Farm Animal Metabolism and Nutrition

Chapter 16

Feed Intake in Ruminants: Kinetic Aspects

W.C. Ellis^1 , D. Poppi^2 and J.H. Matis^3

(^1) Department of Animal Science and (^3) Department of Statistics,
Texas A&M University, College Station, Texas, USA;
(^2) Department of Agriculture, The University of Queensland,
Brisbane, Queensland, Australia

Introduction

As for other mammals, feed intake in
ruminants is regulated by a milieu of
signals, and the level of feed intake
achieved is that which maximizes
benefits versus costs. The thesis of this
chapter is that provision of indispensable
amino acids required by the ruminant’s
tissues is the most basic nutritional
benefit and regulates feed intake.
Assuming that feed intake is not other-
wise constrained, the rate of feed intake
is proposed as that which provides the
flux of net amino acids required by the
ruminant’s tissues. Feed intake may be
constrained by metabolic or physical
transformations of the feed by the
ruminant’s nutrient acquisition system.
This chapter focuses on the dynamics of
the physical and metabolic processes of
the ruminant’s nutrient acquisition
system, processes that are proposed to
both be driven by and contribute to the
nutritional balance of amino acids at the
ruminant’s tissue level.

Ruminant Nutrition

Digestion dynamics

The ruminant has evolved a digestive

system that maximizes efficiency of energy

acquisition from its evolutionary environ-

ment. Characteristic features of this diges-

tive system are its dynamic interactions

with attributes of the feed resulting in the

growth and maintenance of a significant

microbial ecosystem whose end- and by-

products are the primary source of

nutrients for the ruminant’s tissues.

Efficiency of energy acquisition via the

microbial ecosystem is maximized via

interactions that extend the residence time

of feed residues in the reticulo-rumen.

Mean residence time, N/–
e, is slowed by

an Norder of age-dependent escape of feed

residues, –
e, that is considerably less than

the growth rate of the microbial ecosystem.

As a consequence, sufficient residence time

is available for the metabolically significant

microbial ecosystem to be sustained from

the flux of energy derived from

hydrolysable structural carbohydrates (HF)

that are hydrolysed at a relatively slow

rate, kh, i.e. HFkh.

© CAB International2000. Farm Animal Metabolism and Nutrition

(ed. J.P.F. D’Mello) 335