urine and 8 percent as gases of fermentation 
(Byerly 1967). 
Net energy is the most refined measure of 
the usefulness of food for herbivores. It is the 
energy remaining after metabolic energy 
(fecal and urinary and combustible gases of 
fermentation) and heat losses due to nutrient 
utilization have been deducted from the gross 
energy of a feed. The net energy of energy bal- 
ance studies is that available for maintenance, 
work, or secondary production (pregnancy, lac- 
tation, etc). 
Digestion trials can be conducted on experi- 
mental animals housed in open-circuit respira- 
tion chambers, where data are collected on 
food and water consumption, fecal and urine 
production, gases of fermentation, and tissue 
production. In respiration trials, heat loss and 
energy retention are calculated from oxygen 
consumption and from production of carbon 
dioxide and urinary nitrogen. These complex 
data can be used to determine how energy, car- 
bon, and nitrogen in foods are partitioned and 
utilized by animals. Digestibility coefficients of 
foodstuffs are also calculated from these data. 
Such rigorous procedures and advanced 
technology produce extremely useful informa- 
tion. Obviously, however, this quality of infor- 
mation is not often obtained in studies with 
wild herbivores. 
MACRODIGESTION IN CONFINED 
ANIMALS 
Animal Variations and Experimental Design 
Variations in the amount of forage con- 
sumed in feeding trials have been attributed to 
season of the year and to the reproductive and 
production status of test animals. Such differ- 
ences affect food utilization and should be min- 
imized through experimental design. Between- 
animal or within-class variations in the way 
replicates digest a common ration have been 
reported by many authors. Apparently, as few 
as three animals, selected because they are 
similar or identical in size, well-being, produc- 
tion status, tractability, and sex, may ade- 
quately depict the apparent digestibility of ex- 
perimental forages (i.e., provide coefficients of 
variation of only a few percent). The degree of 
between-animal variation may also differ by 
type of herbivore (Van Dyne 1968). 
Many ration-herbivore combinations have 
been tested in digestion trials. The favored sta- 
tistical designs are the Latin square (Lindahl 
1963) and various factorial arrangements 
(Crampton 1963). 
Digestion Trials With Total Excreta 
Collection 
Indigestible residues of ingested food are 
80 
eventually excreted, but the rate of passage 
varies widely among animal species. Prior to 
the actual collection of excreta for chemical 
analyses, the consumption: excretion relation- 
ship must be established. Marker substances in 
the feed of omnivora and carnivora are recov- 
ered in a few hours, indicating rapid passage. 
However, the large digestive tracts of rumi- 
nants may retain a portion of a meal for 1 week 
or more. Thus, digestion trials on ruminants 
require at least 2-3 weeks. Some time is needed 
for stabilizing food intake and for animal reac- 
tion; then for 1 week excretory products are 
collected for analyses. Collected material can 
be frozen until representative aliquots are pre- 
pared for chemical analysis. 
Indigestible food residues have been totally 
collected either with harnesses and bags or in 
metabolism cages of various designs. Har- 
nesses and suspended collection bags, such as 
those pictured by Maynard and Loosli (1962, p. 
300), have been used with most of the domestic 
herbivores and with white-tailed deer (Forbes 
et al. 1941). Animals equipped with collection 
bags can be maintained in small pens, where 
some of the rigors and artificialities of very 
close confinement are removed. 
Difficulties with harnessed and bagged ani- 
mals have also been cited. Stillions and Nelson 
(1968) noted that the equipment on horses 
was difficult to keep clean, repair, and adjust, 
and was cumbersome when the animal was ex- 
ercised. Forbes et al. (1941) observed that deer 
sometimes ate their harnesses in apparent at- 
tempts to correct the nutritional deficiencies of 
their test rations. The differential collection of 
urine and feces from female animals requires 
intricately engineered collection devices, such 
as those described by Gorski et al. (1957). 
Digestion stalls for steers have been de- 
scribed by Nelson et al. (1954). Special con- 
duits may be harnessed or cemented onto heif- 
ers or cows in stalls to provide for the separate 
collection of urine. Metabolism cages for sheep 
are pictured in Morrison (1950); stalls for 
horses are described by Stillions and Nelson 
(1968); and cages for deer are shown by 
Cowan et al. (1969). 
Most total-collection trials with medium- 
sized herbivores under dry-lot conditions have 
been done in metabolism cages. Deer require 
cage arrangements such as those of Cowan et 
al. (1969) to minimize disturbance and han- 
dling of animals. Feces and urine are allowed to 
fall through a mesh flooring and are me- 
chanically separated and collected. Perhaps the 
major difficulty with small digestion cages is 
the periodic weighing of animals. Even though 
body weights can be somewhat misleading be- 
cause animal tissue can be replaced by retained 
fluids, they are desirable for assessment of ani- 
mal condition and response to experimental 
