Farm Animal Metabolism and Nutrition

livestock production has been evaluated

(see reviews by Anderson et al., 1991;

Moloney et al., 1991; Mersmann, 1998).

The compounds most commonly studied

include cimaterol, clenbuterol, L-644,969,

ractopamine, salbutamol and zilpaterol

(Fig. 4.1).

Summary of effects

Phenethanolamines are often referred to as

repartitioning agents because of their

ability to redirect nutrients away from

adipose tissue and toward muscle (Ricks et

al., 1984). In general, the effects of

phenethanolamines are: (i) increased rate of

weight gain; (ii) improved feed utilization

efficiency; (iii) increased leanness; and (iv)

increased dressing percentage. Using

ractopamine in swine as an example, a

summary of the effects of phenethano-

lamines is presented in Fig. 4.2.

Phenethanolamine efficacy has been

demonstrated for lambs, broilers, turkeys,

beef cattle and swine, although not all

phenethanolamines are equally effective in

all species (see Anderson et al., 1991).

Although comparisons across studies are

difficult to make due to differences in

compounds studied, dosage, duration and

response parameters measured, a sum-

marization of the available data on all

phenethenolamines shows that cattle and

sheep give substantially larger responses to

phenethanolamines than swine, with the

smallest response in chickens. Turkeys

appear to be intermediate, although this

represents data from only ractopamine

(Wellenreiter, 1991; Mersmann, 1998; Fig.

4.3). The lower response in chickens may

be due to intensive selection for growth rate

in chickens, thus less potential for improve-

ment; or perhaps due to species differences

66 D.E. Moody et al.

Fig. 4.1.Structure of several phenethanolamines evaluated as leanness enhancers in livestock. Current

classifications of receptor subtype selectivity (
1- or 
2-adrenergic receptor) are indicated.

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