Agriculture Reference
In-Depth Information
laying hens, broiler breeders, roosters), and
a wide range of methods has been used. All
methodologies that estimate requirements
may be criticized because biological phe-
nomena are, in general, difficult to measure
precisely or to explain mathematically. In
addition to different methodologies, there
are also several factors that affect requirements,
such as the scale of measurement. In add-
ition to Armsby and Moulton (1925), many
other authors have attempted to define the
concept of maintenance. According to Leveille
and Fisher (1958, 1960) and Leveille
et al
.
(1960), maintenance is a status in which
laying hens are able to maintain egg produc-
tion and to stay alive, and minimal amounts
of nutrients are needed to maintain the balance
between intake and excretion. The generally
accepted definition of amino acid mainten-
ance requirement of chickens is the situation
where nitrogen intake is exactly equal to the
sum of its losses, thereby keeping body nitrogen
(N) content constant (Owens and Pettigrew,
1989; Sakomura and Coon, 2003; Sakomura
and Rostagno, 2007).
During maintenance, metabolism results
in body protein loss, which must be replaced
by the diet (Bonato
et al
., 2011). Moughan
(2003) lists these processes in chickens: amino
acid losses via skin and feathers; amino acid-
derived nitrogen losses in the urine indicating
body protein turnover inefficiency; endogen-
ous amino acid losses from the gut (mainly
mucus, bile, sloughed cells); irreversible
amino acid loss due to the synthesis of es-
sential non-amino acid nitrogenous metab-
olites (e.g. creatinine); irreversible chemical
changes of amino acids (e.g. lysine to hy-
droxyl lysine); and the loss of free amino
acids in the urine. In order to maintain ni-
trogen balance, amino acids must be supplied
at the same rate as they are lost through me-
tabolism, secretion or excretion from the body
(Sakomura and Rostagno, 2007) and in the
precise ratio in the feed.
Several approaches have been applied
for the determination of maintenance re-
quirements. For instance, the Reading Model
is a factorial approach that estimates main-
tenance by extrapolation (Fisher
et al
., 1973;
Bowmaker and Gous, 1991). The Gottingen
approach also uses the factorial method based
on nitrogen balance (Samadi and Liebert,
2006a,b, 2007a,b, 2008; Liebert, 2008). Ni-
trogen balance trials measuring the point
where retention is equal to zero, that is,
losses are equal to intake, were reported by
Gous
et al
. (1984), Burnham and Gous
(1992), Nonis and Gous (2008), Bonato
et al
.
(2011) and Siqueira
et al
. (2011). The com-
parative slaughter technique has been used
to measure nitrogen retention (Edwards
et al
., 1997, 1999; Kim
et al
., 1997a,b,c).
Each of these approaches may be criticized
because maintenance is a biological phenom-
enon and, therefore, difficult to measure and
to explain mathematically.
Different diet formulations have been
used for the determination of the mainten-
ance requirements of chickens. Leveille and
Fisher (1959, 1960) used a whole egg diet
and an amino acid-free mixture, which con-
tained
13
amino acids at the same levels as
those found in whole egg protein. The curvi-
linear response they obtained for isoleucine
(the test amino acid) suggests that other
amino acids may have been limiting. The
method using supplementation of crystal-
line amino acids does not reflect practical
situations, when amino acids are included
in plant and animal protein, because they
are fully digestible and, therefore, true amino
acid requirements may be underestimated
(Siqueira, 2009). Another method applied is
diet dilution, proposed by Fisher and Morris
(1970), where amino acid ratios are kept
constant at the different evaluated levels.
When determining maintenance require-
ments feeding methods must also be con-
sidered, with both
ad libitum
and force feeding
being commonly applied. However, some pro-
blems may arise. For instance, Leveille and
Fisher (1959, 1960) observed that chickens
rejected a pelleted feed deficient in threo-
nine. This may be explained by the fact that
diets used to estimate maintenance require-
ment must contain extremely low levels of the
test amino acid, casting doubts on the valid-
ity of the results of these trials. Sibbald (1976)
described a force-feeding method in which
feed is introduced directly into the crop of
the bird, which enables feed intake to be
precisely determined. Gous
et al
. (1984) pro-
posed the method of force feeding of the test
