the rumen and is digested in the abomasum in 
the same manner as in monogastrics. The 
young animal, however, through association 
with other ruminants and through the simulta- 
neous intake of plant materials, will soon de- 
velop a functional rumen microbial population. 
Once microbial fermentation begins, the rumen 
will develop very rapidly. While at birth, the 
rumen comprises only about 20 percent of the 
total four-part stomach (rumen, reticulum, 
omasum, and abomasum) capacity, at about 4 
months, the rumen capacity of deer is 80 per- 
cent of the total four-stomach capacity (Short 
1964). 
Bryant et al. (1958) found that calves 
brought up on commercial starter obtained the 
characteristic microflora of the adult bovine at 
9-13 weeks of age. The data of Lengemann and 
Allen (1955) suggest that the usual adult pro- 
portions of the short-chain fatty acids, the 
principal end products of rumen microbial fer- 
mentation, are obtained only after 6 months of 
age. Our data (Nagy, unpublished) show that 
ratios of the short-chain fatty acids of antelope 
fawns do not differ significantly from those of 
adults at approximately 4 months of age. 
Factors Influencing Kinds and Numbers of 
Rumen Micro-Organisms 
One should not think, however, that the 
adult rumen microflora and fauna has a rather 
static composition. On the contrary, it will 
vary throughout the life of the animal. The 
kind of organisms present at any given time, 
their numbers, and their ratios to each other 
will depend on various factors. Some act di- 
rectly on the animal and indirectly on the mi- 
crobial population of the rumen, while others 
act directly on the rumen environment and 
thus on the balance of bacterial species present 
(Kistner 1965). One of the dominant factors 
acting directly on the animal is the human 
influence. Human influence on domestic rumi- 
nants is rather obvious; we continuously 
manipulate the genetic makeup of domestic 
ruminants and also change their ecological dis- 
tribution. For wild ruminants, selective hunting 
undoubtedly influenced big game species of 
Europe, while in the United States the ecologi- 
cal distribution of bison, antelope, elk, and big- 
horn sheep was certainly influenced by the en- 
croachment of _ civilization. Physiological 
changes due to selective breeding can possibly 
influence food selection, manner of food intake, 
capacity of the digestive tract, and ratios of 
certain organs, i.e., the rumen to total body 
weight. Changes in geographic location of the 
ruminant can expose the ruminant to different 
climatic conditions and consequently can alter 
the availability of principal plant species uti- 
lized as food. 
160 
The nature of diet so influenced will, in turn, 
largely determine certain rumen environmen- 
tal conditions, such as the amounts of available 
nutrients for the rumen bacteria, rumen pH, 
and the concentration of end products in the 
rumen. Thus, the nature of diet coupled with 
the environmental conditions the particular 
diet produces in the rumen, and coupled with 
different growth rates of micro-organisms, ad- 
aptation of micro-organisms, and _ different 
rates of microbial removal from the rumen, 
will ultimately determine the balance of mi- 
crobial species in the rumen. Diet, of course, 
will be determined by a variety of the factors 
discussed previously; these include human in- 
fluence, plant species available, food selection, 
ete. Pearson (1965) found that the number of 
a ciliate protozoa (Entodinium spp.) in the 
rumen of white-tailed deer increases during 
rapid vegetation growth and decreases as 
plants dry up at the end of the growing season. 
As the food enters the rumen, depending on 
its chemical nature, different micro-organisms 
will attack it, utilize it, and produce a variety 
of end products which, in turn, will be utilized 
by other organisms and/or by the host animal. 
It is generally agreed that among the short- 
chain fatty acids produced in the rumen, acetic 
acid will predominate under any dietary re- 
gime regardless of the species of ruminant. 
However, ratios of these acids will vary ac- 
cording to the diet. Concentrates which are 
high in easily digestible plant sugars and 
starches will produce proportionately some- 
what lower acetate and higher propionate, 
while diets low in the above substances and 
high in fiber such as cellulose and lignin will 
produce just the opposite effect (Annison and 
Lewis 1959). Wild and domestic ruminants 
seem to exhibit very similar patterns in this 
respect, which suggests that diet and not nec- 
essarily the ruminant itself will mainly deter- 
mine the patterns of microbial fermentation. 
Dietary changes will produce changes in the 
environment of the rumen such as changes in 
pH of the ingesta and amounts and types of 
characteristic metabolites; therefore, these 
changes will influence the types and numbers 
of micro-organisms present. Under a high 
roughage dietary regime, the pH of the rumen 
ingesta will be nearly neutral, and the num- 
bers of cellulolytic bacteria will be relatively 
high. A diet high in concentrate and low in 
roughage will produce lower pH; consequently, 
the numbers of cellulolytic micro-organisms 
will also be lower. A sudden change in diet 
from high roughage to high concentrate could 
cause rather drastic changes in the microbial 
population of the rumen. Thus, if the diet is 
changed suddenly from a low to a high concen- 
trate (containing large amounts of starch and 
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