Agriculture Reference
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protein when given in an elevated quantity.
However, this is a misleading conclusion and
not in accordance with reality.
Consequently, a validated comparison of
both single feed protein sources and mixed
diets required a standardization of the intake
level. Under such circumstances ( Fig. 6.1 ) a
clear difference between the examined pro-
tein sources was obviously achieved. In this
way the application of NPU data standardized
for daily N intake could improve this critical
situation in developing protein evaluation
parameters. However, no validated or scien-
tifically based tools were available for such
a standardized approach. Gebhardt (1966)
proposed a physiologically based description
of the course of body N retention, dependent
on both the quantity and quality of N intake
and using an exponential function conform-
ing to biological laws of growth, as described
by von Bertalanffy (1951).
Liebert and Wecke, 2008; Wecke and Lie-
bert, 2009, 2010, 2013) and are defined as
follows:
NR = NR max T ( 1 - e NI . b )
(6.1)
ND = NR max T ( 1 - e −NI . b ) - NMR
(6.2)
Where NR = daily N retention ( ND + NMR ) in
mg/BW kg
0.67 ; ND = daily N deposition or N bal-
ance = NI - NEX in mg/BW k 0.67 ; NI = daily N
intake in mg/BW k 0.67 ; NEX = daily N excretion
in mg/BW k 0.67 ; and NMR = daily N mainten-
ance requirement in mg/BW k 0.67 ; NR max T = the-
oretical maximum for daily N retention in
mg/BW k 0.67 ; ND max T = NR max T - NMR = theoret-
ical maximum for daily N deposition in mg/
BW k 0.67 ; b = slope of the N retention curve (in-
dicator of dietary protein quality; the slope of
the curve for a given protein quality is inde-
pendent on NI ); e = basic number of natural
logarithm (ln); and BW = body weight.
The attribute 'theoretical' suggests that
the threshold values ( ND max T or NR max T ) are
generally not in the scope of practical per-
formance data but estimate the genetic
potential (Samadi and Liebert, 2006a). In so
doing, the genetic potential is defined as an
unreachable 'theoretical' threshold value.
The growth potential defined in this way can-
not be realized even with an optimized feed-
ing strategy or in ideal environmental con-
ditions. If the ranking of such a threshold
value is clear, no problem exists for further
model applications. Accordingly, individ-
ual AA requirement data are derived using
daily protein deposition data in line with
practical growth data. As a consequence, the
threshold value ( ND max T resp. NR max T ) is used
only as a parameter to relate the real rate of
deposition to the assumed genetic potential.
An example of ND max T prediction in a meat
type chicken is given in Fig. 6.2.
The experimental data for such a pre-
diction result from N response experiments
in which a graded protein supply is used.
Generally the aim is to achieve a very high
variation in the average individual daily NI
by varying the dietary protein content.
Physiological mechanisms set a limit for the
upper range of the achievable daily NI .
Force-feeding could partly extend the upper
NI level, but this practice is prohibited by
Basics of the model development
In fact, the designation 'Goettingen approach'
is partly misleading because the procedure
had already been developed to a significant
extent in the years between 1970 and 1990
at Leipzig University under the scientific
leadership of Günter Gebhardt who held the
post of chair of animal nutrition for 25 years.
However, during the past 15 years the
Division Animal Nutrition Physiology at
Goettingen University has focused on filling
in gaps in the procedure for current applica-
tions in animal nutrition. Because of the
great potential for applying modelling pro-
cedures in modern feed to food conversion
systems with farm animals, both the evalu-
ation of the efficacy of dietary amino acids
and modelling of amino acid (AA) require-
ments by accounting for dietary AA effi-
ciency were of special interest during this
working period.
Model parameters used in current appli-
cations of the non-linear model are based on
recent publications (Liebert et al ., 2000;
Thong and Liebert, 2004a,b,c; Samadi and
Liebert, 2006a,b, 2007a,b, 2008; Liebert
and Benkendorff, 2007; Liebert, 2008, 2009;
 
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