the ruminant ecosystem. Hungate (1966)
has emphasized the potential thermo-
dynamic limitations of the ruminant’s
dependence upon a sequential food chain
of anaerobic and aerobic metabolisms. As
indicated in Table 16.1, the anaerobic
rumen fermentation yields a gross dis-
proportion of energy to MP relative to the
energy and protein requirements of the
ruminant’s aerobic metabolism.
Ruminal efflux of amino acids synthe-
sized in the rumen from purified, low-
protein rations containing cellulose, starch
and urea appears to supply adequate MP
for maintenance and limited milk produc-
tion. However, the flux of amino acids
derived from microbial protein is
insufficient for more productive ruminants.
Rationing systems are required to formulate
rations based on ruminal escape protein
and yield of ruminal efflux protein.
Ruminal efflux of microbial protein and
escaping dietary protein are intimately
related to ruminal kinetics.
Additionally, Hungate (1966) suggested
that the propensity of ruminants to deposit
fat may be the consequence of excessive
proportions of energy relative to amino
acids in the ruminal efflux. Possible effects
of nutrient supply on body composition are
clearly evident but prove difficult to modify
favourably (Poppi, 1990). The consequences
of such obligatory excesses of energy upon
body compositon seem not to have been
considered. If such excesses of energy are
causal of excessive fat deposition or,
conversely, limited protein deposition,
systems for estimating nutrient require-
ments based on body composition will
overestimate energy and underestimate
protein requirements.
The first thesis for this chapter is that
ruminants consume feed to provide for their
tissue requirements for amino acids unless
intake is otherwise constrained. A second
thesis is that level of intake per sealters the
dynamics of ruminal turnover and thereby
the metabolic dynamics and the ruminal
efflux yield of protein and energy. The
cyclic sequence of diet–digestive–metabolic
interactions illustrated in Fig. 16.1 is given
as a framework for examining the literature.
Metabolizable nutrient supply versus demandSupport for the thesis that ruminants
regulate intake to provide the net amino
acids required by their tissue metabolism
was investigated in a collation of data from
the literature. This collation involved 29
reports involving non-lactating cattle and
sheep and 23 reports involving lactating
dairy cows. A total of 240 treatment means
were represented. Reports were selected
that provided data concerning dietary
intake and faecal output of organic matter
(OM) and CP and ruminal efflux of OM,
microbial protein and undegraded intake
protein. To standardize data across reports,
daily intake of truly digested OM (TDOMI)
was computed assuming that faecal nitrogen
consisted of unhydrolysed feed protein and
ruminal efflux microbial protein and that
microbial protein comprised 0.12 of the
faecal OM. Daily MP effluxing the rumen
was calculated as the sum of microbial
protein (0.85 true protein nitrogen of
microbial nitrogen 6.25) and undegraded
intake protein (total non-ammonia nitrogen
minus microbial nitrogen each multiplied
by 6.25) which was assumed 0.9 truly
digestible. Fermented carbohydrates were
computed as the difference between intake
and ruminal efflux of neutral detergent
fibre (NDF) and non-structural carbo-
hydrates. The OM of non-structural carbo-
hydrates, NSC, was computed as OM
minus the sum of NDF, CP and lipids.
When not reported, dietary lipids were
assumed to represent 20 g kg^1 of dietary
OM and to be 0.5 hydrolysable.Feed intake versus balance of
metabolizable nutrientsFeed intake is commonly expressed as
TDOMI. However, TDOMI represents a
mixture of potential nutrients of which
only the first limiting nutrient will limit
intake. Simple calculations given in Table
16.2 illustrate variations in intake of
TDOM that would be expected to occur if
MP first limited feed intake. It is clear that
wide variations in TDOMI may occur in338 W.C. Ellis et al.