Agriculture Reference
In-Depth Information
eating hay but still signifi cantly increased the time spent
browsing lentisk (Table 9.2). Goats offered PEG thus
changed their patterns of diet selection. They also had a
higher intake and maintained weight, whereas those not
given PEG lost weight.
Table 9.3
Indicative summary of some of the
factors shown to infl uence herbage intake in the
free-ranging ruminant.
Plant factors
Animal factors
Species composition of plant
biomass
Capacity to use energy/
physiological state
DIGESTIBILITY AND FEED INTAKE
There are many factors that infl uence the voluntary feed
intake of grazing or browsing ruminants, including many
of the factors that infl uence diet selection. These factors
have been reviewed extensively recently (see for example,
Dove and Mayes, 2005; NRC, 2007). The AFRC (1998)
presents equations for the prediction of intake in housed
goats. More recently, Luo et al. (2004) conducted an exten-
sive review of the feed intake literature for “… lactating,
Angora, growing and mature goats …,” but once again, the
emphasis was on the development of equations for predict-
ing intake in housed animals. There is much less reviewed
information on intake in the free-ranging goat, in part
because of the diffi culty in estimating digestibility and
intake in free-ranging animals (Dove and Mayes, 2005).
Of necessity, much of the information discussed below has
thus been derived from housed goats. A degree of caution
is required in extending this to the pasture or range
situation.
An indicative, but by no means complete, summary of
the factors infl uencing diet digestion and voluntary intake
is given in Table 9.3. Clearly, the amount of plant material
available, its spatial distribution, and its content of nutri-
ents and plant secondary metabolites are major factors
infl uencing intake. However, it is the animal itself that can
be regarded as setting the upper limit to intake, through its
need for and capacity to use nutrients at a particular age,
body condition, and physiological state. This “potential
intake” (CSIRO, 2007; NRC, 2007) is then constrained by
features of the plant biomass on offer, by the animal's
previous experience, by transactions within the gut, and by
the intake of milk or supplement. A cornerstone of the
approach is the notion of a “standard reference weight”
(SRW) for a given breed, that is, the weight of the mature
female with a condition score in the middle of the range
used. Potential intake is scaled to this weight (see CSIRO,
2007; NRC, 2007) but will be different if the animal's
current weight is below the SRW either because it is
younger or if its condition differs, or both.
Both CSIRO (2007) and NRC (2007) took the view that
intake equations derived for sheep could be applied equally
to goats, arguing that published data did not warrant a
separate treatment for goats. This assumption is examined
further later in this chapter.
Mass, height, proportions of
leaf, stem, or thorns in the
plant biomass
Standard reference weight
( “ mature live weight ” )
Spatial heterogeneity —
patchiness, height
distribution
Current age, weight and
body condition
Stage of plant
development — vegetative
versus reproductive
Previous experience with
plant species on offer
Digestibility of the species
on offer
Social interactions with
other animals
Concentrations of plant
secondary compounds in
the plant species
Digesta load
Protein content of the
species on offer
Intake of milk or of
supplementary feed
Forage Digestion and Intake in Goats
A selection of the published comparisons of forage diges-
tion and of intake by goats and sheep is given in Table 9.4.
Almost all of the data are derived from studies with housed
animals. Although details of the breeds and live weights
of the animals are often provided, it is usually very diffi cult
to assess age, body weight, and body condition in terms
that would allow the use of the prediction equations in the
published nutrient requirements. Moreover, the diets con-
sumed by the two species are not always identical so that
diet and animal species are confounded. These points
should be kept in mind when assessing whether goats and
sheep really differ in their digestion and intake of forage.
F
ORAGE
D
IGESTIBILITY
Diets consumed in the reports summarized in Table 9.4
have ranged from high-quality (e.g., Doyle et al., 1984,
diet A) to high-fi ber, low-quality materials such as wheat
straw (Reid et al., 1990). In general, published results do
suggest that higher - fi ber diets are digested better by goats
than sheep, but overall, the data in Table 9.4 do not provide
convincing evidence that diet digestibility is predictably
and substantially higher in goats than in sheep. Tolkamp
and Brouwer (1993), for example, surveyed published data
