Farm Animal Metabolism and Nutrition

Chapter 7

Aspects of Cellular Energetics

N.S. Jessop

Institute of Ecology and Resource Management,

The University of Edinburgh, Edinburgh, UK

Introduction

All animals start life as a single cell from
which complex organisms develop by
processes of cell multiplication, cellular
differentiation and cellular growth. For
mammals, there is an enormous diversity
in final body size and shape, yet all share
similar body constructive architecture. At a
cellular level, these widely differing
animal species are remarkably similar. For
example, cell size in comparable organs or
tissues does not vary appreciably between
animals of widely differing size when
compared at similar stages of maturity.
What does differ is the number of cells
each organism possesses.
In order to develop from a single
fertilized egg into a complex organism, the
supply of considerable quantities of
nutrients is required. One need for
nutrients is to provide a supply of energy
for survival. When considering the energy
requirements of farm animals, it is conven-
tional to divide the total energy require-
ment into those for different processes or
factors such as maintenance, growth,
pregnancy and lactation. At this holistic
level, empirical equations have been

developed that enable the quantification of

these requirements. Further subdivision of

these factors is necessary in order to gain

more understanding of requirements and

how these might change over the course of

an animal’s life. For example, maintenance

costs are quantitatively the most important

• in an animal living out its full life-span
  they will make up around 98% of total
  lifetime energy requirements. Even in
  animals slaughtered before they reach
  maturity, maintenance costs can be around
  50% of total nutrient requirements.
  Maintenance includes many functions.
  Firstly, it includes the basal metabolic rate
  (BMR) which is the energy required in
  order to maintain cells within an animal’s
  body in a functional state, together with
  minimal activity such as respiration and
  circulation enabling the animal to survive.
  It is measured as the heat production in a
  rested animal in the post-absorptive state,
  i.e. when there is minimal processing of
  food within the digestive tract. Next,
  maintenance includes energy costs of
  muscular activity above those included in
  BMR (such as beating of the heart, expan-
  sion of the chest wall). These might
  include locomotory costs or muscular

© CAB International2000. Farm Animal Metabolism and Nutrition

(ed. J.P.F. D’Mello) 149