Farm Animal Metabolism and Nutrition

They can distinguish between the two
foods only by their colour and learn to
associate this with their metabolic feelings
after consuming the foods.

Optimizing Voluntary Intake

In practice, marginal deficiencies of a
nutrient result in an increase in intake but
severe shortage of an essential nutrient
causes intake to decline (Fig. 15.9) (Forbes,
1995), causing a reduction in rate of
growth, milk production or egg production.
Alleviation of the deficiency is followed
very quickly by a return to normal levels of
intake and production. Also, most
nutrients are toxic when present at very
high levels in the diet so that intake and
performance are also reduced when a gross
excess of a nutrient is present.
These general effects of diet composi-
tion also affect the intake of ruminant
animals. However, with the forage feeds
typically available to ruminants, there is
another factor which often imposes a limit

to intake before an animal can satisfy its

appetite for energy or essential nutrients,

and this is the bulk of the food. Although

the ruminant has a very capacious set of

stomachs (up to 150 litres in the cow), the

slow rate of digestion entails each particle of

food remaining in the rumen for a very long

time, typically 20–50 h; the more fibrous the

forage, the slower its digestion. Rumen fill,

acting through stretch receptors in the

rumen wall, is therefore likely to be a

dominant factor in the intake of forages. Any

factor which decreases this residence time

will increase voluntary intake; such factors

include reducing the particle size of the food

itself, increasing the activity of the rumen

microflora by providing additional amounts

of nutrients which limit their metabolism,

and replacing some of the forage by rapidly

digested concentrate supplements.

Conclusions

Voluntary food intake is influenced by

many factors including negative feedbacks

Physiological and Metabolic Aspects 331

Fig. 15.9.General relationship between concentration of an essential nutrient in food and rate of voluntary
food intake (Forbes, 1995).