essentially equal for all four widely
different diets summarized in Tables 16.3
and 16.4 which were in the order of 10 g
feed intake g^1 of ruminal efflux of CP.
Unfortunately, ruminal microbial protein
was not measured for the less digestible
diet (Table 16.3). For the more digestible
diets (Table 16.4), ruminal influx of OM
per efflux of microbial CP was relatively
constant at 17.27 and 19.9 and supports the
generalization that the rate of rumen
microbial synthesis first limited the ruminal
efflux rate of OM. If the levels of ruminal
load and efflux rate of UF are taken as
some indication of an upper limit to the
physical capacity for ruminal flux, physical
factors did not appear to constrain intake
of the diets in Table 16.4.
For cattle fed the more digestible diets
(Table 16.4), the mean effective escape
rates of UF from the sequence of the lag-
rumination and mass action turnover pools,
(^2) UF–
e, were inversely related to mean
ruminal digesta loads of UF of from 2.5 to
9.2 g UF kg^1 BW. In contrast,^2 UF–ewas
directly proportional to ruminal digesta
loads of UF in sheep having ruminal
digesta loads of 14.5 and 22.3 g UF kg^1
BW (Table 16.3). When consuming the
more digestible diets, ruminal digesta load
of UF increased from 2.5 to 9.2 g kg^1 BW
and^2 UF–e decreased by 0.48-fold (from
1.34 to 0.65 day^1 ). In contrast, in sheep
fed the less digestible diets,^2 UF–eincreased
0.77-fold (0.54–0.34 day^1 ) when ruminal
digesta load was increased from 6.2 to
22.3 g UF kg^1 BW. These responses suggest
that^2 UF–e is regulated primarily by a
nutrient requirement-driven intake when
diets are low in UF and contribute a rela-
tively small ruminal load of UF (<9 g UF
kg^1 BW as for bermudagrass in Table 16.4).
As ruminal load of UF increases with con-
sumption of diets higher in UF, the inertia of
larger ruminal loads become more dominant
and less opportunity exists for metabolic
regulation via an MP requirement-driven
intake.
Efficiency of microbial growth would
respond to flux rate of hydrolysable entities
if the escape rates of the microorganisms
were associated with^2 UF–e. However, the
Feed Intake in Ruminants 347
Table 16.4.Ruminal dynamics of unhydrolysable fibre (UF) and crude protein (CP) in cattle grazing
semi-tropical (bermuda) or temperate (ryegrass) forage pasture (adapted from Hill, 1991).
Diet Ryegrass
Item Bermuda Ryegrass Bermuda
Dietary composition, g kg^1 OM
CP 76 250 33
UF 340 117 3
Daily ruminal influx, g kg^1 BW
OM 14.1 23.7 1.7
UF 4.8 2.8 0.6
Ruminal digesta load, g kg^1 BW
UF 9.2 2.5 0.3
Daily mean ruminal escape rates, –e
UF-rare earth, 0.65 1.34 2.1
CoEDTA-Co 1.3 4.8 3.7
Rumen microorganisms-organic bound^35 S 0.41 0.53 1.3
Ruminal efflux composition, g g^1 OM
Total CP 0.169 0.205 1.2
Non-ammonia CP 0.156 0.187 1.2
Microbial CP 0.084 0.111 1.3
Ruminal influx/ruminal efflux ratios, g g^1
Influx of CP/efflux of CP 0.73 1.90 2.6
Influx of OM/efflux of non-ammonia CP 9.24 11.83 1.3
Influx of OM/efflux of microbial CP 17.27 19.90 1.2