This often enables us to increase our food in- 
take if our diet becomes less nourishing. Rumi- 
nants, on the other hand, depend on the rate at 
which micro-organisms can digest food constit- 
uents. Only a few hours are required to liber- 
ate sugars from the plant cells, and to attach 
them and convert them to useful end products 
for the ruminant. Cellulose and lignin, how- 
ever, might stay in the rumen for a few days. 
As the fiber content of the diet increases, rate 
of digestion in the rumen, rate of passage 
through the digestive tract, and, consequently, 
food intake will decrease. Ultimately, the ani- 
mal will starve or nearly starve. This condi- 
tion, coupled with a gradual loss of the func- 
tional rumen microbial population, could lead 
to a point of no return. The animal will die 
even if food is offered to him. 
CONCLUSIONS 
In ruminant nutrition, one of the most im- 
portant concepts to understand is that we are 
really feeding billions of micro-organisms 
whose digestive activities in turn provide most 
of the nutritional needs of the ruminant ani- 
mal. Thus, our chief concern is with providing 
the proper foods and ruminal environment for 
the micro-organisms. 
In the laboratory, experimental alteration of 
the foods and environment of the microbial 
populations has taught us much about the re- 
sponses of the host. We have learned to apply 
some of these findings under feed-lot conditions 
to maximize the performance of the ruminant 
hosts (Virtanen 1966; Raleigh and Turner 
1968). However, we must keep in mind that 
ruminants on the range are dependent upon 
the welfare of their rumen micro-organisms as 
well, and this should be considered an impor- 
tant part of range ecology—for domestic live- 
stock or game animals. 
Ruminant species vary in their ability to use 
forages with different proportions of cellulose, 
lignin, and other constituents, including vari- 
ous bacteriocidal agents. The biological rela- 
tions of range vegetation and ruminant grazers 
are first biological relations of vegetation and 
rumen micro-organisms. Cattle, sheep, goats, 
deer, elk, antelope, and bighorns have each made 
specific adaptations to many kinds of vegeta- 
tion. These adaptations depended upon accom- 
panying evolutionary adaptations of the micro- 
bial populations of their rumen (Silver et al. 
1959; Short et ai. 1965). 
Management of livestock rangelands and 
big-game habitat cannot proceed intelligently 
without an adequate understanding of the 
functioning of the flora and fauna of the ru- 
men. We have only begun to acquire this 
understanding. 
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