variables. In digestion trials deer have been 
housed in individual pens so that they could be 
shunted into tared weighing boxes on platform 
scales (Short 1966). However, use of pens 
without collection devices complicates the col- 
lection of feces. 
If energy and protein intake are adequate 
for maintenance, consistent food consumption, 
excreta production, appearance of excreta, nor- 
mal behavior, and similar weight at the start 
and end of the trial are indicators of accepta- 
ble experimental technique. 
Feed and excreta components analyzed in 
digestion trials vary with the research objec- 
tive. Nitrogen, energy, and carbon balances can 
be determined from digestion trials that mea- 
sure net energy. Proximal analysis, non-nutri- 
tive analyses, or other systems are used to as- 
sess the relative value of experimental forages 
to test animals. The routine analysis of diges- 
tion coefficients and calculation of balance data 
are reviewed in many animal nutrition texts, 
e.g., Maynard and Loosli (1962); Crampton 
and Lloyd (1960). 
Indirect Methods in Dry-Lot Digestion Trials 
When all food eaten and excreta produced is 
not measured, digestion coefficients can be de- 
termined by index procedures. Severe condi- 
tions imposed on indicators by Lindahl (1963) 
include indigestibility and pharmacological in- 
ertness. Determination of digestibility coeff- 
cients is based on the concentration of the indi- 
cator substance in feed and in fecal samples, 
using the standard equation: 
(1) 
Digestibility = 100 — 
Percent Percent 
indicator nutrient 
100 in feed sg in feces 
Percent Percent 
indicator nutrient 
in feces in feed 
Indicators have included chromic oxide, lignin, 
naturally occurring plant chromogens, and 
others (Lindahl 1963). Chromic oxide in either 
a gelatin capsule or an impregnated paper 
bolus is administered orally, preferably several 
times daily, to minimize variable excretion 
rates (McGuire et al. 1966). Digestibility is 
calculated by procedures described by Cramp- 
ton and Lloyd (1960). Apparently, the relative 
accuracy of plant chromogens and lignin as in- 
dicators depends on the nature of the forages 
studied. Errors in indirect methods arise from 
unrepresentative forage or fecal samples. 
Forages Utilized in Digestion Trials 
Digestion coefficients obtained from feeding 
freshly clipped forages in dry lot are meaning- 
ful indicators of range forage quality only if 
the test foods are identical in composition to 
forages selected by range animals. Range foods 
have sometimes been modified by being 
chopped or after clipping by being dried, 
milled, and pelleted into homogeneous rations 
that are easy to feed and to sample for chemi- 
cal analysis. The resulting data represent the 
Bocas materials rather than the range 
plant. 
_ When a foodstuff is incompletely consumed 
in a digestion trial, the total nutrient intake is 
obtained either by measuring uneaten orts 
and subtracting their nutrient levels from 
those of the offered foodstuff or by combining 
the orts and fecal fractions. If the animal is al- 
lowed to select only the most nutritious por- 
tion, the apparent digestibility coefficient will 
be higher than that determined if the entire 
food sample had been eaten. 
The feeding of concentrates or little-eaten 
roughages presents special problems because of 
the small bulk ingested. In such instances, for- 
age digestibility has been determined by differ- 
ence. First, the apparent digestibility of a 
basal ration, such as hay, is determined; then 
that of the concentrate or little-eaten rough- 
age plus the basal ration is decided. The differ- 
ence in total nutrients apparently digested be- 
tween the combined and singly fed ration rep- 
resents the digestibility of the low-bulk item. 
The major drawback is that the nutrient con- 
tribution of the low-bulk item may affect the 
digestibility of the basal ration. 
Rations compounded to simulate a mixture 
of range forages or a single forage at different 
phenological stages can be used to predict food 
quality for herbivores. Such rations generally 
indicate how certain nutrients are digested 
rather than the digestibility of a particular 
plant specimen. 
MACRODIGESTION IN FREE-RANGING 
ANIMALS 
Grazing studies can be conducted in small 
fenced paddocks or on open range. Control 
over animal variables differs with the experi- 
mental situation. The major problems are to 
determine the quality and quantity of the con- 
sumed food and the quantity of the resulting 
excreta. 
Qualitative Estimates of Forage Ingestion 
As with closely confined animals, the compo- 
sition of ingested forage must be known to de- 
