Protracted residence times, N/–e, or,
conversely, slower –e, increase efficiency of
energy capture in the ruminal mixing pool
of digestion because efficiency of energy
capture is determined by kh/(kh+–e).
Maximizing efficiency of energy acquisition
has survival value for ruminants whose
metabolizable nutrient requirements are
primarily for metabolizable energy (ME),
i.e. maintenance and reproduction (Table
16.1). However, more productive ruminants
have proportionally greater requirements
for metabolizable protein (MP) (Table 16.1),
i.e. protein that is derived from the sum of
ruminal efflux microbial crude protein
(MCP) and undegraded escape feed protein.
As in any organism, the efficiency of MCP
synthesis is a function of the rate of protein
synthesis above that required for non-
growth processes and protein losses of the
microbial ecosystem (maintenance). The
absolute growth rate of the rumen
microbial ecosystem is constrained via the
relatively slow flux of monomers derived
from hydrolysis of structural carbohydrates,
i.e. HFkh. Consequently, large proportions
of fermented monomers are utilized for
maintenance of the microbial ecosystem
with relatively small proportional yields of
net microbial synthesis per unit yield of
volatile fatty acids (VFAs). Other factors
being equal, increasing flux of fermentable
monomers via increasing either feed intake
or dietary proportions of more rapidly
hydrolysing carbohydrates will result in
increasing proportions of digested MCP per
unit of VFA produced. These effects can be
demonstrated most clearly via results
obtained from continuous flow culture
systems (Isaacson et al., 1978; Dijkstra et
al., 1998; Baker and Dijkstra, 1999).
The importance of dietary composition
in regulating the dynamics of ruminal
digestion and determining ruminal efflux
yield of amino acids and VFAs is illus-
trated schematically in Fig. 16.1. Dietary
concentration of potentially undigested
neutral detergent fibre (UF) is important in
that it represents mass whose ruminal
efflux is entirely by physical means. A
major determinant of residence time of
feed residues in the ruminal digesta is the
competition of ingested feed residues with
the mass, or load, of feed residues in the
mass action turnover pool. A lag-rumination
pool contributes additional residence time
(Ellis et al., 1994). The effective mean age-
dependent escape rate of UF from the two
pools is^2 UF–e, with –erepresenting the
dilution of UF influx rate by the load of UF336 W.C. Ellis et al.
Table 16.1.Protein and energy required for various metabolic processes in the ruminant.
Metabolic process g of MCP/Mj of ME
- Ruminal anaerobic metabolism yield:
Slow ruminal digesta turnover, 1 day^18 a
Fast ruminal digesta turnover, 1.4 day^115 b - Aerobic tissue requirement, 1/3 mature steer:
Ruminant maintenance, 400 kg 81 c
Specific growth rate, 0.0025 day^1110 c
Specific growth rate, 0.0050 day^180 c - Aerobic tissue requirement, 1/3 mature pig:
Maintenance, 200 kg 79
Specific growth rate, 0.003 day^1117
Specific growth rate, 0.006 day^196
aAssuming a yield of 0.10 microbial CP 1 kJ (^1) of fermented energy and a yield of 0.1 microbial CP that is
0.9 true protein and is 0.8 truly digested and 0.6 of fermented OM is converted to volatile fatty acids at
0.8 kJ kg^1 or 2.59 g metabolizable protein and 18 kJ ME 100 g^1 digestible dietary OM.
bAssuming a yield of 0.24 microbial CP 1 kJ (^1) of fermented energy and a yield of 0.24 microbial CP that is
0.8 true protein and is 0.8 truly digested and 0.4 of fermented OM is converted to volatile fatty acids at
0.8 kJ g^1 or 5.529 g of metabolizable protein and 12 kJ ME 100 g^1 digested dietary OM.
cNational Research Council (1996).