Farm Animal Metabolism and Nutrition

in 
-adrenergic receptor selectivity and

signalling pathways (Mersmann, 1998). In

addition, stage of growth of the chicken at

dosing relative to mammals may contribute

to these species’ differences in response.

There is also a substantial difference in

response between phenethanolamines. The

basis for the differences may lie in the

specificity of a particular compound for

adrenergic receptors (
1 versus 
2 selec-

tivity). For example, the 
2-selective

phenethanolamines clenbuterol, cimaterol

and L-644,969 are particularly effective in

sheep and cattle, but less effective in

swine. The 
1-selective phenethanolamine

ractopamine is less effective in ruminants,

particularly sheep (unpublished data not

shown), but administration to swine has

given consistent increases in both growth

performance and carcass leanness as shown

in Fig. 4.2.

Factors that influence response

to phenethanolamines

Several factors including diet, dosage and

duration of treatment, age, weight and

genetics have been shown to influence the

response to phenethanolamines (Table 4.1).

It will be important to understand inter-

actions of these factors with phenethano-

lamines if the use of phenethanolamines in

the livestock industry is to be implemented

successfully.

Diet

The interaction between dietary protein

content and repartitioning agent treatment

was first reported by Anderson et al.

(1987b). In general, livestock fed

phethanolamines require increased dietary

protein to accommodate increased rates of

lean deposition. In swine, dietary protein

Phenethanolamine Repartitioning Agents 67

Fig. 4.2.Summary of the performance and carcass effects of ractopamine in swine. On average, pigs fed

ractopamine from a body weight of 60 kg will reach 100 kg 4 days earlier and require 18.5 kg less feed

compared with pigs without ractopamine treatment. The carcass of the 100 kg, ractopamine-fed pig will

also have an improved dressing percentage (+0.9 kg hot carcass weight) and more dissectible carcass

lean (+4.5 kg; adapted from Watkins et al., 1990).

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