NET AVAILABLE ENERGY—MAINTENANCE. 435 
complete metabolism experiments, their results as a whole indicate 
clearly that the metabolizable energy of the grains is more valu- 
able for maintenance than that of the coarse fodders, a fact un- 
doubtedly due to the greater expenditure of energy in the digestion 
and assimilation of the latter. 
The maintenance ration of horses, cattle, and sheep, then, as 
ordinarily expressed (i.e., in units of digestible matter or of metabo- 
lizable energy) is not a constant but a variable, depending on the 
availability of the metabolizable energy, and such a statement of it, 
to be definite, must be accompanied by a statement of the kind of 
feed used. 
No similar experiments upon swine appear to have been made. 
The ordinary feed of this animal, however, probably varies less in 
availability than that of ruminants, and it may be presumed that 
no such striking differences would be found. 
VALUE OF CRUDE Fiser.—aAs a result of Wolff’s conclusions con- 
cerning the apparent worthlessness of crude fiber for work production, 
as discussed in the succeeding chapter, and of Zuntz & Hagemann’s 
estimates regarding its digestive work (p. 389), there has been a 
tendency to ascribe the difference between grain and coarse fodders 
to the greater amount of crude fiber in the latter, forgetting that 
what these investigators have actually shown is simply the lower 
value of the digestible matter from coarse fodders, and that their 
conclusions regarding crude fiber are deductions from the observed 
facts. Kellner’s more recent experiments (see p. 182 and Chapter 
XIII, $1) have demonstrated that at least one form of crude fiber 
is nearly as efficient in producing a gain of fat by cattle as is 
starch. A fortiori, therefore, it should be equally valuable for 
maintenance. We have as yet no sufficient evidence to justify us 
in ascribing the difference between grain and coarse fodder to the 
crude fiber as such aside from its influence on the mechanical 
structure of the material. 
INFLUENCE OF THERMAL ENVIRONMENT.—It has been not 
uncommonly assumed that the maintenance requirement of an 
animal is affected by changes in the temperature and other external 
factors which combine to determine the refrigerating effect of the 
environment; in other words, the heat production of the animal 
has been looked upon more or less distinctly as an end in itself. 
