Farm Animal Metabolism and Nutrition

average daily gain began to plateau after 3

weeks (Williams et al., 1994), but benefits

to carcass composition continued as

ractopamine duration increased (Anderson

et al., 1987a; Williams et al., 1994).

Dosage

There is a differential effect of dosage on

the response of meat animals to

phenethanolamines. As an example, when

ractopamine is fed to swine, low dosages

significantly improve weight gain and feed

efficiency, and, to a lesser degree, carcass

parameters. In contrast, higher dosages

improve weight gain, feed efficiency and

carcass parameters (Fig. 4.4). Weight gain is

optimized at low dosages and is

diminished at dosages >20 mg kg^1 due to

reductions in feed intake. Alternately, lean-

ness and dressing percentage continue to

improve through the highest dosage tested

(30 mg kg^1 ). These differential responses

allow a selection of dose to optimize value

under varying economic conditions (Fig.

4.4).

Age or weight

Typically, phenethanolamines are most

efficacious when administered to older,

heavier animals. Vestergaard et al.(1994)

treated bulls with cimaterol for 90 days

beginning at 162, 299 and 407 kg

liveweight. The weight gain response to

cimaterol increased from 10% for the

162 kg liveweight group to 25 and 22% at

299 and 407 kg, respectively, while the

absolute difference in carcass gains were

18, 28 and 37 kg for the three weight

groups, respectively. In pigs, cimaterol

failed to effect growth or carcass traits

in young growing animals (10–60 kg;

Mersmann et al., 1987), and ractopamine

Phenethanolamine Repartitioning Agents 69

Table 4.1.Factors that influence response to treatment with phenethanolamines (see text for references).

Factor Requirement Compounds studied Species studied

Dietary protein Greater response with higher Clenbuterol Pigs, broilers, rats

dietary protein BRL47672

Ractopamine

Duration of treatment Greater response during final Cimaterol Pigs, cattle, sheep

finishing phase Clenbuterol

L-644,969

Ractopamine

Dosage Differential effect on growth and Ractopamine Pigs

leanness

Age or weight Greater response with older, heavier Cimaterol Pigs, cattle

animals Ractopamine

Genetics Effective in both fat and lean Cimaterol Pigs, mice

genetics Ractopamine

Table 4.2.Phenethanolamines are effective for a relatively short period of time before response, measured

as weight gain, reaches a plateau.

Drug Species Time (weeks) until response plateaus Reference

Cimaterol Sheep 3 O’Connor et al.(1991a)

L-644,969 Sheep 2 Pringle et al.(1993)

Clenbuterol Cattle 5 Sillence et al.(1993)

Cimaterol Cattle 10–16 Fiems et al.(1990)

Cimaterol Cattle 8 Barash et al.(1994)

Ractopamine Pigs 3 Williams et al.(1994)

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