Anut 300 500 12 T 23
Anut 300 500 12 T 23
Anut 300 500 12 T 23
Topic 23
Mike Freer
Applied Animal Nutrition: Nutrition Management for Grazing Animals ANUT300/500 –23 - 1
©2009 The Australian Wool Education Trust licensee for educational activities University of New England
23. Factors Influencing the Voluntary Intake
of Food
Learning Objectives
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©2009 The Australian Wool Education Trust licensee for educational activities University of New England
>60% in herbage. For hand-fed animals in feedlots or dairies, the
main reason for wanting to know their voluntary intake is to
ensure that a weighed ration of a formulated diet is within the
animal’s capacity. Most other ruminants in Australia subsist
almost entirely on grazed pasture and here the situation is quite
different.
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23.2 Potential intake of feed by the animal
1.6
Potential intake (kg DM/d)
1.4
1.2 Sh
1.0
0.8
0.6
0.4
0.2
0.0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Relative size
10
Potential intake (kg DM/d)
9
8 C
7
6
5
4
3
2
1
0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Relative size
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Animal size and condition
I = b M Z(1-Z) F
(1)
where: b has suggested values of 0.04 for sheep and 0.025 for
cattle
Physiological state
Pregnancy
The development of the conceptus requires an exponential
increase in the additional energy demand of the animal but the
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potential intake of food does not increase. This is thought to be
because the increasing space occupied in the body cavity restricts
the capacity of the reticulo-rumen. Indeed, feed intake is
maintained at the earlier level only by a faster rate of clearance of
digesta from this organ. The decline in intake that is usually
observed during the last few days of gestation is probably related
to endocrinological changes.
Lactation
Potential intake increases in proportion to energy demand during
lactation but lags behind it in time as the capacity of the reticulo-
rumen slowly increases after parturition. A peak or plateau in
intake is not reached until about 4 months after calving or about 6
weeks after lambing, a situation that obviously has important
consequences for the maintenance of the energy balance of the
lactating animal. In high yielding dairy cows, the peak value may
be more than double that of the dry cow, in beef cows or ewes
with one lamb there may be a 50% increase and in ewes with twins
an 80% increase. As the lactation progresses, the increase in
intake follows a similar pattern to the lactation curve and will be
depressed if milk production is restricted by poor feed supply or
low body condition, particularly at parturition. An example of this
pattern for lactating ewes is shown in Fig. 23-2; the intake factor
predicted here is used as a multiplier for potential intake in
equation 1. Again, a wide range of conditions for ewes and cows
may be tested in the Explorer programs for their effects on this
multiplier.
Unweaned young
The potential intake of solid food by unweaned lambs and calves
in the first few weeks of life depends on rumen development
rather than body weight. The appropriate proportion, s, of the
potential intake that would be calculated from equation 1 is
predicted from equation 2 and again the value is used as a
multiplier in the earlier equation.
T = age (days)
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Climatic factors
Climatic factors leading to thermal stress in the animal will also
affect voluntary food intake, but the response will depend on the
extent of insulation and level of metabolic activity of the animal
and on the quality of the diet. Indoor studies show a consistent
increase or decrease in food intake as the ambient temperature
falls or rises, respectively, beyond the thermal neutral zone of the
particular animal. However, few measurements have been made
under grazing conditions, where the flexibility of grazing
behaviour allows animals to mitigate the effects of climatic
extremes and it is difficult to make predictions for grazing
animals. The adjustment for high temperatures that is used in the
GrazFeed program operates when the average daily temperature
exceeds 25°C and the night temperature exceeds 22°C. The
potential intake of herbage by cattle, other than Brahman types, is
then reduced by 2% for each rise of 1°C in average daily
temperature (Fox 1987); for other stock the reduction is 1% per °C.
If the ambient temperature falls below the animals lower critical
temperature, potential intake is increased by 1% per °C (Fox 1987);
an effect that is reduced with rainfall, to disappear at 20 mm per
day.
