April 2013 - AFMA

enal osmotic malfunction and/or digesta
viscosity in the GUT.
Animals
It is common knowledge that most genetic
strains/lines within pigs and poultry are
directly selected for enhanced feed efficiency
(FE), as well as an improved protein
deposition rate (PDR) and subsequently
lean growth. This selection for improved
FE might result in direct selection of a
lower VFI due to the lower maintenance
requirements of the genetically improved
animals.
VFI also depends on the age and production/reproduction
stage of the animal
due to the physical development of the
GIT. Young rapidly growing broiler birds
would most probably consume as much
feed as physically possible, while laying
hens will regulate their VFI according to
their nutrient requirements in terms of
egg output.
Although chickens have fewer taste
buds than pigs (300 versus 15 000) in their
oral cavity, it doesn’t necessarily make
them less selective when it comes to feed
acceptability, and they have a well-developed
feed perception method. Certain
putative taste genes have been identified
in chickens and their ability to select feed
by means of sensory perception (particle
size, colour, hardness/flexibility of pellets
etc.), is well-known.
A condition called “neophobia” could
occur when the physical appearance in
terms of size and colour of a diet suddenly
changes, resulting in the refusal of feed.
Feed ingredients may reduce palatability,
while mycotoxins/moulds would result
in a significant depression of VFI. Additionally,
chickens prefer to eat “meal size”
portions, which mean they will eat a “full
meal” every hour if they are allowed to
do so. However, with an increase in stocking
density (>30kg/m 2 ) and growth rate,
a subsequent increase in competition for
space at feeder trays and drinker nipples
develops, which negatively influences VFI.
Some of the other animal factors that
might influence the VFI of pigs and chickens,
include social behaviour and dominance
or pecking order, and the availability
of “cage/pen” enrichment to reduce
boredom and stimulate exercise.
One aspect that is often neglected when
it comes to predicting VFI during diet formulation,
is the immune status of animals.
The general health of animals could either
render them incapable to obtain or ingest
feed due to physical strength/fever, or the
body’s own response to antigen formation
would increase the maintenance requirements
of the animal since nutrients
(amino acids) and energy are diverted for
this process. Therefore the general nutritional
status of animals must be borne in
mind when vaccinating animals, since a
few hours’ response might result in a few
grams lost within a 35-day growing period.
Environmental constraints
Environmental temperature (and relative
humidity) is probably the most important
environmental factor that influences VFI.
An increase in temperature above the upper
critical temperature (UCT) would decrease
VFI, especially if the CF of the diet
is high. This drop in VFI could partly be
offset by an increase in nutrient density,
but would only be a short-term method
to enhance nutrient consumption.
Also, the increase in feed price due to
an increased nutrient density would most
probably not warrant this practice for
broilers in the current financial climate.
Although cold temperatures would also
result in a poorer FE, the magnitude might
be less than that of hot temperatures.
Lastly, cleanliness, wind draught and
speed, floor isolation, NH 3
gas, photoperiod,
feeder and water line design could
all play a less important role in the prediction
of VFI for pigs and chickens. Years of
experience in terms of animal husbandry
and plenty of common sense will ensure
that you develop a set of rules for a specific
species and its specific nutrient requirements.
Ruminants
Chemostatic control
Control of intake by effects of blood metabolytes
on the appetite centre is known
as chemostatic regulation of feed intake.
An energy deficit is thought to generate
hunger signals, the intensity which is
directly related to the size of the deficit.
The signals controlling feed intake at the
metabolic level in ruminants are likely to
be different from those in monogastric
animals. Therefore it would seem unlikely
that a glucostatic mechanism of intake
control could apply to ruminants.
A more likely mechanism might involve
the volatile fatty acids (VFA) absorbed
from the rumen. Intra-ruminal infusions
of acetate and propionate have been
shown to depress VFI of concentrate diets
for ruminants. These same infusions into
the hepatic portal vein may also reduce
intake via signals send by the liver to the
hypothalamus.
Butyrate seems to have less of an
effect on feed intake than acetate and
propionate. With diets consisting mainly
of roughages, infusions of VFAs have had
less of a definite effect on VFI.
Effect of food characteristics
Ruminants are adapted to utilise “bulky”
feeds, hence fibrous foods have remain in
the digestive tract longer in order for their
digestible components to be extracted. If
foods are detained in the digestive tract,
the animal’s throughput (daily intake)
will be reduced. It is postulated that the
digesta load generates satiety signals, the
intensity of which is directly related to the
size of the load beyond a threshold value.
It has long been recognised that in
ruminants there is a positive relationship
between the digestibility of foods (resistance
to degradation) and their intake. In
previous experiments it was found that
dietary intake of lambs more than doubled
as the energy digestibility of the food
(hay source) increased from 0,4 to 0,8.
When supplementing a roughage
source with concentrates, the concentrate
added to a roughage source of low
digestibility tends to be eaten in addition
to the roughage, but when added to
roughage of high digestibility it tends
to replace the roughage. Therefore food
that is digested rapidly, and is also highly
digestible, promotes high intakes.
Neutral-detergent fibre (NDF) is in this
regard the primary chemical component
of foods that determines their rate of
Processing
AFMA MATRIX ● APRIL 2013 31