Farm Animal Metabolism and Nutrition

amino acids required by mammalian tis-
sues is generally first limiting at the rumi-
nant’s tissue level. Thus the flux of ME:MP
surplus to the ME required for indispens-
able metabolic requirements (such as vital
activities and tissue protein synthesis) may
be considered incidental in regulating
intake. This priority for dietary CP is
manifested in feed selection trials where
mammals in general and ruminants in
particular exhibit preferences for combina-
tions of protein that tend to maximize
productivity (Kyriazakis and Oldham,
1997).
Voluntary intake commonly is related
to the dietary level of CP in a curvilinear
manner, as shown in Fig. 16.11. Figure
16.11 summarizes the relationship between
voluntary intake by lambs of various forage
Sorghum sp. varieties and cultivars of
different maturities with and without
supplemental levels of a purified soybean
protein (Buentello and Ellis, 1969). When

fitted to a Michaelis–Menten type response

function, voluntary intake appears related

to dietary concentration of CP in a manner

analogous to Michaelis–Menten kinetics,

i.e. rapid increases in response rate that

diminish as supply of substrate approaches

and saturates the responding mechanism(s)

and achieves its maximal rate. Based on

the Michaelis–Menten function estimated

from levels of CP ranging from 40–190 g

kg^1 DM, maximum intake was not

achieved even when projected levels of CP

exceeded 1000 g kg^1 DM.

Clearly, voluntary intake of forage

responds to dietary levels of CP that

considerable exceed those commonly

investigated, just as MCPE responds to

levels of CP and RHP. The projected

response in voluntary intake beyond the

range of dietary CP in Fig. 16.11 is consis-

tent with implications that the amino acid

requirement of the ruminant’s tissues may

never be met via normal feeding practices

Feed Intake in Ruminants 359

Fig. 16.11.Relationships between voluntary intake by lambs of forage Sorghum sp. varieties (, Buentello
and Ellis, 1986) with and without supplemental purified protein, expected voluntary intake based on
Michaelis–Menten kinetics (–––) and the voluntary intake of grazed forages by early weaned lambs (,
Cruickshank et al., 1992).