these two dietary entities with greater
precision than could voluntary intake or
digestibility. Such empirical observations
are consistent with the negative effects of
UF upon digesta kinetics, the positive
effects of CP upon nutrient acquisition of
MP and the general insufficiency of MP for
ruminant tissues in the metabolizable
nutrients which exit the rumen. The
superior empirical description of liveweight
gain by this model and failure reliably to
predict intake and digestion reflect the
complexity of dietary–digestive–metabolic
interactions described in this chapter.
Conclusions
The anaerobic metabolism of the rumen
microbial anaerobic fermentation yields
deficient proportions of metabolizable
amino acids relative to the amino acid and
energy requirements of the aerobic
metabolism of ruminants’ tissues.
Nutritionally, feed intake appears to be
regulated in an attempt to provide the flux
of amino acids required by the animal’s
genetic potential. However, voluntary in-
take by ruminants appears constrained by
the metabolic capacity of its nutrient acqui-
sition system. Such constraints appear
related to inefficient utilization of amino
acids for microbial growth in addition to
thermodynamic limitations inherent in
anaerobic metabolism. These constraints
are accentuated further by levels of
undigested entities in the diet that increase
ruminal residence time and, thereby,
ruminal efflux yield of metabolizable
amino acids. Reflecting such constraints,
voluntary intake of forages by ruminants
appears not to be maximized even by levels
of dietary protein well beyond those which
they normally contain.Acknowledgement
This chapter is based, in part, upon work
supported by the Texas Advanced Research
(Technology) Program under Grant No.
517–0170–1997.Feed Intake in Ruminants 361References
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