mined by digestion trials, the equations used 
by this agency differ according to the type of 
forage studied as follows: 
Legume Forage: 
TDN =74.43 + 0.35 CP — 0.73 CF 
Grass Forage: 
TDN =50.41 + 1.04 CP — 0.07 CF 
Mixed Hay Crop Forage: 
TDN =65.14 + 0.45 CP — 0.38 CF 
The current studies on deer forage are con- 
cerned with the application of these or simi- 
larly developed equations in the estimation of 
nutritive values of browse and other deer 
foods. Calibration of such experimental evalua- 
tion techniques requires actual measurement of 
digestibility of many different samples of 
browse and other deer food. Therefore, sheep 
are used as test animals because of the obvious 
advantages of using domesticated animals for 
such studies. Similarities of the digestive ca- 
pacities of sheep and deer have been demon- 
strated and will be discussed later. Deer of suf- 
ficient uniformity and docility for use in diges- 
tion trials are rare. 
To avoid difficulties such as those encoun- 
tered by Ullrey et al. (1964) in feeding single 
species of browse in digestion trials, browse 
materials fed in these studies will be mixed 
with complete diets of known digestibility, and 
their digestibility will be determined by differ- 
ence. 
A minimum of such laborious conventional 
digestion trials will be conducted; the number 
will be that needed to establish a sound re- 
lationship between actual digestibility values 
and those determined by various small-sample 
techniques such as the in vivo nylon bag tech- 
nique and the in vitro digestion methods of 
Tilley and Terry (1963). One of these small- 
sample methods may then be selected for cali- 
bration of a still more simple and practical 
chemical analysis. 
CRUTE PROTEIN AS AN INDEX TO 
FEEDING VALUE 
It is well established that forage crops 
grown for feeding domestic animals have their 
highest feeding value at the rapidly growing 
stage. At this stage, crude protein and mineral 
content is highest, and the energy of the car- 
bohydrate portions of the plant are most read- 
ily available to the animal. Lignification of cell 
wall structures has not progressed sufficiently 
to limit their accessibility to the digestive ac- 
tion of rumen microbes. In general, then, a 
50 
high crude protein content is considered a de- 
sirable attribute in a forage only partly be- 
cause of the need for protein in the diet for 
growth and other metabolic functions. It is 
also desirable because high-protein forages are 
usually succulent and palatable and because 
their nonprotein (carbohydrate) constituents 
are highly digestible. Also, the high content of 
nitrogen and minerals in such forages permits 
the maintenance of a healthy, vigorous popu- 
lation of rumen microbes. Consequently, the 
animal has the capacity to digest more fibrous 
foods, which may at times constitute a rela- 
tively large portion of the diet of the deer. 
Thus, even if the high-protein forage is availa- 
ble in relatively small amounts, it may make a 
valuable contribution to the diet by serving to 
supplement other foods. Such foods, if eaten 
alone, would be so indigestible as to constitute 
a starvation diet. 
Emphasis on the importance of the crude 
protein content as an index of feeding value of 
forages does not mean that the importance of 
an adequate supply of available (digestible) 
energy should be overlooked or minimized. 
Much more nutriment is required to maintain 
the normal energy metabolism of livestock 
than for all other purposes combined; and if 
this need is satisfied, probably all other re- 
quirements will be incidentally covered (Swift 
1957). However, when considering forages and 
browse-type foods, the availability of their en- 
ergy, and the amounts consumed, are closely 
related to such factors as stage of maturity 
and succulence. Such factors are in turn asso- 
ciated with nitrogen (crude protein) content in 
such a way as to give relatively high positive 
correlations between nitrogen content and di- 
gestibility of energy. Thus, it may be safe to 
predict that if the combination of natural 
foods eaten by a deer contains an adequate per- 
centage of crude protein, other nutritional re- 
quirements probably will be incidentally cov- 
ered. In most browse forage species, crude pro- 
tein is negatively correlated with crude fiber 
content (cell wall constituents). Therefore, 
equations similar to those used in forage evalu- 
ation for domestic animals should provide con- 
venient and sufficiently accurate means of com- 
puting feeding values of foods sampled from a 
given range. 
A deer’s diet is usually varied, and studies of 
food preference may not just reflect differences 
in palatability of various foods but also in- 
stinctive efforts to balance the ration. A simpli- 
fied, hypothetical model of this situation is il- 
lustrated by the use of selected data from 
sources fully described later in this paper. 
These data indicate that black cherry browse 
is about twice as rich in crude protein as red 
maple browse (fig. 1). In this theoretical situa- 
tion, we will assume that only these two spe- 
