Farm Animal Metabolism and Nutrition

fact, they eat discrete meals with relatively
long inter-meal intervals unless the rate of
eating is so slow, due to the scarcity of
pasture, that they must eat all day to obtain
a reasonable level of intake (Chapter 17).
This implies that there are other chemical
and metabolic factors involved in intake
control in ruminants, as in simple-
stomached animals.

CHEMICAL FACTORS. The stretch receptors in
the rumen wall are also sensitive to
chemicals, including the acids produced
by rumen fermentation. Figure 15.4 shows
the effects of infusion into the rumen of
sodium acetate or propionate, and of
distension, on the intake of hay or silage by
lactating cows prepared with rumen
fistulas (Anil et al., 1993). It is now clear
that some of the effect of introduction of
salts into the rumen is due to their osmotic
properties, and it has been demonstrated
that sodium chloride and other substances
which increase the osmotic pressure of
rumen fluid without providing nutrients
can markedly depress intake when given
into the rumen (Carter and Grovum, 1990).
The relative importance of changes in
osmotic pressure of rumen fluid in the
control of feed intake by ruminants is not
yet clear, however, and mole for mole

sodium acetate depresses intake more than

sodium chloride when infused into the

rimen.

Liver

The liver is the first organ which has a

more or less complete picture of the

nutrients yielded by a meal as the venous

drainage of the digestive tract is funnelled

into it. The liver is the first site after

absorption which is able to monitor the

results of eating a meal, although lipids are

absorbed into the lymphatic system and

might not, therefore, be able to exert their

full effect on the liver before being utilized

by some other tissues.

It was shown in dogs in 1963 that infu-

sion of glucose into the hepatic portal vein

depressed intake to a much greater extent

than infusion of the same amount into the

general circulation via the jugular vein.

Subsequently, a great deal of evidence has

accumulated that there are receptors in the

liver sensitive to glucose and to other

metabolites that are oxidized in the liver

(Forbes, 1988); the mechanism is probably

an activation of the sodium pump which

changes the transmembrane potential of

the hepatocytes resulting in a change in the

324 J.M. Forbes

Fig. 15.4.Intake of grass silage by lactating dairy cows with a balloon inflated to different volumes (–––,
level of treatment = litres) for 3 h or infused with sodium acetate (....) or sodium propionate (----) at different
rates (level of treatment = mol 3 h
^1 ) (Anil et al., 1993).

0 5 10 15 20

2.5

2.0

1.5

1.0

0.5

0

DM intake (kg 3 h

^1

)

Level of treatment (l)