2.0
1.8
Intake factor
1.6
1.4
1.2
1.0
0 10 20 30 40 50 60 70 80 90 100
Day of lactation
Diseases
Diseases reduce the potential intake of the animal and parasitic
infestations are of particular relevance to grazing animals. Tests
with different intestinal parasites indicate a complex pattern of
responses depending on the level of infection and the
development of resistance (Coop and Sykes, 2002) and, as a
result, it is not possible at present to make quantitative
predictions.
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Table 23-1. Predicted mean potential intake (kg DM/d) by
growing sheep and cattle of different Standard Reference
Weight (SRW) (from data in Freer et al. 2005)
(kg)
20 30 40 50 60 70 80 90
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Table 23-2. Effect of maturation on the attributes that affect relative
ingestibility and voluntary intake of phalaris and subclover by sheep
a
Hogan et al. (1969)
b
Weston and Hogan (1971)
Relative ingestibility
Within an upper limit set by the energy demand of the animal, the main
characteristics of plant material that determine its intake by ruminants
are those that limit the rate at which it can pass through the gut. About
70% of the digesta are in the reticulo-rumen, from which they disappear
by microbial digestion and by onward passage through the reticulo-
omasal orifice when the feed particles are sufficiently small (ca. 1 mm)
and dense to flow out in the fluid phase. The rate of breakdown of the
feed, through chewing (during eating and rumination) and microbial
action, depends very largely on chemical and physical features of the
structural carbohydrates in the plant tissue that change as the plant
matures. Progressive crystallization and lignification of the cell wall
material make the feed more and more resistant to breakdown, increase
the time spent chewing per kg of food and reduce relative ingestibility
(see Table 23-2).
These chemical changes that affect the clearance rate of digesta from
the rumen are crudely reflected in the overall apparent digestibility of
the diet (Table 23-2), a characteristic that is much more readily
estimated than the cell wall structure and has been used widely as a
predictor of relative ingestibility. A review (Freer, 1981) of several
experiments with different pasture species fed to sheep and cattle in
pens, showed a linear relationship between apparent digestibility and
voluntary intake over the full range of maturity to be found in herbage
(digestibility from 30% to >80%). For a 50 kg SRW sheep, voluntary
intake increases at about 20-25 g DM per unit increase in digestibility
and the relationship appears to be proportionately the same for cattle
(Hodgson, 1977). Variation in this relationship occurs between different
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pasture species and also if the diet is deficient in specific nutrients
essential for microbial activity in the rumen.
1.0
0.9
Relative ingestibility
0.8
0.7
0.6
0.5
Tropical
0.4
Temperate
0.3 Legume
0.2
30 40 50 60 70 80
Species differences
When different pasture species have been compared, the slope of the
regression of relative ingestibility on digestibility has been similar but
there are significant differences in the intercept values (see Fig. 23-3).
One of the main examples is the greater intercept for most pasture
legumes (ca. 0.17 on the relative ingestibility scale) compared with
temperate grasses. There is no suggestion here of a dietary preference
for legumes (the evidence for that is quite equivocal), merely that
digesta from legumes are broken down more rapidly in the rumen and
therefore have a higher clearance rate. This enables a greater intake of
feed, depending on the proportion of legume in the available herbage.
The effect will diminish with herbage weight as declining availability
becomes more important than gut clearance rate in constraining intake.
Nutrient deficiencies
Relative ingestibility may be depressed if the diet is deficient in certain
chemical constituents, particularly those that are essential nutrients for
the rumen microbial population, on which the optimum clearance rate of
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the digesta depends. The most common deficiency is in rumen-
degraded protein (RDP), made up of nitrogenous compounds that can
supply the ammonia essential for microbial synthesis. The requirement
for RDP ranges between 7 and 11 g/MJ of the metabolizable energy (ME)
intake that is fermentable in the rumen, i.e. after deducting the ME in
fat, undegraded protein and silage acids. If the RDP intake is less than
the requirement, feed intake will fall and the microbial population will
be maintained at a lower level of activity by the recycling of circulating
urea back into the rumen.
