hydrolysis of cotton. The data of Moir and
Harris (1962) together with those sum-
marized in Fig. 16.5 indicate that levels of
CP in the order of 80 g kg^1 DM are
required to maximize apparent dry matter
digestion in the rumen (Buentello and
Ellis, 1969). Protein supplementation of
forages containing <80 g CP kg^1 results in
increased rate of digestion of structural
carbohydrates and increased voluntary
intake (Buentello and Ellis, 1969).
The effects of the level of dietary CP
on ruminal dynamics are illustrated in
Table 16.3. Increasing the level of dietary
CP from 52–123 g kg^1 was associated with
essentially equal increases in voluntary
intake of UF (1.4-fold), ruminal digesta
load of UF (1.6-fold) and the CP content of
ruminal efflux DM (1.8-fold). Voluntary
intake of DM per unit of ruminal efflux of
CP was essentially identical for the two
diets (9.77 versus 9.78 g DM intake g^1 of
ruminal efflux CP), strongly implying that
intake was being regulated by the quantity
of ruminal efflux CP. Voluntary intake of
DM and CP and ruminal efflux rate of CP
was significantly negatively related to
plasma levels of 3-methylhistidine and to
increases in plasma methionine and lysine,
but no other amino acid (Wylie, 1987).
The voluntary intake responses to
dietary CP in Table 16.3 were interpreted
to be the result of a nutrient requirement-
driven intake to defray nutritional
deficiencies at the ruminant’s tissue level
of the first limiting nutrient, as evidenced
by reductions in plasma 3-methylhistidine.
Feed intake was not constrained by
physical capacity to harbour or process UF.
Results of protein supplementation of
growing calves grazing bermuda or annual
ryegrass pastures are summarized in Table
16.4. Compared with results with less
digestible roughage in Table 16.3, cattle
consuming these more digestible diets
exhibited much smaller ruminal loads of
UF and more rapid ruminal escape rates for
UF. In spite of large differences in ruminal
load and escape rates for UF, the voluntary
influx of OM relative to ruminal efflux of
non-ammonia CP did not differ significantly
for bermudagrass versus ryegrass (11.83
versus 9.24 g influx OM g^1 efflux non-
ammonia CP). If allowances are made for
differences in ash, the relationships between
dietary influx and ruminal efflux are346 W.C. Ellis et al.
Fig. 16.5.Effects of level of dietary CP upon apparent digestibility of dry matter (DDM) estimated in vitroas
forage with supplemental casein hydrolysate as compared with DDM observed when the same forage was
fed to lambs without CP supplementation.