sugars, starch, nonprotein nitrogen, and solu- 
ble protein, are soluble in neutral detergent. 
Because this system separates the totally di- 
gestible, partially digestible, and indigestible 
portions of plants, it is much better than the 
crude fiber method. 
Forage Digestibility 
Although data on the proximate composition 
of plants indicate their probable nutritive 
value, feeding trials are needed to provide a 
more definitive reference (King and McClure 
1944). Atwood (1948) reported that compari- 
son of the gross composition of a plant to its 
ability to combat starvation shows a routine 
feed analysis alone is unreliable as an indicator 
of nutritive value, and that this analysis must 
be supplemented by digestion trials. Digestion 
trials have shown that older, more mature fall 
and winter forage is less readily digested than 
newer, tender spring growth; thus, any protein 
deficiency in the fall and winter may be much 
greater than that shown by routine chemical 
analysis (Biswell et al. 1945). Some of the 
measurements employed in digestibility studies 
are described in the following paragraphs. 
The in vivo digestion trial is conducted (1) 
to record the amounts of feed consumed and 
the wastes excreted by an animal, (2) to deter- 
mine the nutrient percentages of both feed and 
excreted substances by chemical analysis, and 
(3) to calculate the apparent digestibility of 
the individual nutrients. A 7-day conditioning 
period usually precedes a 7-day collection pe- 
riod; however, shorter intervals are sometimes 
used. Indicators are often used in conjunction 
with the digestion trial to facilitate the deter- 
mination of feed intake and nutrient digestibil- 
ity. In vitro digestion trials, using rumen li- 
quor in artifical rumens to estimate forage di- 
gestibility, eliminate the need for handling 
large animals in digestion cages and feeding 
large amounts of forage. 
Digestion Coefficient 
The digestion coefficient of a nutrient is the 
percentage of a nutrient in a feed apparently 
digested by an animal. It is calculated from the 
amount of nutrients in the feed minus that re- 
covered from the feces. 
Digestible Protein (DP) 
The percentage of protein in the ingested 
feed that remains in the animal body is called 
“digestible protein.” It is listed separately in 
nutritional studies because of its importance 
in maintaining bodily functions. 
Total Digestib!e Nutrients (TDN) 
The term TDN indicates the sum of all di- 
gestible organic nutrients in a feed. The per- 
centage of digestible fat is multiplied by a fac- 
tor of 2.25 due to its higher energy value, and 
is added to the percentages of digestible pro- 
tein, fiber, and nitrogen-free extract. TDN esti- 
mates the digestible energy value of the ration. 
Nutritive Ratio (NR) 
The ratio between the digestible protein and 
the sum of the digestible nonnitrogenous nutri- 
ents is called the nutritive ratio. 
Digestible Energy (DE) 
Digestible energy is calculated from the 
gross energy of feed minus the energy in the 
feces. It represents apparent digestibility, 
without correction for fecal constituents of 
metabolic origin. The digestible energy of feed 
is comparable to total digestible nutrients. 
Techniques are available for converting DE 
data to TDN (Morrison 1957). DE (and also 
TDN) is relatively unaffected by the plane of 
nutrition, environmental temperature, and 
physical activity of the animal. Many nutri- 
tionists consider DE one of the most practical 
and meaningful measurements of the nutri- 
tional value of a feed (Swift 1957). 
Metabolizable Energy 
The gross energy of a feed minus the energy 
lost in methane, urine, and feces is called metab- 
olizable energy. It is more difficult to obtain 
than digestible energy and more subject to ex- 
perimental error (Swift 1957). 
Net Energy 
The net energy of a feed is the gross energy 
minus the energy lost in feces, urine, methane, 
and in heat production. Theoretically, it is the 
ideal measurement of the nutritional value of a 
feed because it is the energy available to the 
animal for growth, maintenance, fattening, 
and other processes and activities. Actually, it 
is expensive and difficult to determine because 
of the need for exacting control over experi- 
mental procedures and conditions. Also, its 
applicability is limited because the results de- 
termined in the laboratory under carefully con- 
trolled conditions are not equally applicable to 
farm and ranch conditions (Swift 1957). Net 
energy values are subject to the limitations 
that the heat increment per unit of food intake 
varies according to the balance of the nutrients 
in the ration, the level of intake, the produc- 
tive function, the species, and other factors. 
Thus, net energy values of feed cannot be con- 
sidered constant (Maynard and Loosli 1956). 
Because of its great potential importance in 
the field of animal nutrition, future research 
will undoubtedly be directed toward simplify- 
ing and standardizing net energy techniques. 
It is important to remember, when consider- 
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