R . Baumont et al. Livestock Production Science 64 2000 15 –28
19
tionships between forage characteristics, short-term creases the amount of cell content, which is soluble,
control of intake, feeding behaviour and finally rapidly degraded and has almost no fill effect, and
forage intake. Modelling intake behaviour is a good increases the amount of cell walls. Consequently,
tool to test our knowledge of what controls intake. forage retention time in the rumen and thus fill effect
Forbes 1980 developed the first mechanistic model increases. In addition, tissue lignification increases
of intake in ruminants. More recently, Sauvant et al. the undegradable fraction of the cell walls and
1996 proposed a mechanistic model of intake and decreases the degradation rate of the degradable
chewing activities that integrates relationships be- fraction Grenet and Demarquilly, 1987. The time
tween feeding behaviour and digestive processes. needed to reduce particle size before ruminal escape
Decisions between eating, ruminating and resting are is also increased. Retention time in the rumen
taken according to the relative values of the func- depends mainly on the degradation rate of the
tions of motivation to eat and of satiety, which degradable fraction and on the proportion of the
integrate the signals described above. The forage is undegradable fraction, since its outflow rate does not
characterised by the cell wall content and its po- vary widely among plant species Baumont et al.,
tential digestibility, and by the proportion of large 1997. Residence time of dry matter in the rumen is
particles. A palatability index and a coefficient of closely related to forage ingestibility Baumont et al.,
heterogeneity take
into account
non-nutritional 1996.
characteristics and selection possibility in the forage and
are combined
to estimate
instantaneous 3.2. Sensory properties
palatability of the forage. This model accurately predicted intake kinetics of sheep fed different types
The sense of touch plays a role in the response of of hay indoors.
the animal to the feed Arnold, 1966. Physical characteristics of the forage such as dry matter
content and particle size, and resistance to fracture
3. Role of plant characteristics on forage are known to affect ease of prehension and thus
´ ingestibility
intake rate Inoue et al., 1994. Sheep generally prefer the feeds they can eat faster. With dried
Forage ingestibility is defined as the maximum forages, relative preferences for mixtures with vary-
quantity of the feed that can be eaten by the animal ing proportions of long and short particles were
when this is supplied ad libitum as the sole feed. closely related to the differences in intake rates
When given indoors, ingestibility of green forage Kenney and Black, 1984.
depends mainly on its nutritive value and fill effect Small ruminants are also sensitive to the four
and on its sensory properties, assuming it does not primary tastes: sweet, salty, bitter and sour Goatcher
contain toxic compounds Fig. 1. Taking into and Church, 1970. However, animals’ response to
account the main variations in forage ingestibility added chemicals varies with feed management
allowed the development of the fill unit system to Grovum and Chapman, 1988 and animals in the
predict feed intake for ruminants fed indoors Jarrige same flock display a very wide range of response to
et al., 1986. the four taste components Morand-Fehr et al.,
1993. The effects of various odoriferous compounds 3.1. Nutritive value and fill effect
naturally present in plants were analysed by Arnold et al. 1980 by sprinkling the chemicals onto cotton
For a given plant, ingestibility, like digestibility, is wool pads placed in the manger. Effects of odorifer-
dependent on the vegetation stage and the number of ous compounds are difficult to interpret because they
the vegetation cycle. During the first vegetation can vary in amplitude and sometimes in sign accord-
cycle, ingestibility decreases with the age of the ing to whether the animals are in choice situations or
plant Demarquilly et al., 1981. The decrease in not. Moreover, several compounds recognised to
ingestibility with age of forage is the consequence of decrease intake by their odour or by their taste
the increase in its fill effect. As the plant ages, its tannic acid, gramine, etc. were found to decrease in
morphological and histological development de- vitro digestibility of the feeds.
20 R
. Baumont et al. Livestock Production Science 64 2000 15 –28
Low intake of silage is often attributed to low 1996. When an animal begins to eat, it selects a
palatability, since digestibility is only slightly differ- feeding site, and then a patch within that site.
ent from that of green forage. Effects of smell and taste on silage intake were studied with anosmic and
4.1.1. Representation of the grazing process and agustatory sheep Michalet-Doreau, 1975. The in-
theoretical bases of foraging behaviour crease in silage intake by anosmic compared with
Two approaches to foraging decisions have been normal sheep was more pronounced with poorly
proposed. Synthetic approaches assume animals preserved 1 33 than with well preserved silages
organise their behaviour towards an objective, 1 6.4. Silage intake by agustatory sheep was not
whereas analytical approaches assume that behav- modified. Acetic acid added to the silage had a clear
iours arise from cause–effect relationships. The basic negative effect on intake Buchanan-Smith, 1990.
axiom of the main synthetic approach, Optimal Amines are suspected to decrease palatability, be-
Foraging Theory OFT, is that present-day animals cause in sheep initial eating rate at the beginning of
forage optimally as a result of natural selection, the meal was depressed by addition of amines in
because optimal foraging enables the animal to silage Van Os et al., 1995. Low palatability of
maximise its reproductive output ‘fitness’ Krebs silages probably results from learning the negative
and McCleery, 1984. Fitness maximisation has post-ingestive signals due to high amounts of fer-
often been translated into efficiency of foraging, mentation end products.
which, for practical reasons, has often been equated with short-term dry matter intake rate. The recent
model of Newman et al. 1995 base foraging
4. Role of vegetation characteristics on grazing decisions on maximisation of fitness and include a
