age digestibility. The method of determining 
total digestible nutrients (TDN) should be re- 
placed by energy determinations, because TDN 
does not adequately express the relative nutri- 
tive effect of roughages compared with concen- 
trates (roughages are overevaluated). For di- 
rect or in vivo feeding trials, determinations of 
digestible protein and either digestible energy 
or metabolizable energy are recommended. Di- 
gestible energy is recommended over metabo- 
lizable energy if the forage does not contain ap- 
preciable amounts of essential oils because the 
technique is less complex; however, results 
are highly correlated to animal performance. 
Furthermore, in determining metabolizable en- 
ergy, losses in methane gas must usually be 
computed rather than determined directly, 
which can further increase experimental error 
(Swift 1957). 
Indirect methods can be used to calculate 
forage digestibility, which is often a decided 
advantage because, (1) much time and expense 
can be saved and (2) grazing animals can be 
studied under range conditions. Indicators are 
used in both the ratio and fecal index tech- 
niques, but there is some doubt about the effec- 
tiveness of indicators in determining digesti- 
bility coefficients. The use of chromogens as 
internal markers was unsatisfactory on some 
range plants evaluated by Cook and Harris 
(1951). Harris et al. (1959) found that the use 
of the lignin-ratio technique combined with 
chromic oxide capsules gave promising results 
during range forage intake and digestibility 
studies on sheep. Smith et al. (1956) had diffi- 
culty in determining lignin percentages of 
woody materials accurately and stated that 
there is no basis for using the lignin-ratio ap- 
proach in studying winter diets of mule deer. 
The use of artificial rumens—the in vitro 
technique—is a promising method for indi- 
rectly determining plant digestibility. Forage 
dry matter digestibility obtained by incubation 
with fresh rumen liquor usually compares well 
with in vivo digestibility. Protein can also be 
digested in the laboratory by the enzyme, pep- 
sin (Miller 1961). Despite the advantages of 
the indirect methods, the most valid informa- 
tion is gained by relating results obtained from 
in vitro trials to those obtained from in vivo 
trials. 
Rumen assays as described by Short (1966) 
are gaining popularity. The determination of 
volatile fatty acids (VFA) is a newer tech- 
nique which greatly assists in evaluating 
quality of forages. A knowledge of diges- 
tion end products, particularly the proportion 
of the volatile fatty acids, produced from feed 
in the rumen will permit a more accurate esti- 
mation of animal response such as milk yield, 
fat gain, etc. (Shaw 1959). 
Mott (1959), in discussing the factors in- 
volved in a grazing trial, said “output per ani- 
mal is a function of nutritive value and rate of 
intake; nutritive value is a function of chemi- 
cal composition and digestibility; and rate of 
intake is a function of palatability, chemical 
composition, digestibility, grazing pressure, 
and animal response to the environment.” 
Because a feed has a high nutritive content 
and is highly digestible does not necessarily 
mean that it satisfies the physiological de- 
mands of the animals ingesting it (Dietz 
1965). Nutritional requirement and animal re- 
sponse studies are needed to obtain this type of 
information. 
In summary, the quality of forage for range 
animals is determined by: (1) The palatibility 
of the forage and related intake by the animal; 
(2) the levels of important nutrients in the 
portion of the plant eaten; (3) the ability of 
animals to digest these nutrients; and (4) the 
efficiency of the digested nutrients to meet the 
physiological demands of the animal for main- 
tenance, growth, reproduction, fattening, and 
other processes and activities. In assessing for- 
age quality for ranges primarily used by rumi- 
nant animals, the end results are measured by 
animal performance. This performance will be 
determined largely by the ability of the rumen 
micro-organisms to utilize efficiently the nutri- 
ents in the ingested feed. 
LITERATURE CITED 
Anderson, Arthur K. 
1953. Essentials of physiological chemistry. Ed. 4, 
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Annison, E. F., and Lewis, D. 
1959. Metabolism in the rumen. 184 pp., London: 
Methuen and Co. Ltd., and New York: John 
Wiley and Sons, Ine. 
Atwood, Earl L. 
1948. A nutritive knowledge shortcut. J. Wildl. Man- 
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Biswell, H. H., Collins, R. W., Foster, J. E., and Bog- 
gess, T.S., Jr. 
1945. Native forage plants. N.C. Agr. Exp. Sta. 
Bull. 358, 27 pp. 
Bonner, James, and Galston, Arthur W. 
1952. Principles of plant physiology. 499 pp., San 
Francisco: Freeman and Co. 
Cook, C. Wayne, and Harris, L. E. 
1950. The nutritive value of range forage as af- 
fected by vegetation type, site, and stage of 
maturity. Utah Agr. Exp. Sta. Bull. 344, 
45 pp. 
and Harris, L. E. i , 
1951. A comparison of the lignin ratio technique and 
the chromogen method of determining di- 
