April 2013 - AFMA

Factors affecting the
voluntary feed intake of livestock
By Foch-Henry de Witt and Ockert Einkamerer, University of the Free State, Bloemfontein
We often assume that
because there is 15%
protein or 60% carbohydrate
in a ration,
that the animal has to
use all of it. This assumption is not true.
In fact, we must consider that even the
ingested nutrients are still “outside” the
body until it has been absorbed. When
any animal is presented with food, for
some reasons that may not be known,
the animal may consume 95 units of this
material and refuse the balance.
Increasing intake of high-producing
animals is essential, but care must be
taken to prevent metabolic disorders
and excessive fat accumulation closer to
the end of the feeding period. Feeding is
a complex activity which includes such
actions as the search for food, recognition
and movement towards it, sensory
appraisal, the initiation of eating and ingestion.
It is necessary to consider why, in
most mature animals, body weight is
maintained more or less constant over
long periods of time, even if feed is available
ad libitum. Hence, the concepts of
short- and long-term control of food
intake must be considered. The former
concerns initiation and cessation of individual
meals, and the latter the maintenance
of a long-term energy balance.
Although thought to be similar, there are
important differences between species.
This depends mainly on the structure
and function of their digestive tracts.
Non-ruminants
The most problematic factor in diet formulation
for monogastric animals is
probably the correct prediction of voluntary
feed intake (VFI). Although diet formulators
are normally skilled in the use
of software programmes for “least-cost”
formulation, they should continuously
strive to improve their biological knowledge
regarding the factors influencing
VFI. The followings aspects will be briefly
discussed to indicate their importance in
VFI of pigs and chickens.
Feed
Due to the limited nutrient contribution
of microbial fermentation in monogastric
animals, pigs and chickens normally consume
feed in accordance with the diet’s
first limiting nutrient – thus any shortage
of amino acids, minerals, vitamins and
energy would provoke an increase in VFI.
Additionally, the amino acid profile as
well as the lysine against apparent metabolisable
energy (AME) ratio, must concur
with the requirements of the specific
requirements of the animal.
Generally, it is assumed that a decrease
in dietary energy would result in
an increase in VFI to compensate for the
energy loss, until GUT capacity becomes
the limiting factor. It is indicated that the
threshold energy value for poultry is between
10,1 and 10,8 MJ AME/kg, depending
on the environmental temperature,
thereby suggesting that an energy intake
below these values would negatively influence
lean protein growth in favour of
adipose tissue deposition.
The physical form of the diet (wet/dry
mash, crumbs and pellets), as well as particle
size and the distribution of particles
(% particles ≥1,0 mm & ≤3,5 mm), would
not only influence VFI but also total available
nutrient intake, rate of passage and
digestibility coefficients of a given diet.
Nutrient density or “bulkiness” of diets,
which are mostly linked to the AME and
crude fibre (CF) concentration of diets,
plays an important role in VFI, especially
in young animals with limited GUT capacity
and a high rate of passage due to
the rate of nutrient metabolism.
In addition to that, the water-holding
capacity (WHC) and non-starch polysaccharide
(NSP) component of certain feed
sources (especially if no synthetic enzymes
are included and the diet was not
exposed to heat/steam treatment) will
impede VFI due to its interference with
the rate of passage and digesta viscosity.
“It is highly unlikely that feed
with such a high PV will be fed to
pigs and chickens under
commercial conditions.
It is also known in poultry that
the relationship between water
consumption and feed intake is
linear”
Feed acceptability due to rancidity
“off odours” or “sourness” from wet fermentation
of diets represent a classical
interaction between “feed qualities versus
animal tolerance” which will influence
VFI – mostly in a negative manner.
Although some literature indicates that a
dietary peroxide value (PV) of 75 to 150
milli-equivalent peroxide per kilogram
fat had no negative effect on feed intake
and production performances of birds, it
remains open for debate.
It is highly unlikely that feed with
such a high PV will be fed to pigs and
chickens under commercial conditions.
It is also known in poultry that the relationship
between water consumption
and feed intake is linear. Any factor that
influences water intake (WI) would subsequently
influence VFI. Simultaneously,
water quality in terms of chemical and
microbial contaminants will influence WI
and eventually VFI of the animals due to
Processing
AFMA MATRIX ● APRIL 2013 29