Agriculture Reference
In-Depth Information
thesis of vitamin K and most B complex vitamins, and (4)
conversion of metabolically recycled phosphate to micro-
bial nucleic acid and membrane phospholipids.
increase in feed intake may counterbalance this negative
impact on digestibility and lead to improvements in both
rate and effi ciency of gain.
E
FFECT
OF
D
IET
ON
M
ICROBIAL
F
ERMENTATION
The amount and composition of feed consumed can alter
the microbial population and distribution of types found in
the reticulo-rumen. Thus, diet can impact rate of digestion,
rate of passage, and retention time of digesta in the rumen.
Feeds rich in protein can alter the microbial populations to
predispose an animal to fermentation disturbances such as
ammonia toxicity or other toxins in the feeds. Diets rich
in protein stimulate growth of protein-digesting (proteo-
lytic) microorganisms. High fi ber diets will increase the
proportion of cellulose digesting (cellulolytic) microor-
ganisms within the rumen, and high starch diets promote
growth of starch-digesting (amylolytic) microorganisms.
Diets rich in protein and starch (up to 80% concentrate
diets) are associated with higher fermentation rates, higher
digestibility of dry matter, and possibly improved intake.
However, diets that contain less than 15% roughage tend
to have a negative impact on fermentation and digestion
(Vieira et al., 2008) by goats. High concentrate diets lead
to greater production of fermentation acids in the reticulo-
rumen and place a greater load on rumen buffering system.
Goats like other ruminants have more tolerance to down-
ward than to upward shifts in rumen pH (Silanikove,
2000 ). Generally, methane - producing (methanogenic) and
cellulolytic microorganisms in the rumen are more sensi-
tive to lower rumen pH and have a reduced prevalence
under such conditions.
Nitrogen, sulfur, and essential minerals must be sup-
plied for optimum microbial fermentation in the reticulo-
rumen both for microbial growth and protein synthesis and
for fi ber digestion. Saliva provides buffering via bicarbon-
ates of sodium and potassium. Through hydrolysis to
ammonia, salivary urea also provides buffering for rumen
contents. These components must be at optimum levels for
optimum rates of digestion and feed intake.
Passage rate of feed residues from the rumen is associ-
ated with the plant cell wall content of the diet, action of
microorganisms, and rate of particle size reduction. When
passage rate is high, more slowly available substrates such
as cellulose will escape ruminal digestion leading to a
lower extent of fi ber digestion. Grinding or pelleting feeds
will increase passage rates of particles and thereby will
negatively impact fi ber digestion relative to feeding long
particle forage. Because performance generally is propor-
tional to intake of digested dry matter, a compensatory
T
YPES
OF
M
ICROORGANISMS
IN
THE
R
UMEN
Microorganisms in the rumen are either facultative or strict
anaerobes. A limited supply of oxygen can be tolerated so
long as it is actively removed by facultative anaerobes.
Because ruminal microbes are obligate anaerobes, inocula-
tion or cross-inoculation from one animal to another via
feed, saliva, or air is limited. Inoculation of protozoa can
be prevented by maintaining an animal in isolation, but
inoculation by bacteria is impossible to prevent. Fecal con-
tamination appears to be the major source of anaerobic
fungi in the reticulo-rumen.
Active fermentation in the reticulo-rumen requires semi-
continuous infl ux of substrates and a specialized group of
bacteria. Although the majority of rumen bacteria are obli-
gate anaerobes, some facultative anaerobes exist in the
rumen and may play roles in rumen dysfunction. Although
bacteria at some 10
10
to 10
11
bacteria per ml, can account
for about half of the rumen biomass, they are responsible
for most of the fermentation within the rumen. Larger
organisms (protozoa at 10
4
to 10
6
/ml) can account for the
other half of the biomass in the rumen, but they play a
smaller but yet signifi cant role. In addition, fungi can be
found within the rumen (Van Soest, 1994).
Bacteria within the reticulo-rumen are diverse and spe-
cialized according to the substrates used, the products
formed, or their nutrient requirements. The primary groups
of rumen bacteria are those that degrade diet components
while a secondary group will use end-products of the
primary groups as substrates. Functions within these
primary groups overlap considerably. The secondary
groups (more than 60% of the total) are very important for
adjusting fermentation output, providing growth factors
for the primary groups, and maximizing effi ciency of fer-
mentation. Major primary bacterial and protozoan species
found in the rumen are shown in Figure 8.1.
F
ERMENTATION
OF
F
IBER
The primary cellulolytic bacteria have the unusual capacity
of being able to hydrolyze the
1,4 linkages of plant poly-
saccharides and convert released cellobiose (2 glucose
units with a
β
1,4 linkage) and glucose into volatile
fatty acids (VFA) and a transient intermediate formate.
Cellulose and starch are converted to glucose, fructosans
to fructose, and hemicellulose and pectin to xylose, as
transient intermediates. These intermediates are converted
β
