DETERMINING FORAGE QUALITY 
Digestibility Trials: In Vivo Techniques 
HENRY L. SHORT? 
The way the nutrient composition of food 
changes during animal digestion is a good indi- 
cator of forage value and range quality. The 
measurement of these changes and the evalua- 
tion of their meaning are discussed in this 
paper. Factors affecting the digestibility of 
forage, such as plant species phenological 
stage, and forage nutrient quality, are not con- 
sidered. 
Macro- and microdigestion techniques are 
covered separately. Macrodigestion data are 
obtained from standard digestion trials with 
animals confined in a dry lot, barn, or range. 
In in vivo microdigestion studies forage sam- 
ples are suspended within the digestive tract of 
experimental animals, and changes in the nu- 
trient composition of these samples are mea- 
sured. 
SOURCES OF ANIMAL VARIATION 
Anatomical and physiological differences in 
the digestive systems of animal species are re- 
sponsible for large variations in ability to 
digest foodstuffs. The storage and fermentative 
organs in digestive tracts of many animals are 
discussed by Moir (1965). Stomach capacity is 
about 60-70 percent of the total digestive ca- 
pacity of cats, dogs, sheep, goats, and cows 
(Dukes 1955). The digestive tract of herbi- 
vores, in both absolute terms and in relation to 
body volume, is many times larger than that of 
carnivores. Ruminants have a very large stom- 
ach; other herbivores, such as the pig and 
horse, have capacious small intestines and co- 
lons. 
Animals with such diverse digestive systems 
as the rat, guinea pig, sheep, pig, and man 
have similar ability to digest foods with low 
fiber content. However, as diets become in- 
creasingly fibrous, ruminants, with their volu- 
minous digestive tracts and symbiont microbial 
*The author is Wildlife Biologist at the Wildlife 
Habitat and Silviculture Laboratory, Nacogdoches Tex. 
The Laboratory is maintained by the Southern Forest 
Experiment Station in cooperation with Stephen F. 
Austin State Univ. 
populations, have major digestive advantages. 
Differences in digestive ability between 
breeds of cattle, between cattle and sheep, and 
between other ruminant species are reviewed 
by Hungate (1966). Because of their larger ru- 
men reticulums, cattle probably digest fibrous 
forage more thoroughly than do deer of the 
genus Odoceilus (Short 1963). Roe deer (Cap- 
reolus capreolus) apparently are less able to 
digest fibrous forages than are the larger red 
deer (Cervus elaphus) (Briiggemann et al. 
1965). 
Although suitable domestic analogues can 
sometimes be found for particular wild species, 
data that are extrapolated to little-studied her- 
bivores are always suspect. The additional ex- 
pense of dealing with the wild herbivore can 
probably often be justified. 
MEASUREMENTS OF FORAGE UTILIZATION 
The first approximation of animal use is di- 
gestible energy or total digestible nutrients 
(T.D.N.). Digestible energy is the gross energy 
of the foodstuff minus the gross energy of the 
feces. If bomb calorimetric data are not availa- 
ble, the apparent digestibility of forage compo- 
nents can be arithmetically equated to T.D.N. 
In most digestion trials with wild herbivores, 
digestible energy has been measured. 
Metabolizable energy is the difference be- 
tween the gross energy of a feed and the gross 
energy of feces, urine, and gases of fermenta- 
tion from the gastrointestinal tract. Urine is 
collected with a harness and collection bag or 
with a gravity delivery system in a metabolism 
cage or respiration chamber. Gas from fermen- 
tation processes is collected and measured ei- 
ther in respiration chambers or with face 
masks and spirometers. An advantage in mea- 
suring metabolic energy in forest and range 
animals is that some important forages contain 
volatile substances that are excreted mostly in 
the urine. Measuring apparent digestible en- 
ergy of these forages overestimates their use- 
fulness to herbivores. Ruminants frequently 
lose about 10 percent of digestible energy in 
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