dium carbonate (fig. 4). The filtering process 
was first described by Alexander and McGowan 
(1961). 
All in vitro digestion data are recorded on 
automatic data processing forms. Fortran com- 
puter programs are used to compile individual 
dry-matter digestibilities and triplicate tube 
digestion summaries. 
USEFULNESS 
Digestibility 
The usefulness of the in vitro fermentation 
technique in evaluating forages must be judged 
by the accuracy with which it can predict the 
value of the forage to the animal. In vitro fer- 
mentation results are usually reported in terms 
of digestible dry matter or digestible cellulose. 
Van Soest (1967) cautioned researchers against 
using only one component, such as protein, cel- 
lulose, or crude fiber, to predict digestibility, 
since all nutrient digestibilities should be in- 
cluded in such evaluations. These component 
digestibilities are, in turn, related to in vivo 
measurements such as total digestible nutri- 
ents (TDN), digestible dry matter (DDM), di- 
gestible energy (DE), and several other meas- 
ures of feed value (Baumgardt et al. 1962a, 
1962b; McCullough 1959; Moir 1961). Most 
workers have not related in vitro fermentation 
values directly to animal responses, but it ap- 
pears that in vitro values should be evaluated 
by their correlation with animal production, 
rather than by their correlation with other 
laboratory measures. In vitro digestion re- 
search has attempted to estimate in vivo 
values, which are themselves only estimates of 
animal production. These in vivo measure- 
ments are also meaningless without direct or 
inferred relationships to animal production. 
In vitro digestible forage consumed was re- 
lated to yearling Hereford cattle gain on pon- 
derosa pine ranges from 1963 to 1967. These 
preliminary data indicate that the relationship 
can be expressed as a straight line: 
Y = —1.018 + 0.257X 
where X is in vitro digestible dry matter con- 
sumed in pounds per acre and Y is beef gain in 
pounds per acre (fig. 5). 
In vitro techniques can be used to evaluate 
the nutritive value of forage consumed by both 
domestic and wild animals (Short 1963). Plant 
fractions—leaf, stem, and seed—can be evalu- 
ated by this method (Pearson 1967b). Plant 
breeders can evaluate the nutrition of newly 
developed strains and varieties. The effect of 
management practices and the effect of fertil- 
izers and herbicides can also be tested. 
90 
Y=-1.013+0.257X 
BEEF GAIN 
(POUNDS PER ACRE) 
— 4 SSS el 
° 20 40 60 80 100 
DIGESTIBLE FORAGE CONSUMED 
(POUNDS PER ACRE) 
FIGURE 5.—Beef production as related to digestible 
forage consumed. 
Volatile Fatty Acids 
Another phase of in vitro work is evaluating 
the volatile fatty acids produced by various 
feeds. Rice et al. (1962) in Wyoming related in 
vitro volatile fatty acid production to the feed- 
ing value of forage as expressed by conven- 
tional TDN determinations. Knowledge of the 
volatile fatty acids produced could help in de- 
termining the type of feed needed for a parti- 
cular purpose. For instance, feeds that produce 
low proportions of acetic acid in the rumen 
will yield milk of low fat content but will show 
a high feed conversion to milk (Tilley et al. 
1960). Rations high in production of propio- 
nate and butyrate may be expected to result in 
more efficient fat production than rations high 
in acetate (Shaw 1959). From analyses of vol- 
atile fatty acid and gas composition during in 
vitro fermentations, Short (1963) demon- 
strated that deer and cattle utilize an alfalfa- 
corn diet similarly, but that browse species 
were digested better by deer. Nagy et al. 
(1967) compared the inoculum from deer, cat- 
tle, and sheep to determine their ability to di- 
gest alfalfa hay, and found all similar in vola- 
tile fatty acid production when the animals 
were on an adequate diet. 
Intake 
To determine the value of a feed, we must 
know both its digestibility and the amount of 
forage an animal consumes voluntarily. Recent 
research has indicated that this aspect of nu- 
trition may be evaluated by in vitro fermenta- 
tion trials.° As indicated earlier, short periods 
of digestion—6 to 12 hours—are best related to 
intake. This theory hypothesizes that the speed 
at which a particular forage is digested and re- 
°Crampton 1957; Crampton et al. 1960; Donefer et 
al. 1960; Barnes 1966. 
