Agriculture Reference
In-Depth Information
even-toed ungulates from the suborder
Ruminantia
of the
mammal order
Artiodactyla
(meaning an even number of
toes). Among the ungulates, ruminants are the most numer-
ous, widespread, and diverse grass-eating mammals. Of
about 250 known genera, 68 still exist, of which 4 (cattle,
sheep, goat, and bison) are widely domesticated agricul-
tural species (Leek, 1993b). The ecological success of this
group can be attributed to predigestive fermentation. This
microbial fermentation process results in catabolism of
fi brous plant cell walls (including cellulose) into energy-
rich fermentation acids, release of nutrients enclosed
within plant cells, and synthesis of microbial protein from
nutrients recycled to the rumen such as nonprotein nitro-
gen (urea) and phosphorus. In addition, the fermentation
process yields vitamin K and B complex vitamins provided
there are adequate precursors (cobalt for B
12
; sulfur for
thiamin and biotin) for synthesis.
The term “ ruminant ” refl ects the capacity of an animal to
ruminate (chew their cud). Anatomically, ruminants
possess a unique multicompartment (typically subdivided
histologically into three or four sections) fermentation vat
generally called the rumen (or the reticulo-rumen) anterior
to their gastric stomach. This nonsecretory forestomach
and associated organs are designed to support microbial
fermentation. Following fermentation, the digesta fl ows to
the ruminant's secretory stomach and intestines for diges-
tion. Subsequent to the rumen, digestion generally parallels
that of nonruminant animals. The capacity for this microbi-
ally assisted digestion within the rumen permits herbivores
like goats to consume diets very rich in fi brous cell walls.
Generally, over half of the dry matter consumed will be
fermented to microbial products within the rumen reducing
the quantity of material that passes to the gastric stomach
(abomasum) for postruminal digestion. Anatomical and
physiological adaptations of ruminants are geared to pro-
viding a favorable ruminal environment for maintaining a
high degree of symbiosis between ruminal microorganisms
and the host ruminant. These alterations of their GI tract
permit ruminants to consume and digest feed resources of
much wider variety than nonruminants (Hofmann, 1988).
end of the spectrum are the coarse grazers or grass/rough-
age eaters (GR) that include cattle and sheep. This class
can consume very large quantities of food because of their
large forestomach capacity that allows slow but effi cient
fermentation of high fi ber feeds. They eat intensively but
only a few times of the day, and they ruminate less fre-
quently but for longer periods than CS. Most agriculturally
important pasture species are readily consumed by the GR
group. Between these two classes are the intermediate
group/type eaters (IM) including goats. Within the IM
class, the goat is most similar to the CS class having a
relative small forestomach when compared to cattle, with
less capacity for fermentation of roughage. However, they
are extremely adaptable to different environments. When
given a choice, goats select and consume the less fi brous
portions of plants as well as shrubs and browse. But when
fed fi brous feeds and pasture, the volume of the forestom-
ach of goats will increase.
PREHENSION, MASTICATION, SALIVATION
Prehension
Grazing ruminants use both visual cues and taste when
selecting a diet. Ruminants will exhibit preferences for
certain plant parts and specifi c forage when grazing and
also may select and sort feed components when fed dried
forage (hay), but preferences can differ with feeding condi-
tions. Smell and taste appear responsible for selectivity or
“palatability” of a diet. Palatability is measured by allow-
ing animals to choose among various diets. Palatability
should not be confused with total feed consumption. When
animals are not given a choice among a more and a less
preferred feed, total dry matter consumption typically is
no greater for a feed that is more preferred in a “palat-
ability” study. Much remains to be learned about physical
and chemical factors that are responsible for differences in
food preference, diet sorting, and total dry matter intake.
Some results from preference trials are noted below. Cattle
and goats appear to recognize and have preference for a
sweet taste. Given a choice, cattle dislike a salty taste, but
goats prefer it mildly with pygmy goats having a higher
preference for salt (Goatcher and Church, 1970). Goats can
distinguish between bitter, sweet, salty, and sour tastes,
and are more tolerant than sheep and cattle to a bitter taste
(Bell, 1959). Goats enjoy a wide variety of plants that are
distasteful to other ruminants presumably because of their
greater tolerance to a bitter taste. Grazing goats prefer
grasses to legumes and clover over alfalfa (De Rosa et al.,
1997, 2002). Goats also show a preference for forage
DIVERSITY OF RUMINANTS IN FEED TYPE
Ruminant species can be subdivided into classes based on
their ecological spectrum and feeding type (Hofmann,
1989). At one end of the spectrum are the very selective
feeders or concentrate selectors (CS) such as antelopes and
giraffes. This class consumes frequent meals that consist
of highly digestible nutrients, they ruminate frequently but
for short periods of time, and relative to other ruminants,
they have a small forestomach compartment. At the other
