Farm Animal Metabolism and Nutrition

select for lysine, methionine, thiamine
(vitamin B1), vitamin B6, ascorbic acid
(vitamin C), calcium and zinc (see Forbes,
1995). Pigs show appetites for several
specific nutrients, while ruminants have a
well-developed appetite for sodium. Figure
15.8 shows the ascorbic acid intake chosen
by young broiler chicks by selecting

between an unsupplemented food and one

supplemented with 200 mg kg
^1 ascorbic

acid (Kutlu and Forbes, 1993). When kept in

heat-stressing conditions, which are known

to increase the metabolic requirement for

ascorbic acid, the chicks voluntarily take in

almost twice as much of the vitamin as

when in a thermoneutral environment.

330 J.M. Forbes

25 35 45 55 65

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Proportion of higher protein food eaten

Time after hatching (days)

Fig. 15.7.Proportion of high- and low-protein foods taken by chickens from 27 to 63 days after hatching
(Shariatmadari and Forbes, 1993). ––––: choice between foods containing 225 and 65 g kg
^1 ;– – –: 280 and
65 g kg
^1 ; .....: 280 and 225 g kg
^1 ; –..–: 320 and 280 g kg
^1. For discussion, see text.

Fig. 15.8.Intake of ascorbic acid (vitamin C) by young chicks given a choice between two foods, coloured
differently, containing 0 or 250 mg g
^1 ascorbic acid. Cool: thermoneutral zone; Hot: 35°C for 10 h day
^1.
*, significantly different from random choice between foods (Kutlu and Forbes, 1993).