Farm Animal Metabolism and Nutrition

lactating and non-lactating ruminants were
not evident. The relationships in Figs 16.7
and 16.8 were interpreted to indicate that
ruminants regulate ruminal flux of UF
primarily via^2 UF–
ewhen consuming diets
of 0.2 or less of UF and via increasing UF
load when consuming diets of greater UF
content.
The relationships illustrated in Fig.
16.8 are consistent with UF intake and
ruminal dynamics being driven by an
unfulfilled requirement for MP (Fig. 16.1).
With concomitant increases in flux of HF
and HCHO, the efficiency of microbial
growth rate might be expected to be
increased due to the combination of
increased flux of hydrolysable carbo-
hydrates and to increased turnover or the
microbial ecosystem associated with
increased^2 UF–
e.

Physical limitations to ruminal efflux of UF

As indicated in Equation 16.4, the physical
capacity of the rumen to process UF is
determined by both capacity to harbour

(UNDFL) and to process feed residues for

efflux (UF efflux). The relationships between

ruminal load and efflux of UF above a

UNDFL of 6 g kg^1 BW in Fig. 16.7C

appear linear throughout the range of the

data. Some evidence for a non-linear

approach to a physical limit to processing

UF would be expected if such a physical

limit for ruminal efflux of UF was

approached by the range of the current

data. Thus, together with other evidence to

be reviewed, it is concluded that processes

other than physical limitations appear to

constrain ruminal efflux of UF and,

thereby, feed intake.

Microbial metabolic regulation

of residence time

Mechanisms have been suggested whereby

ruminal residence time of feed residues

might be related to the degree of microbial

colonization of feed residues and/or by rate

of hydrolysis and metabolism of derived

monomers. One example of this is the rela-

tively complete hydrolysis of HF observed

Feed Intake in Ruminants 351

Fig. 16.8.Relationships between daily flux of metabolizable protein (MP) and mean load of neutral
detergent fibre undigested in the rumen, UF (A), mean effective escape rate,^2 UF–
e(B) and efflux rate (C).
Load, escape and efflux rates are for UF loads ≤6g kg^1 BW for lactating (□) and non-lactating ruminants
(
) or ≥6g kg^1 BW for lactating () and non-lactating ruminants (). Note the inverse relationships
between ruminal efflux rate of metabolizable protein (MP) and rumen UF load (A), mean effective escape
rate (B) and ruminal efflux rate (C) depending upon rumen loads of UF. These differential responses are
interpreted to reflect interactions between achieved ruminal load of UF and the drive to acquire the MP
requirements of the ruminant’s tissues.