Agriculture Reference
In-Depth Information
with higher available carbohydrate content (Burns et al.,
2001). Why goats prefer some feeds and avoid others is
not known, but they appear to have a preprogrammed
capacity to recognize nutrients and toxins (Euphagia)
and to consume certain plants because of their pleasant
smell and taste (Hedyphagia). In most cases, body mor-
phology and size may be determinant factors for food
preference. Goats have a smaller size, and thus prefer more
digestible and nutritious feeds. When grazing a tall-
standing forage, ruminants do not graze to ground level.
Presumably, this allows the grazer to keep their eyes above
the grazing horizon to be on the outlook for predators.
Consequently, grazing animals with a longer snout or with
eyes atop their head would have a selective advantage.
ideal for microbial growth. The HPO
3
in saliva allows
phosphate to be recycled for rumen microorganisms to
synthesize nucleoproteins, phospholipids, and nucleotides.
Recycled urea, being up to 77% of total salivary nitrogen,
also provides ammonia for formation of microbial protein.
Urea, recycled in saliva, together with effi cient kidney
renal tubular urea resorption, appears critical for survival
of ruminants consuming forage or feeds with very low
protein content (NRC, 2007; Leek, 1993a). In addition,
recycling of urea reduces water excretion; combined with
very effi cient resorption of water by the large intestine,
which helps ruminants survive when the quantity or quality
of water is limited.
As browsing ruminants, goats have relatively large sali-
vary glands and higher rates of secretion than sheep. The
primary functions of saliva are to provide a copious and
continuous supply of alkaline buffers to counterbalance the
volatile fatty acids (VFA; primarily acetate, propionate,
and butyrate) produced during fermentation and to provide
an aqueous suspension for rumen solids. Secondary func-
tions of saliva include urea recycling as a source of non-
protein nitrogen (NPN) for microbial protein synthesis and
phosphate for synthesis of microbial nucleic acid and
membrane phospholipids. Saliva also acts as a wetting
agent for ingesta, provides an antifoaming agent to prevent
frothy bloat, and supplies proline-rich proteins that bind
and deactivate dietary tannins.
Saliva provides a medium for short-term adaptation to
changes in diet composition, namely, the presence of plant
secondary metabolites such as tannins. Salivary proteins
infl uence taste and digestive function. Proline-rich proteins
in saliva that bind tannins are present in saliva of browsers
but not grazers. Lamy et al. (2008) showed that protein
profi les of saliva in the 25-35 kiloDalton (kDa) range
differ between goat and sheep saliva (25 - 35 kDa range).
Austin et al. (1989) reported that tannin-binding proteins
were present in the saliva of deer but not of sheep and
cattle. Hofmann et al. (2008) indicated that in ruminants,
the mass of salivary glands is correlated positively with
body mass but negatively correlated with the ratio of grass
to browse in the diet, perhaps refl ecting the need for
complex salivary compounds, such as tannin-binding pro-
teins, by browsing ruminants. Silanikove (2000) confi rmed
that the ratio of salivary gland mass to body size was high
for goats indicating their capacity to consume browse
plants.
Mastication
The mouth is designed to harvest food, mechanically
reduce particle size, and mix it with saliva. Saliva is essen-
tial as a lubricant to facilitate swallowing. Goats have a
pointed tongue and jaws, mobile thin lips, and a deep
mouth. Grazed forage is gathered between the labial
surface of the lower incisor teeth and the upper dental pad
(ruminants lack upper incisors), and forward movement of
the muzzle cuts through the forage. Chewing of food is
irregular with variable amplitude. In contrast, during rumi-
nation, the cud is chewed more slowly and regularly.
Premolars and molar teeth aid in mastication. The upper
jaw is wider than the lower jaw, so lateral (circular) jaw
movement and the shape and spacing of the molar teeth
result in shredding of tough plant fi bers.
Mastication results in particle size reduction to increase
in ports of entry for fermenting microorganisms. In addi-
tion, movements of teeth excite mechanoreceptors in the
mouth that provide stimuli for production of saliva by sali-
vary glands. Mouth movement also increases the rate and
amplitude of primary and secondary cycle contractions of
the reticulo-rumen. Feedstuffs that fail to promote chewing
(low particle size) result in reduced rumination, saliva
secretion, and forestomach motility.
Salivation
Although salivary production is relatively continuous,
volumes are greater when ruminants are eating and rumi-
nating. Daily saliva production averages 6-16 liters by
sheep and may be greater by goats. Saliva produced by
ruminants contains no enzymes, but it is particularly rich
in buffers (HCO
3
and HPO
4
); these account for its alkalin-
ity (pH = 8.1). Being swallowed, saliva aids in preventing
ruminal pH from becoming too low for microbial growth.
In the rumen, pH is maintained within a range (5.5-7.0)
FUNCTIONS OF RUMEN AND RETICULUM
The major physiological activities related to rumen and
reticulum will be addressed here. Readers are referred to
