10
Rumen Function
A. Bannink
1
and S. Tamminga
2
1
Division of Nutrition and Food, Animal Sciences Group, Wageningen
University Research Centre, P.O. Box 65, 8200 AB Lelystad, The Netherlands;
2
Animal Nutrition Group, Wageningen Institute of Animal Sciences,
Marijkeweg 40, 6709 PG Wageningen, The Netherlands
Introduction
Under natural conditions the compartmentalization of the digestive tract of
ruminants is a vital adaptation to the utilization of the biomass they select with
grazing or browsing. The evolution of the reticulorumen made it possible to
retain fibrous material in the rumen for long periods, and to sustain a microbial
population that lives in symbiosis with the ruminant as the host. This has
evolved in distinct morphological characteristics of the multiple-stomach sys-
tem among ruminant species (Van Soest, 1994). Differentiation among spe-
cies, and even breeds, supports the idea that next to dietary factors, rumen
factors may also be important determinants of microbial activity and rumen
function as a whole.
As a result of microbial fermentation, biomass that otherwise could not
have been digested enzymatically by the host, becomes degraded and is con-
verted to digestible microbial matter, volatile fatty acids (VFA), fermentation
gases and heat. The major end-products of fermentation deliver most of the
metabolizable energy and metabolizable protein to the host. This emphasizes
the importance of rumen function, as an essential link in the chain of feed
ingestion, microbial fermentation, intestinal digestion and metabolic utilization.
In current practice, nutrient supply to the host is expressed in terms of energy

the different fractions and with the level of feed intake.
This chapter deals with the effects of dietary changes on the fermentation
processes in the rumen and their consequences for the amount and type of
nutrients delivered to the ruminant host, as well as the mathematical descrip-
tion of these processes. In addition to the fermentation in the lumen, the tissues
in the rumen wall are also of importance for rumen function (Bergman, 1990).
Therefore, in this chapter some effort is also made to identify the interactions
between the functioning of the rumen wall and events taking place in the
lumen.
Carbohydrate Degradation
When discussing carbohydrate fermentation, three distinctly different types of
carbohydrates are distinguished: fibre, starch and a fraction defined by organic
matter minus crude fat, crude protein, starch and fibre. The latter fraction is
highly heterogeneous and in the remainder of this chapter the fraction will be
referred to with the term soluble carbohydrate. In this section, an extensive
collection of data (data set used by Bannink et al., 2000) from rumen digestion
trials with lactating Holstein Friesian dairy cows, covering a large variety of
dietary treatments, is used to discuss degradation of different types of
carbohydrates.
Fibre degradation
In general, ruminant diets contain forages with a relatively high content of cell
wall material and concentrates also contain limited amounts of cell walls. Cell
walls, also known as structural carbohydrates, or simply fibre, are chemically
264 A. Bannink and S. Tamminga
characterized as insoluble in neutral detergent and hence are called neutral
detergent fibre (NDF). This NDF is considered to consist of cellulose, hemicel-
lulose, lignin and a small amount of nitrogen-containing material. Part of the
pectic substances also contributes to NDF. The main role of the rumen is the
fermentation of dietary fibre. Several factors influence the fermentation char-
acteristics of the NDF in forage, such as stage of maturity, growing season and

(2000). Variation of the fractional passage rate of particulate material influ-
ences the retention time and hence the amount of NDF available for microbial
degradation. Several reviews (Owens and Goetsch, 1986; Clark et al., 1992)
indicate that fractional passage rate affects the concentration of microorgan-
isms present, and also the efficiency of microbial growth. Thus, fractional
passage rate may be positively related to fractional degradation rate. Fractional
degradation rate itself determines the time required for a feed particle to reach
the appropriate specific weight to flow out of the rumen. Using
13
Cas
an internal marker for NDF, Pellikaan (2004) indeed demonstrated a relation-
ship between rate of degradation and rate of passage. It then also becomes
apparent why particle size and rate of particle comminution are important for
Rumen Function 265
the degradation rate of NDF (Kennedy and Murphy, 1988). The size of rumen
particles influences the surface area available for microbial attack, their reten-
tion time in the rumen and the concentration of fibrolytic microorganisms
attached to them. Baldwin et al. (1987) attempted to represent the effects of
particle dynamics on rumen function. Interactions also exist between amylolytic
and fibrolytic activity in the rumen. Large amounts of starch and soluble
carbohydrates not only reduce fibrolytic activity (via rumen pH as mentioned
above), but also affect the availability of ammonia and protein as nitrogen
sources for the growth of fibrolytic microorganisms (Dijkstra et al., 1992).
Yet, current feed evaluation systems largely ignore the effects of variation in
rumen pH and passage rates, and the fractional rates of degradation and
passage are as yet considered independent of each other. If considered at all,
current feed evaluation treats the amylolytic and fibrolytic activity in the rumen
as fully independent of each other.
From analysing the database with reported rates of NDF degradation, it
appears that the extent of rumen NDF degradation varies from as low as 13%

