Farm Animal Metabolism and Nutrition

in the rumen digesta. Physiologically,^2 UF–
e
may be viewed as an equilibrium rate
between the opposing forces of a nutrient
requirement-driven intake of feed contain-
ing UF versus the inertial force of the mass
of UF in the ruminal digesta mixing pools.
These physical effects of UF determine the
flux of nutrients derived from potentially
hydrolysable entities such as carbohydrates
and crude protein (CP) and, consequently,
the ruminal efflux proportions of amino
acids and energy change with level of
intake. Because the processes illustrated in
Fig. 16.1 involve interactions among
driving and driven processes, estimation of
intake involves an iterative cycling of
the processes portrayed in Fig. 16.1.
Biologically, the level of intake is suggested
to involve similar iterative successive
approximations of processes to arrive at a
level which either supplies the net amino
acid requirements or is constrained by
either the physical or metabolic capacity

of the nutrient acquisition system or

imbalances in the flux of metabolizable

nutrients.

Sequential ecosystems

The nutrition of ruminants should be

viewed as the nutrition of two sequential

ecosystems. The nutrition of the initial

ecosystem, that of the rumen micro-

organisms, is derived via anaerobic meta-

bolism of monomers derived from

hydrolysed polymers of plant tissues. The

nutrition of the second ecosystem, the

ruminant’s tissues, is derived primarily

directly from metabolism of absorbed VFAs

and indirectly from amino acids produced

from intestinal digestion of ruminal efflux

microbial protein. The two ecosystems

have distinctively different metabolic and

nutritional characteristics, differences that

have important nutritional implications for

Feed Intake in Ruminants 337

Fig. 16.1.The central role of ruminal load of undigested fibre (UF) in determining the dynamics of ruminant
digestion. UF turnover,^2 UF–
e, is an equilibrium between the influx forces of an amino acid requirement-
driven feed intake and the inertial forces constraining escape of UF from the ruminal digesta mixing pools.

(^2) UF–

eregulates flux of hydrolysed carbohydrates (HCHO) and CP, and thereby the metabolic rate of the
rumen microbial ecosystem and thus the flux proportions of energy, VFAs and amino acid (AA) to
the ruminant’s tissues. The feed intake rate finally achieved is determined iteratively as that which provides
the requirements for AA or that which is constrained by insufficient ruminal efflux of AA.