amount of amino

acids consumed (g) and ADis the amount

of amino acids excreted in ileal digesta or

faeces (g).

However, the indicator technique

avoids the necessity of total collection of

ileal digesta or faeces and the measurement

of feed consumption. With the indicator

technique, an indigestible marker (e.g.

Cr 2 O 3 ) is included in the assay diet and

representative samples of diet and ileal

digesta or faeces are taken; the digestibility

values are calculated according to Equation

13.2.

DD= 100% 
((IDAF)/(IFAD))

100% (13.2)

where DDis the apparent digestibility value

of amino acids in the assay diet (%), IDis

the indicator concentration in the assay diet

(g kg
^1 ), AFis the amino acid concentration

in ileal digesta or faeces (g kg
^1 ), IFis the

indicator concentration in ileal digesta or

faeces (g kg
^1 ) and ADis the amino acid

concentration in the assay diet (g kg
^1 ).

Apparent amino acid digestibility

values do not take into account amino acids

in ileal digesta or in faeces of endogenous

origin. True amino acid digestibility values

are calculated on the basis of apparent

digestibility values and the estimated

amount of endogenous amino acids in ileal

digesta or faeces according to Equation 13.3.

DT= DA+ (AE/AD) 100% (13.3)

where DTis the true ileal or faecal amino

acid digestibility value in the assay diet

(%), DAis the same definition as described

for Equation 13.1, AEis the amount of

endogenous amino acids recovered in ileal

digesta or faeces (g kg
^1 dry matter diet)

and ADis the amino acid concentration in

the assay diet (g kg
^1 dry matter diet).

Faecal versus ileal amino acid

digestibility values

Amino acid digestibility values can be

determined according to the ileal or faecal

Methods for Measuring Ileal Amino Acid Digestibility 281