Agriculture Reference
In-Depth Information
been similar to those seen in sheep. In co-grazing sheep
and goats, the time spent eating and the grazing time as a
proportion of total time both usually increase as stocking
rate increases, while ruminating time and idle time tend to
decrease.
reader is referred to excellent reviews by Foley et al.
(1999) , Mueller - Harvey (2006) , and Shimada (2006) .
Reference is often made to goats being able to consume
more tannin-rich browse than sheep under similar condi-
tions. Similarly, goats are said to somehow exhibit resis-
tance to tannins or at least are less affected by plant
secondary compounds than other animals. At times, this
ability to cope with tannins has been linked to goats having
proteins in their saliva that can bind to the tannins, though
this remains a controversial area. For example, Snyder
et al. (2007) asserted that goats are more tolerant of
tannins “… because they have been reported to have
proline-rich proteins … in their parotid salivary glands.
…” They cited Mehansho et al. (1987) in support of this
assertion, though there is no mention of goats in the latter
paper.
There are actually two components embedded within
comments like that made by Snyder et al. (2007). The fi rst
is that goats do have tannin-binding salivary proteins
(TBSPs) and the second is that these TBSPs are in fact the
proline-rich salivary proteins (PRP) that function as spe-
cialist tannin-binding proteins in some animals. It is true
that TBSPs occur in the saliva of true browsers like deer
and moose. It is perhaps because of this fact and the
fact that goats often browse, that the idea has arisen that
goats also cope with dietary condensed tannins through
tannin-binding proteins in saliva. This idea merits closer
examination.
There is no doubt that, through their greater intake of
browse that may contain condensed tannins, goats may
encounter a greater dietary intake of plant secondary com-
pounds than sheep or especially cattle. Moreover, goats
have a higher tolerance of bitter fl avor than did sheep or
cattle.
What is not clear from the research to date is whether
TBSPs are present in goats, whether they are proline-rich
proteins (PRPs), and whether they are functionally signifi -
cant. As Foley et al. (1999) emphasized, the word “func-
tionally” is most important. Even if TBSPs are detected, if
they are present in very small amounts or have very low
affi nities for condensed tannins, then their functional sig-
nifi cance is questionable.
Austin et al. (1989) found that there were TBSPs in
the saliva of deer but not in the saliva of sheep and
cattle (goats were not tested). Mole et al. (1990) subse-
quently tested for the presence of PRP in the saliva of
lagomorphs, rodents, marsupials, and sheep and cattle.
While salivary PRPs were regarded as “detectable” in the
saliva of sheep and cattle, when scaled to body weight,
these species had only 0.01-0.02% of the amount in the
S
EPARATE
V
ERSUS
C
O
-
GRAZING
: P
REVIOUS
E
XPERIENCE
The results of a number of studies indicate that the species
of animal that previously grazed an area can potentially
infl uence the diet choice of later animals by, for example,
altering the legume content of the biomass as described
above. Similarly, previous experience, preconditioning, or
adaptation of animals to particular plant species can alter
their future diet choice (e.g., Distel and Provenza, 1991).
This should be taken into account when designing or inter-
preting studies of ingestive behavior and diet selection.
Ingestive behavior and diet selection can differ depend-
ing on whether the animal species involved are co-grazing
or grazing as separate species. A detailed discussion of the
merits or otherwise of co-grazing is outside the scope of
this chapter. For a recent discussion of the major issues as
they relate to goat production, the reader is referred to the
excellent review by Animut and Goetsch (2008). One
major advantage of co-grazing in relation to investigations
of ingestive behavior is that it removes the confounding,
which can occur between grazing site and animal species,
when separate grazing occurs. For example, the use of co-
grazing of the same area by ewes, does, and cows (Celaya
et al., 2007) allowed a direct assessment of the degree to
which their respective diet compositions overlapped.
F
ORAGE
O
DOR
/F
LAVOR
AND
D
RY
M
ATTER
C
ONTENT
The diet choice of goats can certainly be infl uenced by
fl avor and by odor, as demonstrated by De Rosa et al.
(2002). The DM content of forage can also infl uence inges-
tive behavior, though the response is not always clear-cut.
Bateman et al. (2004) offered goat kids fresh forage from
cereals and brassica crops and found that there was a 3-fold
range in intake, which was strongly correlated with forage
DM content (r = 0.958;
P
0.01). Their results also pro-
vided some evidence that goats prefer those feeds that
allow a higher short-term intake rate.
<
C
ONDENSED
T
ANNIN
C
ONTENT
OF
THE
F
ORAGE
Condensed tannins are a major class of plant secondary
compounds that are often found in the browse species
consumed by goats. It is beyond the scope of this chapter
to present a detailed treatment of the effects of condensed
tannins or other secondary compounds on animal nutrition
and health. For a detailed discussion of these aspects, the