294 W.C. Sauer et al.
Table 13.5.The linear relationships between the apparent ileal digestibility values and the contribution
levels of crude protein and amino acids for the determination of apparent ileal digestibility values of amino
acids in barley with the regression method (Fan and Sauer, 1995a).
Items Regression equationsa,b r^2 Sy. xc Pd
Crude proteine Y= 57.7 + 6.6X 0.52 3.39 0.001 0.05
Amino acids
Indispensable
Argininee Y= 68.4 + 11.1X 0.69 2.49 0.001 0.01
Histidinee Y= 70.5 + 8.2X 0.48 2.71 0.001 0.05
Isoleucine Y= 64.5 + 2.6X 0.01 3.81 0.001 0.66
Leucine Y= 67.6 + 1.5X 0.01 3.75 0.001 0.78
Lysinee Y= 59.3 + 12.5X 0.72 3.54 0.001 0.01
Phenylalanine Y= 68.6 + 0.6X 0.01 4.41 0.001 0.91
Threonine Y= 62.3 + 1.9X 0.01 3.68 0.001 0.52
Valine Y= 63.9 + 1.5X 0.01 3.94 0.001 0.79
Dispensable
Alaninee Y= 53.9 + 12.7X 0.74 4.04 0.001 0.05
Aspartic acid Y= 60.1 + 1.9X 0.01 3.75 0.001 0.75
Glutamic acide Y= 72.2 + 7.4X 0.55 3.02 0.001 0.03
Glycinee Y= 43.5 + 18.5X 0.76 5.51 0.001 0.01
Serine Y= 61.8 + 1.3X 0.01 3.45 0.001 0.79
Tyrosinee Y= 58.2 + 6.3X 0.54 3.54 0.001 0.04
aY= apparent ileal digestibility values of crude protein or amino acids (%) in the assay diets.
bX= the contribution levels of crude protein or amino acids from the basal feed ingredient (canola meal) to
the content of the assay diets.
cStandard error of estimate of the regression equation.
dThe probabilities of significance for the intercept and the slope of the regression equation.
eThe linear regression equation is significant (P< 0.05, n= 20).
phenylalanine, serine, threonine and
valine could not be estimated. This
resulted from the fact that the differences
in ileal digestibility values of these amino
acids between the assay and the basal feed
ingredients were not large enough to create
linear variations (Table 13.5).
The apparent ileal digestibility values
of crude protein and amino acids in barley
were also determined by the direct method
according to Equation 13.5 from diet 1. The
digestibility values are compared in Table
13.6.
Among the three methods, the regres-
sion method is in principle the most
accurate. The apparent ileal digestibility
values of crude protein and amino acids in
barley estimated by the regression method
are considered to be the most precise
values for evaluating the results obtained
by the other two methods. As shown in
Table 13.6, the ileal digestibility values in
barley estimated by the regression method
were not different (P> 0.05) from values
determined by the difference method at the
barley inclusion level of 675 g kg^1 (diet 4).
However, determined by the difference
method, the ileal digestibility values of
amino acids in barley were underestimated
at the lower inclusion levels (225 and 450 g
kg^1 ). This shows that the difference
method is only reliable when the inclusion
levels of the assay amino acids from the
test feedstuff in the diets are high.
Furthermore, the ileal digestibility values
of crude protein and amino acids deter-
mined by the difference and regression
method were usually higher (P< 0.05 or
0.10) than those determined by the direct
method. This shows that the direct method
is not a valid approach for the deter-
mination of amino acid digestibility values
in feedstuffs with a low protein content.
On the other hand, for some low-protein
feedstuffs that have a limited level of
inclusion in the assay diets because of poor