Farm Animal Metabolism and Nutrition

levels ≥167 g kg^1 were required to achieve

a significant response to ractopamine

(Dunshea et al., 1993). Similarly, Mitchell

et al.(1994) reported greater improvements

in gain and carcass leanness in pigs that

consumed an 180 relative to a 120 g kg^1

crude protein diet in combination with the

phenethanolamine BRL47672, and Jones et

al.(1988) concluded that rations contain-

ing 160–200 g kg^1 crude protein or lysine

equivalent were needed in swine to

optimize the growth performance response

to ractopamine. However, in contrast to the

protein levels needed for maximum gain,

increased carcass leanness in ractopamine-

fed pigs may be achieved at lower crude

protein levels (Ji et al., 1991; Jones et al.,

1992). In broilers, a linear response to

dietary protein levels (220, 240 and 260 g

of protein kg^1 ) was observed in combina-

tion with clenbuterol treatment (Hamano et

al., 1998). These authors concluded that

increased protein consumption is impor-

tant to achieve maximal gains in body

weight gain and protein to DNA ratio in

clenbuterol-fed birds. Together, these

studies indicate that increased dietary

protein levels are essential to achieve the

maximum benefit of treatment with

phenethanolamines in swine and poultry.

Models can be used for estimating the

specific level of dietary protein required

based on anticipated rates of protein accre-

tion (National Research Council, 1998).

Data describing the effect of protein on

the response to phenethanolamines in

ruminants are limited. However, Beermann

et al.(1986) observed improved perform-

ance in cimaterol-treated sheep that were

given supplemental by-pass protein. More

work is needed in this area to confirm the

interactions between phenethanolamines

and nutrition in ruminants.

Duration of treatment

Phenethanolamines are often effective for a

limited time period due in part to the

desensitization or down-regulation of

activated 
-adrenergic receptors (see dis-

cussion in Mode of Action). The length of

treatment before effects on weight gain

begin to plateau varies for different com-

pounds and species (see Table 4.2), but

will probably limit the use of phene-

thanolamines to the final finishing phase in

most cases. In ractopamine-treated swine,

68 D.E. Moody et al.

175

150

125

100

75

50

25

0

Percentage response relative to cattle

Cattle Sheep Pigs Chickens Turkeys

Gain Feed efficiency Leanness

Fig. 4.3.Species differences in gain, feed efficiency and leanness with phenethanolamine administration.

Values are relative differences compared with cattle. (Adapted from Mersmann, 1998; Wellenreiter, 1991.)

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