Relative Availability
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Fig. 23-4. Relative availability and its component attributes, for
sheep, (for the first herbage class, where the unsatisfied
appetite of the animal has a relative value of 1), in relation to
the weight of herbage dry matter; upper line is relative time
spent grazing, lower line is relative rate of eating and middle
line is the product, relative availability (from Freer et al. 2005)
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Selective grazing
In semi-arid grazing areas, where the vegetation is very variable and the
animals graze only some of the plants, this method for predicting diet
digestibility would be inappropriate. The most promising alternative is
through the analysis by near infrared spectroscopy of faecal samples
from the grazing animals, calibrated against known standards (Coates
1999). However, some meaningful estimate of the relative availability of
the feed base is still required for the prediction of relative intake.
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Table 23-3 Predicted intake of feed by a lactating Merino ewe
with a potential intake of 2.13 kg DM/d and the digestibility
of its selected diet from a pasture with 0.8 t DM/ha of green
herbage, mean digestibility 70%, and 0.4 t DM/ha dead
herbage, mean digestibility 45% (from data in Freer et al. 2005)
Herbage pool
1 2 3 4 5 6
Dry matter digestibility (%) 80 70 60 50 40 30
Relative ingestibility 1.0 0.83 0.66 0.49 0.32 0.15
Weight of herbage (t DM/ha) 0.24 0.36 0.23 0.16 0.15 0.06
Relative availabilitya 0.39 0.34 0.11 0.05 0.03 0.01
Relative intake 0.39 0.28 0.07 0.02 0.01 0.00
Cumulative relative intake 0.77
Pasture intake (kg DM) 1.63
Mean digestibility of dietb (%) 73
a
After adjusting for the proportion of appetite satisfied by more
digestible pools
b
Weighted mean for the herbage eaten from all pools
Dietary preferences
Availability of water
Lynch et al. (1972) found that the feed intake by sheep grazing
improved temperate pastures was not reduced in the absence of
drinking water unless they were lactating or the environmental
temperature was high. In semi-arid areas, however, the distance that the
animals have to walk to water may place a severe constraint on the area
that the animals can reach in a grazing day and this problem is
exacerbated where the vegetation has a high salt content or when the
land close to the water supply has already been denuded (O’Reagain and
McMeniman 2002).
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23.4 Supplementary Feeding
When supplements of grain or processed meals are offered to hand-fed animals
eating a basal diet of roughage, the intake of roughage is usually depressed.
The depression in the dry matter intake of the roughage divided by the dry
weight of supplement eaten is called the substitution rate. This depends on the
relative quantities and qualities of the supplement and roughage (Dove 2002).
For grazing animals, the prediction of the substitution rate is complicated by its
interaction with the availability of the pasture. With high quality supplements on
high quality abundant pasture, substitution rates are close to 1.0, but on
abundant pastures of only 50 per cent digestibility it may be as low as 0.65
(Allden 1981). As the weight of pasture falls, and with it the intake of
unsupplemented pasture, so does the substitution rate (Stockdale 2000).
1.0
0.9
0.8
Substitution rate
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0 0.5 1.0 1.5 2.0 2.5
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23.5 Putting it all together
Reference has been made throughout this lecture to the difficulty of predicting
the feed intake by grazing animals through simple hand calculations, because
of the many interactions between the attributes of the animals, the pasture and
possible supplements in any particular situation. Yet this information may be of
significant economic importance to a grazier who is uncertain about the need for
expensive supplements if a target weight gain or milk yield is to be achieved.
The GrazFeed program is an attempt to integrate the factors discussed above
and provide sensible predictions for the user. It is also a useful tool for the
student to explore how changes to the specifications of pastures and animals
affect the predicted intake of feed. These effects may examined more fully using
the plotting routines that allow the interactions between different variables to be
predicted.
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Readings
The following readings are available on CD.
!
• Weston, R.H. 1996, ‘Some aspects of
constraints to forage consumption by
ruminants’. Australian Journal of Agricultural
Research, 47, 175-197.
References
Allden, W.G. 1981, ‘Energy and protein supplements for
grazing livestock’, in Grazing Animals (Ed. F.H.W.
Morley), World Animal Science B1, Elsevier,
Amsterdam, pp. 289-308.
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Owens), Proceedings of Conference at Oklahoma
State University, November 1986, pp. 193-207.
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Journal of Agricultural Research, 47, 175-197.
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©2009 The Australian Wool Education Trust licensee for educational activities University of New England