dependent and SBi the independent
variable. The regression coefficients of A
and Bcan be obtained by fitting the simple
linear regression model.
DA= A (13.12)DB= DA+ B= A+ B (13.13)The assay nutrient digestibility values (DA
and DB) in the basal and the assay feed
ingredients can be determined from the
regression coefficients shown in Equations
13.12 and 13.13, respectively.
Studies were carried out to determine
the effect of the aforementioned methods
on apparent ileal digestibility values of
amino acids (Fan and Sauer, 1995a,b).
Barley, canola meal and peas were chosen
to be the representative feed ingredients for
cereal grains, protein supplements and
legume seeds, respectively.
Cereal grains
Five barrows, average initial body weight
35 kg, were surgically fitted with a simple
T-cannula at the distal ileum and fed five
diets according to a 5 5 Latin square
design. Five maize starch-based diets were
formulated. Diet 5 contained 900 g kg^1
barley providing the sole source of amino
acids. In diets 2, 3 and 4, barley and canola
meal were the only amino acid sources and
were included at three levels, 225, 450 and
675 g kg^1 , respectively for barley and 366,
305 and 244 g kg^1 , respectively for canola
meal. Diet 1 contained 427 g kg^1 canola
meal providing the sole source of amino
acids. Except for diet 5, all other diets were
formulated to contain 160 g kg^1 crude
protein. Chromic oxide was included in
the diets as the indicator for calculating
ileal amino acid digestibility values in the
assay diets (Fan and Sauer, 1995a).
The apparent ileal digestibility values
of crude protein and amino acids in barley
were determined according to Equation
13.7 at three levels of inclusion of barley
(diets 2, 3 and 4). There were increases
(P< 0.05) in the ileal digestibility values of
crude protein and the majority of amino
acids in barley as the level of inclusion was
increased from 225 (diet 2) to 675 g kg^1
(diet 4). With an increasing level of inclu-
sion of barley, there were large increases in
the contribution levels of crude protein
and amino acids from barley to the assay
diets. For crude protein, the increase in
contributions ranged from 14.2 to 42.6%.
Of the indispensable amino acids, the
increases in contributions were smallest for
lysine, ranging from 9.5 to 32.1%, and
largest for phenylalanine, ranging from
18.5 to 50.6%. Of the dispensable amino
acids, the increases were smallest for
tyrosine, ranging from 9.9 to 33.1%, and
largest for glutamic acid, ranging from 18.0
to 49.6%. The accuracy of determination of
ileal nutrient digestibility values by the
difference method is dependent on the
contribution levels of crude protein and
amino acids from the assay feedstuff to the
contents of the assay diets. The higher the
contribution level, the more accurate the
estimation of ileal digestibility values (Fan
and Sauer, 1995a). This was reflected by
decreases in the standard error of the
means of ileal digestibility values in
response to the increase in inclusion level
of barley in the assay diets. A similar
observation was reported by Van Leeuwen
et al. (1987) in studies with maize soybean
meal diets.
To estimate the ileal digestibility
values of crude protein and amino acids in
the assay ingredient (barley) by the regres-
sion method, the linear relationships
between the apparent ileal digestibility
values of crude protein and amino acids in
the assay diets and the contribution levels
of crude protein and amino acids from the
basal feed ingredient (canola meal) to the
assay diets were established according to
Equation 13.11 from diets 2, 3, 4 and 5.
The regression equations are presented in
Table 13.5. The principle of the regression
method is illustrated in Fig. 13.5 for lysine.
The ileal digestibility values of crude
protein and amino acids in barley by the
regression method were obtained from the
equations presented in Table 13.5 accord-
ing to Equations 13.12 and 13.13. The
digestibility values are presented in Table
13.6. However, the ileal digestibility values
of aspartic acid, isoleucine, leucine,Methods for Measuring Ileal Amino Acid Digestibility 293