Farm Animal Metabolism and Nutrition

set of circumstances, this does not seem to
be a tenable point of view. Consider, for
example, sheep offered a poor-quality
forage, whose intake is thought to be
limited by rumen capacity, in which
progressively increasing the digestibility of
the feed leads to progressive increases in
feed intake (see Forbes, 1995). Eventually,
metabolic control of intake limits the
amount eaten to less than that which the
animal is physically capable of putting in
its rumen, so we say that intake is no
longer physically limited. It does not seem
feasible that the stretch receptors in the
rumen are no longer subject to any stimula-
tion, nor does it seem likely that the CNS
suddenly ignores the signals from the
stretch receptors. Rather, it seems most
likely that the stretch signals are integrated
with the metabolic signals, and there is
evidence that this integration is by simple
addition. Figure 15.6 shows that the effects
of distending a balloon in the rumen are
additive with those of infusing sodium

acetate and sodium propionate (Mbanya et

al., 1993) and there is also evidence for

such additivity in sheep and of glucose and

lysine given into the liver of chickens.

The implications are that voluntary

intake is not controlled only by physical

factors, even in ruminants, and that it is

the sum total of the strengths of signals

received by the brain from many types of

receptors in many parts of the body which

determines how much an animal eats.

Thus we have an explanation for the fact

that lactating cows have a greater amount

of digesta in the rumen than non-lactating

cows even when they are both fed on a

forage whose intake traditionally would be

supposed to be limited physically (Forbes,

1986). The fact that the lactating cow has a

much greater demand for nutrients to

support lactation causes a more rapid

removal of chemical and metabolic feed-

back factors from blood, liver and digestive

tract, thus providing a weaker set of signals

from metabolic receptors which gives the

328 J.M. Forbes

Fig. 15.6.Intake of silage DM by lactating cows with or without infusion of sodium acetate (9.0 mol 3 h
^1 ,
A) or sodium propionate (4.9 mol 3 h
^1 , P) or distension of a balloon (10 l, D) in all combinations (Mbanya
et al., 1993).

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

C A P D AP PD AD APD

Treatment

Silage intake (kg 3 h

^1

) *

**

*