ibility is calculated using equation 1. Feed sam- 
ples for indicator and nutrient analyses are ob- 
tained from esophageally or ruminally fistu- 
lated animals, and fecal samples are obtained 
from collection bags or “grab samples.” Vari- 
ous internal indicators have been used to deter- 
mine the digestibility of a range forage. Lignin 
was preferred over forage silica by Van Dyne 
and Lofgreen (1964), and plant chromogens 
have been advantageous in some studies. How- 
ever, Bohman and Lesperance (1967) cited re- 
sults indicating that plant chromogens were 
unreliable in at least one instance where sheep 
were grazing on winter range. High concentra- 
tions of essential oils in certain plants appar- 
ently caused absorption of chromogens, and 
thereby, distortion of digestibility values. Die- 
tary cellulose was used with some success in a 
study with mule deer (Short and Remmenga 
1965). 
Both consumption and digestibility of feed 
have been estimated with chromic oxide as an 
external indicator in combination with some 
naturally occurring plant indicator. The 
method for calculating digestibility is that of 
Crampton and Lloyd (1960). Forage intake is 
estimated from the naturally occurring indica- 
tor in feed and fecal samples (equation 2). 
“Grab” fecal samples are satisfactory for esti- 
mates of range forage digestibility from either 
internal or external indicators. Forage intake 
estimates, however, require careful determina- 
tion of total fecal excretion. 
Metabolic and Net Energy Determinations 
Metabolic energy was calculated for range 
sheep by Morris et al. (1965). Gross energy 
was computed from total forage intake, as 
measured by the lignin ratio technique, and 
from the mean caloric content of forage sam- 
ples collected through esophageal fistulas. En- 
ergy loss in urine and feces was determined 
from collection-bag samples; that in gases of 
fermentation was estimated from data of pre- 
vious work. 
Net energy is determined for grazing ani- 
mals with the same basic data needed to com- 
pute metabolic energy. However, the energy 
expended on the range also must be measured. 
For this measurement, an animal that has a 
tracheal cannula is used (Flatt et al. 1958). 
Respired gases are collected and analyzed to 
determine oxygen consumed and carbon diox- 
ide produced. Caloric expenditure is calculated 
from oxygen consumption, and from produc- 
tion of carbon dioxide and urinary nitrogen. 
Such determinations compare the quantity of 
net energy available for secondary production 
in livestock grazing pastures of varying qual- 
ity. 
MEASUREMENTS OF IN VIVO 
MICRODIGESTION 
Scientists have been suspending food sam- 
ples in the rumen through a fistula for more 
than 30 years (Barnett and Reid 1961). The 
advantage of this procedure is that the ru- 
minal environment is probably little changed by 
the inclusion of such materials, and the com- 
plexities of duplicating ruminal conditions, as 
in in vitro studies, are avoided. Samples of sev- 
eral forages can be simultaneously suspended 
in a single rumen. 
Measured quantities of forage or feed are 
usually placed within porous, indigestible bags. 
The bags, weighted to submerge in the ingesta, 
are inserted through the fistula and are at- 
tached to the fistula plug to assure retention in 
the rumen. Van Dyne (1968) reviewed in vivo 
procedures and stated that the retention time 
needed to maximize microdigestion varies with 
size, grind, and type of sample. He further 
noted that the size of bag mesh and the fine- 
ness of sample particles may affect loss from 
the bags. In vivo microdigestion data have fre- 
quently been closely correlated with macro- 
digestion coefficients obtained from standard 
digestion trials. 
In vivo microdigestion studies have been 
conducted with white-tailed deer at Pennsylva- 
nia State University. Castrates were more doc- 
ile than either bucks or does and could be al- 
euee to forage within paddocks or while teth- 
ered. 
Microdigestion techniques may be useful for 
estimating digestibility of range-forage sam- 
ples by wild ruminants. For wild animals mi- 
crodigestion data have not yet been adequately 
correlated with macrodigestion data, and little 
information is available indicating appropriate 
retention times for bags within the rumen. 
SUMMARY 
In vivo measurements of digestion are poten- 
tially valuable for future Forest Service re- 
search. In dry-lot conditions, digestibility of 
foodstuffs of varied nutrient content can be 
evaluated. Such data, when used in combina- 
tion with information about range forage qual- 
ity and animal habits, will help explain how 
different herbivore species compete on common 
ranges. 
Range digestibility trials seem destined to be 
more useful in studies with domestic than with 
wild herbivores. Maintaining wild animals 
equipped with excreta collecting facilities and 
fistulas may prove difficult. In vivo microdiges- 
tion studies with very tractable specimens 
tethered or kept in small enclosures could ma- 
terially increase our knowledge of wild ani- 
mals. 
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