Farm Animal Metabolism and Nutrition

had greater effects in heavier animals

(Anderson et al., 1988; Jones et al., 1990).

These results demonstrate that the greatest

responses would be obtained from the use

of phenethanolamines in heavier weight

animals. This, along with limited duration

of effectiveness, may direct the use of

phenethanolamines to the final finishing

phase of production systems.

Genetics

Phenethanolamines have been proven

effective across a variety of genetic back-

grounds. In particular, ractopamine has

been shown to be equally effective in

Landrace and Hampshire crossbred pigs

(Mills et al., 1990) and in Meishan,

Meishan crossbred and US crossbred pigs

(Yen et al., 1991). Cimaterol also increased

lean and decreased fat similarly in genetic-

ally lean and obese pigs (Yen et al., 1990).

A significant interaction between

ractopamine and genotype was reported

for lean growth rate, but not for growth

performance or carcass merit, by Gu et al.

(1991a,b). In this study, a greater response

to ractopamine was observed in pigs from

leaner genotypes. Bark et al.(1992) also

reported a significant interaction between

genotype and ractopamine since

ractopamine caused a greater increase in

muscle accretion in pigs with a high

genetic capacity for lean growth relative to

a low genetic capacity. Finally, Eisen et

al. (1988) reported a significant line 

cimaterol interaction in mice. In this

study, mice from lines selected for rapid

3–6 week post-weaning gain showed a

greater response to cimaterol treatment

than did mice from unselected control

lines. Thus, it appears that phenethanol-

amines will be effective across divergent

genetic backgrounds, but greater responses

may be obtained from animals selected for

superior genetic potential for lean tissue

accretion.

Phenethanolamines and meat quality

The effects of phenethanolamine reparti-

tioning agents on meat quality can be both

positive and negative. For the purposes of

this discussion, meat quality is defined as

70 D.E. Moody et al.

120

100

80

60

40

20

0

Percentage of maximal response

0 2.5 5 10 20 30

Ractopamine dose (mg kg^1 )

ADG

F/G

Lean

DP

Fig. 4.4.Ractopamine dose–response relationship in swine. Beneficial effects on average daily gain (ADG)

and feed efficiency (F/G) are achieved at lower dosages. However, higher doses are required to optimize the

benefits on carcass leanness (Lean) and dressing percentage (DP). Data are adapted from Watkins et al.

(1990).

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