April 2013 - AFMA

digestion. As a consequence of this, foods
equal in digestibility, but which differ in
NDF content, will promote different feed
intakes. The more NDF a diet contains, as
well as level of lignification, the more VFI
will be depressed.
“Another important concept
is that ruminant animals
consume feeds to maintain a
constant amount of DM in the
rumen”
Mechanical grinding of roughage
accelerates breakdown by destroying
the structural organisation of cell walls,
thereby accelerating their breakdown in
the rumen and increasing intake. Note
that fine particles pass rapidly out of the
rumen, leaving room for more food, but
allowing some digestible material to
escape undigested.
The deficiency of essential nutrients
or the presence of deleterious substances
that reduce the activities of rumen microorganisms,
are liable to reduce feed intake.
The most common is a protein/nitrogen
deficiency. Other nutrients whose deficiencies
are liable to restrict food intake
in ruminants are phosphorus, sulphur, sodium
and cobalt.
Although the concept of palatability
is not easily defined, it is not thought to
be an important factor determining intake,
except where the food is protected
against consumption or contaminated in
some way. However, if a particular flavour
of food becomes associated in the minds
of ruminants with unpleasant consequences
(especially sheep), they tend to
avoid food with that flavour.
Animal factors
Consumption of feed dry matter (DM)
usually ranges from 1,5 to 2% of body
weight in older animals on a maintenance
ration, to about 2,5 to 3,5% for finishing
lambs and beef cattle. It may also be as
much as 3,8 to 4% for dairy cows during
peak production.
Intake seems to be restricted due to
the capacity of the gastro-intestinal tract,
i.e. rumen, with stretch and tension receptors
in the rumen wall signalling the degree
of “fill” to the brain. What constitutes
the maximum (critical) fill of the rumen
is still uncertain. Circumstances that may
change the relationship between the size
of the rumen and the size of the whole
animal (abdominal fat depots, stage of
pregnancy, proportionality to metabolic
body weight, W0.75) are likely to affect
intake as well.
The notion that voluminous foods (hay
and straw) will fill the rumen to a greater
degree than concentrates has received
some attention, although after rumination,
these voluminous foods are not as
“bulky” as in the feeding trough. There is
also evidence that foods with high water
content (about 900g/kg) promote a lesser
DM-intake than comparable foods with
lower water content.
Another important concept is that ruminant
animals consume feeds to maintain
a constant amount of DM in the
rumen. Ruminants appear to limit feed intake
in relation to its capacity to dispose
of energy via the pathways of oxidation
and synthesis. Prolonged under-nutrition
results in an enhanced capacity to utilise
energy, as often reflected in compensatory
growth, and its attendant increase in
feed intake.
The feed intake of pregnant (hormonal
changes) or lactating animals, apart from
volume restrictions mentioned above,
may increase due to the increased need
for nutrients, the latter being mainly
physiological in origin.
According to food choice, when two
feeds differ in the concentration of one
nutrient in relation to energy requirements,
the animal’s interests are best
served by eating the two in such a ratio
that the intake of the nutrient in question
is optimised. As mentioned above,
ruminants may also exhibit “neophobia”,
a reluctance to accept a new food.
Regarding extensive production systems,
ruminants attempt to graze for the
shortest period of time possible because
the energy expenditure in eating is related
to time rather than to ingested mass.
Environmental factors
Voluntary intake of pasture by grazing
ruminants is not only influenced by
chemical composition and digestibility,
but also by its physical structure and
spatial distribution (walking distance).
Therefore its intake is primarily determined
by bite size, bite rate and grazing
time. Ruminants tend to choose long
fibrous foods rather than finely ground
foods for normal rumen functions, as a
lack of fibre leads to fluctuations in rumen
pH and an absence of rumination.
Grazing animals also tend to prefer
rapidly digested leaves to more slowly
digested stems. Some plants may be
rendered unpalatable and ultimately
be rejected due to protected spines or
contamination with excreta. In intensive
systems, frequent feeding of a total
mixed ration (TMR) does not necessarily
increase VFI, but may help to stabilise rumen
fermentation.
Another feature regarding the environment
is the effect of day length on VFI.
Small ruminants tend to decrease their intake
as day length declines, where short
days coincide with a shortage of food.
Like in monogastric animals, feed
intake is also influenced by environmental
temperatures, water intake and ill health.
Prediction of intake
Animals are commonly fed according to
appetite, and it is not possible to predict
their performance using feeding standards
without an intake estimate. For pigs
and poultry it may be relatively simple,
but predictions for ruminants are more
difficult as many food variables have to
be taken into account.
There will always be a significant proportion
of variation in behaviour of individuals
that cannot be predicted from
knowledge of their physiological state
and the composition of foods on offer.
Due to the complexity of modulation and
prediction equations, it cannot be fully
discussed here (animal features and environmental
effects).
References available from the authors.
Please email EinkamererOB@ufs.ac.za
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AFMA MATRIX ● APRIL 2013 33