266 A. Bannink and S. Tamminga
Starch degradation
Although starch is not a major constituent of most forages, it may be a
significant component of many ruminant diets through the use of grain-based
supplements. Such supplements with a high energy density may have profound
0
10
20
30
40
50
60
70
80
90
24681012
NDF intake (kg/day)
Rumen degradability of NDF (% of intake)
Fig. 10.1. Relationship between NDF intake (kg of NDF per day) and rumen degradability of
NDF (% of NDF intake). Only reported values have been used. The drawn lines indicate the
results of linear regression for individual experiments. Regression of the full data set resulted in
the relationship: NDF degradation ¼À1:37  NDF intake þ 56:90 (R
2
¼ 0:03).
0
10
20
30
40
50

Soluble carbohydrate intake (kg/day)
Rumen degradability of NDF (% of intake)
Fig. 10.3. Relationship between soluble carbohydrate intake (defined as organic matter minus
fat, crude protein, starch and NDF, kg of soluble carbohydrate per day) and rumen degradability
of NDF (% of NDF intake). Only reported values have been used. Regression of the full data set
resulted in the relationship: NDF degradation ¼ 5:33 Â soluble carbohydrate intake þ
37:74 (R
2
¼ 0:23).
Fig. 10.4. Relationship between
intake of sugar or soluble
carbohydrate (defined as organic
matter minus fat, crude protein,
starch and NDF, kg of soluble
carbohydrate per day) plus starch
(kg of starch per day) and rumen
degradability of NDF (% of NDF
intake). Only reported values have
been used. Regression of the full
data set resulted in the relationship:
NDF degradation ¼À4:02 Â
soluble carbohydrate and
starch intake þ 75:17 (R
2
¼ 0:38).
0
10
20
30
40

above 2 kg/day, a highly variable fraction of consumed starch was degraded
(from 10% up to almost 100%) and many trials showed a relatively low starch
digestibility and high escape from rumen fermentation. With high levels of
−150
−125
−100
−75
−50
−25
0
25
50
75
100
024681012
Starch intake (kg/day)
Rumen degradability of starch (%)
Fig. 10.5. Relationship between starch intake (kg of starch per day) and apparent rumen
starch degradability (% of starch intake). Only reported values have been used. Regression of the full
data set resulted in the relationship: starch degradation ¼ 2:96 Â starch intake þ 36:25 (R
2
¼ 0:05).
Rumen Function 269
starch consumption, above 8 kg of starch per day, this variation seems to be
smaller and both the escape and the degradation of starch appear to be mostly
between 40% and 60% (Fig. 10.5). With small differences in starch intake
among treatments, no consistent effects were observed. In the two studies with
the widest range in starch intake among treatments, starch degradation be-
came reduced with increased starch intake. However, in both studies starch
intake was confounded with starch source. In the study where starch intake

