Chapter 10
Gas Production Methods
P. Schofield
Department of Animal Science, Cornell University,
Ithaca, New York, USAIntroduction
The energy metabolism of ruminant
animals is based on their ability to digest
plant structural carbohydrates such as
cellulose. This digestion is carried out
under anaerobic conditions in the rumen
by a complex consortium of bacteria, fungi
and protozoa. Cellulose is an exceptionally
stable polymer of glucose units with a
structure resembling reinforced concrete in
some respects. It is therefore digested
slowly. Limitations on the body size of
ruminants mean that fibrous feeds can be
retained in the rumen for only a limited
period of time, 24–48 h for cattle, before
passing to the lower tract where most fibre
digestion ceases. There is thus a continuing
competition between the processes of
ruminal digestion and passage. For this
reason, a study of ruminant nutrition must
include the topics of fibre digestibility and
digestion rates – how they are measured
and interpreted. The goal of this chapter is
to survey the application of in vitrogas
production measurements to the study of
these topics.
We first consider some quantitative
aspects of gas production and their bearing
on the historical development of the
technique. We summarize general factors
that affect gas measurements and discuss
the merits and demerits of alternative
techniques. Following a survey of equip-
ment needs, we examine alternative
models for fitting and interpreting gas
curves and explore the relationship
between the substrate pools hypothesized
by these models and actual plant carbo-
hydrate fractions.
Gas and microbial yields per unit of
substrate digested are considered next. We
survey applications of the gas method
including the detection of plant secondary
compounds that may interfere with diges-
tion, the role of plant soluble carbo-
hydrates in the fermentation process, and
practical applications such as intake pre-
diction. The appendix contains a brief
review of non-linear curve fitting and a
description of pressure sensor structures.Quantitative Aspects
The process of fermentation involves a
series of energy-yielding reactions catalysed
by microbial cells in which organic
© CAB International2000. Farm Animal Metabolism and Nutrition
(ed. J.P.F. D’Mello) 209