analysis method, as defined previously in
Equations 13.1 and 13.2. The faecal
analysis method, developed by Kuiken and
Lyman (1948), has been used extensively in
studies with pigs (e.g. Eggum, 1973). These
studies, in particular those by Eggum
(1973), opened the field of research on the
topic of amino acid digestibility.
The ileal analysis method should be
considered an improvement over the faecal
analysis method. The original studies by
Zebrowska (1973) showed that both in-
tact and enzymatically hydrolysed casein
infused into the distal part of the ileum of
pigs fed a protein-free diet was fermented
and absorbed; however, the absorbed mater-
ial was excreted in urine rapidly and almost
completely. When casein was given orally,
the levels of free amino acids in portal blood
were high. Other reports (e.g. Wünsche et
al., 1982) showed that protein or amino
acids infused into the large intestine make
little or no contribution to the protein status
of the pig. Some contribution could occur
under dietary conditions when nitrogen per
seis limiting for the synthesis of the dis-
pensable amino acids, thereby sparing the
utilization of the indispensable amino acids.
A series of events occur when
undigested protein, from both dietary and
endogenous origin (including peptides and
amino acids not absorbed before reaching
the end of the small intestine), enters the
large intestine. A certain proportion of
dietary protein passes through the large
intestine and is excreted in faeces; the
remainder is fermented by the microflora.
Nitrogen will be either absorbed, primarily
in the form of ammonia (a small proportion
in the form of amines and amides), or incor-
porated into microbial protein. Some of the
microbial protein will be fermented and the
nitrogen absorbed, primarily in the form of
ammonia. The remainder will be excreted in
faeces. The fate of endogenous protein is
likely to be similar to that of dietary protein.
Additional evidence of bacterial fermenta-
tion in the large intestine is shown by the
large amount of bacterial nitrogen present in
faeces. Mason (1984) showed that bacterial
nitrogen can amount to 62–76% of total
nitrogen in faeces. The factors that affect
microbial activity in the large intestine,
including the amount of available fer-
mentable carbohydrates, were discussed
previously by Mason (1984) and Sauer and
Ozimek (1986).
Evidence that the ileal rather than
faecal analysis method should be used for
determining amino acid digestibility values
was provided by Dierick et al. (1988) in
studies in which the performance of pigs
was related to digestibility measurements.
There was a higher correlation between aver-
age daily gain and ileal rather than faecal
protein digestibility (r = 0.76 versus r=
0.34). In the same order, for feed conversion
efficiency (kg feed consumed kg^1 carcass
gain), the correlation coefficients were 0.87
and 0.65, respectively. These results fur-
ther support the view that nitrogen absorbed
in the large intestine does not contribute sig-
nificantly to protein synthesis.
Amino acid digestibility values ob-
tained with the faecal analysis method are,
for most amino acids in most feedstuffs,
higher than those determined with the ileal
analysis method. However, net synthesis of
methionine and lysine has been reported in
the large intestine in several studies
(Zebrowska, 1978; Low, 1980; Sauer et al.,
1982; Tanksley and Knabe, 1982). Therefore,
depending on the amino acid and on the
feedstuff, digestibility values obtained by
the faecal analysis method overestimate
(which is usually the case) or underestimate
those obtained by the ileal analysis method.
Lysine, the sulphur-containing amino acids,
threonine and tryptophan can be considered
as the most important amino acids in
practical diet formulation, as these are often
the first, second or third limiting amino
acids. Of these amino acids, cysteine,
threonine and tryptophan usually disappear
to a large extent in the large intestine
(Zebrowska, 1978; Low, 1980; Sauer et al.,
1982; Tanksley and Knabe, 1982).Apparent versus true ileal amino acid
digestibility valuesMany studies have shown that proteins in
the body are in a dynamic state and that282 W.C. Sauer et al.