these changes and hence this will not be discussed further.
Soluble carbohydrates
Compared with the dietary content of fibre and starch as carbohydrate sources,
water-soluble carbohydrates (WSC), including lactate as a major component in
silages, normally form a modest fraction of up to 15% of the dry matter. An
assumption generally made is that WSC are fermented in the rumen almost
270 A. Bannink and S. Tamminga
instantaneously after ingestion. This is supported by the observation that only
very small concentrations of WSC are found in rumen fluid. Fractional degrad-
ation rates of 300% per hour have been suggested (Russell et al., 1992). With a
fractional passage rate of rumen fluid of 15% per hour, about 5% of the WSC
ingested would escape from the rumen. In such a situation and assuming a daily
intake of 20 kg DM containing 15% WSC, only 150 g/day of WSC would flow
to the duodenum. But, as was argued for fibre and starch, in reality the
fractional degradation rate of WSC must also be a function of rumen microbial
activity rather than a constant value of 300% per hour. Despite this, the
amounts escaping the rumen will remain small under normal feeding condi-
tions. Large quantities of WSC may however induce fluctuations of rumen pH.
This could notably be the case with sugars that are immediately available, such
as in molasses. Such WSC may have consequences for the fibrolytic activity, as
well as the protozoa in the rumen, with a subsequent influence on predation
rate and apparent efficiency of microbial growth on the whole rumen level. The
WSC present in roughages such as grasses or sugarcane have to be released
first from the plant cells before they are available for microorganisms, and
therefore are less likely to cause severe fluctuations in rumen pH.
Next to the dietary content of fibre, starch and soluble sugars, a significant
fraction of organic matter (generally more than 10%) remains unaccounted for
in standard feed analysis. The size of this fraction is often close to that of the
WSC and hence, may not be neglected in attempts to understand the effect of
nutrition on rumen function or on ruminant performance. The types of chem-

synthesized in the rumen constitutes the major part of the duodenal entry of
non-ammonia N. In addition, a variable portion of feed non-ammonia N
escapes rumen degradation, the size of which depends on the intrinsic degrad-
ation characteristics of the protein source involved, and on additional aspects of
rumen function as already discussed for carbohydrates fermented in the rumen.
Finally, some endogenous protein flows to the duodenum, but quantities
remain relatively small.
There are a number of reasons why intrinsic degradation characteristics
obtained from in situ or in vitro incubations are inadequate to assess the real
protein value. The type of N source influences the energy cost of microbial
protein synthesis (Stouthamer, 1973) and therefore a distinction between
amino acid N and ammonia N has to be made. Further, fermented protein is
part of the fermentable organic matter. However, the efficiency of microbial
growth on fermented protein as source of energy is lower than that on protein-
free organic matter (Dijkstra et al., 1996; Bannink and de Visser, 1997). Based
on theoretical considerations the ATP yield per g of fermented protein
was estimated as about half the amount derived from the fermentation of
carbohydrates (Tamminga, 1979).
Microbial Metabolism
Hexose utilization in relation to microbial growth
The fermentation of hexoses to VFA, carbon dioxide and methane generates
metabolic energy for microorganisms (ATP) (see Chapter 9). Hexoses and
fermentation intermediates are also used as precursors for biosynthetic pro-
cesses in microbial growth. In addition, the so-called spilling of energy may
occur as well as the storage of polysaccharides during conditions of a surplus of
available energy in the rumen environment. Furthermore, microbial protein
synthesis on preformed monomers such as amino acids requires less energy
than growth on ammonia as source of N (Baldwin et al., 1987; Dijkstra et al.,
1992), affecting efficiency of microbial growth.
In vivo efficiencies of microbial growth, derived from observed outflows of

urea with saliva and transferred through the rumen wall) to use the energy
from the starch efficiently. Young leafy forages high in N have a positive N
balance, and the surplus of N in the form of ammonia is absorbed through the
rumen wall. Rates of degradation are calculated from the measured ingredient
characteristics, table values, presumed passage rates and so on. However, such
feed evaluation systems all have in common that important aspects of rumen
function known to influence the rate of degradation and the efficiency of
microbial N synthesis are not represented. This may lead to wrong conclusions
on the N balance for the rumen as a whole. It is questionable whether in this
way accurate estimates of actual losses of N as ammonia absorbed from the
rumen are obtained.
An analysis on the rumen N balance was made of observations available in
the database used in the present study. The data indicate that rumen N balance
increases with an increased dietary crude protein content (Fig. 10.6), but more
clearly with an increase in the quantity of N consumed (Fig. 10.7). Variation
among different studies remained very large, however. Only for the extreme
cases with a dietary content of crude protein less than 15% or more than 19%,
positive and negative N balances, respectively, seem to be lacking. For inter-
mediate protein contents the N balance varies from À150 to þ150 g of N per
day. From this it may be concluded that other factors are also important to
Rumen Function 273