that related fecal output of nutrients per unit 
of feed consumed to feed composition. 
Digestibility 
The measure of digestibility commonly used 
is apparent digestibility, which is calculated by 
the formula, [(forage consumed—fecal out- 
put) /forage consumed] X100. This can be ex- 
pressed as a percentage digestibility of dry 
matter, organic matter, or energy. 
The standard procedure for estimating for- 
age quality has been the conventional in vivo 
digestibility trial. However, such trials are 
time consuming, expensive, and require large 
amounts of forage. Therefore, replication be- 
comes a limiting factor. Once again, people 
looked for other methods; consequently, the in 
vitro rumen fermentation and nylon bag tech- 
niques were developed. 
Increasing attention is now being paid to 
herbage digestibility, particularly since the 
two-stage rumen liquor-acid pepsin in vitro 
technique was introduced by Tilley and Terry 
(1963). Researchers then calculated various 
regression equations relating digestibility to a 
number of other factors. However, regression 
equations for in vivo digestibility on nitrogen, 
crude fiber, or lignin content have standard er- 
rors that generally are too high to detect inter- 
species differences (Milford and Minson 1966). 
A review by Barnes (1965) comparing several 
in vitro methods showed that the two-stage 
technique of Tilley and Terry is probably the 
most accurate technique for predicting in vivo 
digestibility, but precision is still a problem. 
An in vivo technique utilizing a nylon bag in 
the rumen has been reviewed recently by Har- 
ris et al. (1967). Essentially, the method con- 
sists of suspending a finely ground forage sam- 
ple contained in a nylon bag in the rumen for 
varying incubation periods (usually 24 hours). 
Estimates of cellulose digestibility of range 
forages were found by California investigators 
to be slightly but consistently higher than 
those obtained by the in vitro technique. Sev- 
eral investigators have reported nonsignificant 
correlations between nylon bag estimates of 
dry matter and cellulose digestibility and di- 
gestibility determined by total collection meth- 
ods. The nylon bag technique appears to be 
more accurate but more variable than the in 
vitro technique for evaluating forage digest- 
ibility (Van Dyne and Weir 1964). 
With further refinement, both of these ‘‘mi- 
crodigestion” techniques seem to be promising. 
Some investigators, however, have apparently 
overlooked several disturbing questions: 
1. Is the behavior of the fistulated animals 
the same as that of the intact animals grazing 
the same range? 
2. For the in vitro technique, are the inocula 
obtained from donor animals the same as those 
in intact animals grazing the same range? 
3. For both techniques, are the forage sam- 
ples used in vitro and in nylon bags the same 
as the forage being consumed by the grazing 
animals? 
These two techniques definitely do not solve 
the animal replication problem. However, it is 
possible that increased precision can be ob- 
tained by using the nylon bag or in vitro tech- 
nique. 
RATE OF INTAKE 
We will not attempt to break down the other 
major function of forage quality, rate of in- 
take, into its components because the emphasis 
in research on digestibility has caused work on 
intake to be neglected, except for a few cases. 
Thus, we do not know very much about some 
of the factors involved in the intake rate. 
Workers at Macdonald College in Canada 
have developed a Nutritive Value Index, which 
is the product of digestibility and intake 
(Crampton et al. 1960). They found that Nu- 
tritive Value Indices show correlations with 
body weight changes ranging from 0.88 to 
0.94. Intake and digestibility contributions to 
the numerical value of the index were roughly 
70 and 30 percent, respectively. The same peo- 
ple (Donefer et al. 1966) later predicted both 
digestibility and intake from in vitro determi- 
nations that reflected rate of digestion rather 
than total digestibility. 
The use of indigestible external indicators 
for measuring dry matter intake has been 
studied extensively. Although chromic oxide 
has seemed the most promising external in- 
dicator, the excretion of chromic oxide has 
large intra- and inter-day variations and large 
between-cow variations. These, then, limit the 
practical use of this method for determining 
intake. Intake measurements based on the 
Cr,0; technique with free-grazing animals 
should be limited to comparative measures 
over short periods and the average of several 
animals, rather than attempting to determine 
accurate data for individual animals (McCul- 
lough 1959). 
Other techniques have also been developed 
for measuring forage intake. Hyder et al. 
(1966) relate mean air temperature, moisture 
content of food, and water drunk to dry matter 
intake. Estimates by the water-intake method 
were less variable than those obtained by her- 
bage clipping. However in this case and others, 
it is impossible to determine the accuracy in 
approximating the herbage eaten by cattle on 
pasture. 
STATISTICAL CONSIDERATIONS 
The measurement of forage quality, however 
defined, presents a major problem in _tech- 
33 
