Agriculture Reference
In-Depth Information
is of considerable importance. In most fac-
torial models these maintenance require-
ments are based on body weight, but be-
cause body lipid does not need to be
maintained (Emmans and Fisher, 1986), a
more accurate basis for calculating these re-
quirements would be the body protein con-
tent of the bird. Emmans and Fisher (1986)
and Fisher (1998) have raised this issue in
the past, and the concept has been success-
fully incorporated into some broiler (EFG
Software, 1995) and pig (Ferguson
et al
.,
1997) growth models. But little useful infor-
mation is available on the carcass protein
content of layers or broiler breeders during
lay or the extent to which this varies over
time, to enable such calculations to be made
of the maintenance requirements of these
birds.
Changes in body composition of the
broiler breeder hen have been measured
during the rearing period (Bennett and Leeson,
1990), at the end of the rearing period (Blair
et al
.,
1976; Pearson and Herron, 1980,
1981, 1982; Spratt and Leeson, 1987; Renema
et al
.,
1999), during the period when the
ovary and oviduct are developing (Bow-
maker and Gous,1989), and at the end of
the laying period (Blair
et al
.,
1976; Pearson
and Herron, 1980, 1981, 1982; Spratt and
Leeson, 1987; Attia
et al
.,
1995; Wilson
et al
.,
1995). However, the pattern of changes in
body composition over time in the ma-
ture hen has not been rigorously investi-
gated.
Pearson and Herron (1980) found that
feather-free body protein content of a broiler
breeder hen continued to increase between
22
and
34
weeks of age. However, a large
part of this increase will be in the growth of
the ovary and oviduct during the period
when the pullet reaches sexual maturity
(Bowmaker and Gous, 1989). Differences in
age at sexual maturity between birds in the
flock will also contribute to the variation in
the apparent increase in body weight during
this period, with early maturing birds no
longer growing, and perhaps even losing
weight because of the reproductive de-
mands placed on her, while those not yet
sexually mature continue to grow until they
have laid their first egg.
In laying hens it is well established
that body protein content is maximal at sex-
ual maturity and that little further protein
growth occurs during lay (Fisher and Gous,
2008). It could be argued that broiler breeder
hens are further from their somatically ma-
ture protein weight at sexual maturity than
are laying hens, and hence that body pro-
tein growth may continue when the oppor-
tunity arises. Such might be the case in poor
egg producers, where body protein may be
deposited if the number of pause days be-
comes excessive, but there is no evidence to
substantiate this. Provision for slow body
weight gain in broiler breeder hens is often
recommended, assuming a mean gain of
about 650 g from 50% egg production to
the end of lay. However, it has been demon-
strated in mammals that protein growth
does not occur when the animal is in a lac-
tating state, equivalent to the egg produc-
tion state in hens. Sows, for example, show
very little protein growth, if any, during ges-
tation (Shields and Mahan, 1983; King, 1987),
while they may lose considerable amounts
of body protein during lactation (Whittemore
and Yang, 1989) unless adequately fed (Coop
and Kyriazakis, 1999).
As the weight of body protein remains
relatively stable throughout the laying period,
and as any growth in body protein may be
regarded as taking place among non-laying
hens only, it should not be necessary to as-
sume that protein growth is obligatory when
determining nutrient requirements of laying
hens or broiler breeder hens. Also, because
changes in body lipid content are the conse-
quence of the way in which the hen has
been fed, it is unnecessary to make provi-
sion for any obligatory gain in body protein
or lipid during lay. Maintenance require-
ments may thus be considered to be con-
stant over the laying period for those birds
that continue to lay in closed cycles, and
these should be based on the body protein
content at the age of first egg.
Predicting Food Intake
To be of any real value, models that attempt
to optimize the feeding of laying hens and
