The use of methods for the determina-
tion of nutrient digestibility values in an
assay feed ingredient is dependent on the
level of inclusion of the ingredient incor-
porated into the assay diet. There are three
methods for determination (Fan and Sauer,
1995a,b).
1.Direct method: the assay diet is
formulated in such a manner that the assay
ingredient provides the sole dietary
nutrient in question. Therefore, the nutrient
digestibility in the assay feed ingredient is
equal to the corresponding value in the
assay diet and can be calculated from
Equation 13.5.
DA= (DD100%)/(AASI) (13.5)where DAis the apparent digestibility of a
nutrient in the assay feed ingredient (%),
DDis the apparent digestible content of a
nutrient in the assay diet (g kg^1 ), AAis the
nutrient concentration in the assay feed
ingredient (g kg^1 ) and SIis the inclusion
level of the assay feed ingredient in the
assay diet (%).
2.Difference method: this method involves
the formulation of both a basal and an assay
diet. The basal diet contains the basal feed
ingredient which provides the sole assay
nutrient in the diet, whereas the assay diet
consists of a mixture of the basal and the
assay feed ingredient. If there is no inter-
action in nutrient digestibility between the
basal and the assay feed ingredient, their
relationship in the assay diet can be
expressed according to Equation 13.6.
DD= DBSB+ DASA (13.6)where DDis the apparent digestibility of a
nutrient in the assay diet (%), DBis the
apparent digestibility of a nutrient in the
basal feed ingredient (%), SBis the con-
tribution level (%) of a nutrient from the
basal feed ingredient to the assay diet, DA
is the apparent digestibility of a nutrient in
the assay feed ingredient (%) and SAis the
contribution level (%) of a nutrient from
the assay feed ingredient to the assay diet.
The apparent nutrient digestibility in
the basal feed ingredient is determined
from the basal diet according to the direct
method from Equation 13.5. The nutrient
digestibility in the assay feed ingredient
can be determined by difference according
to Equation 13.7 which is derived from
Equation 13.6.
DA= (DDDBSB)/SA (13.7)
3.The regression method: this method
evaluates the basal and the assay feed
ingredient simultaneously. If both the basal
and the assay feed ingredient provide the
sole assay nutrient in the assay diets and if
there is no interaction between the two
feed ingredients, the ingredients can be
mixed at various graded levels and more
than two assay diets are, therefore, formu-
lated. The relationship between amino acid
digestibility in the assay diets (DDi) and the
contribution levels (SAi and SBi) of the
nutrient from the basal and the assay feed
ingredient to the assay diets can be
expressed according to Equation 13.8. The
relationship between SAi and SBi is
expressed according to Equation 13.9.
DDi= DBSBi+ DASAi (13.8)
SBi+ SAi= 100% (13.9)
where DDiis the apparent digestibility of a
nutrient in the ith assay diet (%), DAis the
apparent digestibility of a nutrient in the
assay feed ingredient (%), SAi is the
contribution level (%) of a nutrient from
the assay feed ingredient to the ith assay
diet, DBis the apparent digestibility of a
nutrient in the basal feed ingredient (%)
and SBiis the contribution level (%) of a
nutrient from the basal feed ingredient to
the ith assay diet. When replacing SAiby
SAi= 100%SBi, Equation 13.8 can also
be written in the form of Equation 13.10.
DDi= DBSBi+ DA(1 SBi)
= DA+ (DBDA) SBi (13.10)
According to the previous definitions, DA
and DB are the digestibility coefficients
estimated for the assay and basal feed
ingredients. We define A=DA, B=DBDA,
so Equation 13.11 can be derived from
Equation 13.10.
DDi= A+ BSBi (13.11)
Equation 13.11 is actually a simple
linear regression model in which DDiis the292 W.C. Sauer et al.