Biological Relations of Rumen Flora and Fauna 
JULIUS G. NAGY ? 
INTRODUCTION 
Depending on their digestive systems, we 
can divide higher mammals into three general 
groups: Monogastrics, with small cecum such 
as carnivores and omnivores; monogastrics, 
with enlarged cecum such as many herbivores 
(for example, horses, beavers, and rabbits) ; 
and ruminants such as elk, deer, antelope, 
sheep, goats, and cattle. During their evolu- 
tion, all three of these groups developed a 
digestive system best suited for the particular 
diet available to them. Monogastrics with a 
small cecum have the simplest digestive sys- 
tem: it consists of one pouch, the stomach, and 
usually a small cecum. Their diet is the most 
concentrated of the three groups and is the 
easiest to digest. It consists of protein, sugars, 
starches, fat, and comparatively small amounts 
of fiber. At the other extreme of specialization 
are the ruminants; they have a complex system 
consisting of a rumen, reticulum, omasum, and 
abomasum. The abomasum is the true glan- 
dular stomach. The diet of a ruminant is 
bulky, high in lignin and cellulose, and usually 
low in easily digestible plant materials. For 
these reasons cattle have roughly nine times 
the digestive tract capacity of man on an equal 
weight basis. At least half of the energy sup- 
ply of the ruminant, and all of the essential 
amino acids, is obtained from the by- and end- 
products of bacterial and protozoal fermenta- 
tion in the rumen. Monogastrics with enlarged 
cecum also depend on bacterial fermentation, 
but not as much as ruminants. Food is digested 
in the glandular stomach, and then some of the 
digested materials will undergo microbial 
digestion in the cecum. 
SIGNIFICANCE OF RUMEN 
MICRO-ORGANISMS 
Although all three digestive systems use mi- 
cro-organisms, in one way or another, to pro- 
duce nutrients for the host, inter-dependence 
between host and micro-organisms has been de- 
veloped to the highest degree in ruminants. No 
matter what kind of a digestive system an ani- 
mal has, food intake is governed by one or 
more of the following requirements: Food 
must be available at the time it is needed, it 
must be palatable to the animal, and it must be 
digestible and utilizable by the animal. Rumi- 
nants, of course, have one more important re- 
s Assistant Professor of Wildlife Biology, Department 
of Fishery and Wildlife Biology, Colorado State Uni- 
versity, Fort Collins, Colo. 
quirement: food must be digestible by the 
rumen micro-organisms. 
The importance of studying ruminant nutri- 
tion can hardly be overestimated. Domestic 
ruminants occupy a predominant position in 
modern agriculture: most of our big game spe- 
cies are ruminants; and, because of their close 
food requirements, domestic and wild rumi- 
nants often compete. 
The presence of a “fermentation vat,’”’ where 
billions of micro-organisms compete and work 
with each other and with the host, offers a 
unique opportunity to study a microbial ecosys- 
tem, and at the same time offers challenges for 
an investigator. Food must be broken down in 
the rumen by micro-organisms in order to pass 
further down to the digestive tract. Most of 
the ingested carbohydrates will be converted to 
short-chain fatty acids (acetate, propionate, 
butyrate, iso-butyrate, valerate, and iso-valer- 
ate) and constitute the major source of energy 
for the ruminant (Annison and Lewis 1959). 
Since most of the short-chain fatty acids are 
absorbed through the rumen wall and enter 
the bloodstream, the concentration of blood 
sugar in an adult ruminant’s blood is about 
half of that found in nonruminants. Blood 
short-chain fatty acid levels in ruminants are 
higher than in monogastrics. 
The presence of micro-organisms in the 
rumen alters the amino acid requirements of 
the ruminant strikingly. Because micro-organ- 
isms are able to attack nitrogenous substances 
from protein as well as from nonprotein 
sources, i.e., urea, they can supply the host, re- 
gardless of the nitrogen source, with all the es- 
sential amino acids, providing other nutrients 
are not limiting. Thus, the host, in contrast 
with monogastrics, is freed from selecting 
foods where the essential amino acids are al- 
ready present. 
One of the major functions of the rumen mi- 
cro-organisms is, of course, the conversion of 
cellulose in the diet of the host. Since mam- 
mals do not secrete cellulase in their digestive 
fluids, the digestion of cellulose by rumen mi- 
crobial populations enables the ruminant to 
live on coarse, fibrous foods on which monogas- 
trices are unable to subsist. 
DEVELOPMENT OF THE RUMEN 
At birth, the digestive system of the young 
ruminant functions the same way as that of 
monogastrics. There is no rumen fermentation. 
Milk through the esophageal groove bypasses 
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