per gram of absorbed nitrogen) of consumed proteins. This appears to be the result of direct effects on the liver to decrease the extraction ratio and net removal of amino acids from the circulation (an amino acid sparing effect). It is not currently possible to define requirements or formulate diets that deliver the appropriate balance and quantity of amino acids to match the increase in protein accretion rate in ruminants, as can be done for nonruminants. Quantity and balance of amino acids exiting the rumen are not easily predicted or determined because of the alterations that occur during rumen fermentation. The magnitude of protein accretion rate and decrease in lipid accretion rate are also influenced by dietary energy levels, gender, age, and breed of cattle and sheep. To allow for nutrient adequacy assessment, future studies must include careful consideration and detailed description of diets and animals used. Ultimately, studies designed to assess protein and energy requirements using empty-body protein (amino acid), lipid, and mineral accretion rates as response variables must be conducted to accurately and adequately define nutrient requirements in growing ruminants. Because the mechanism(s) by which ST alters growth performance and composition of gain appear to be similar in growing ruminants and pigs (Chapter 2), dietary manipulations for use of ST in ruminants should be based on the responses to alterations that have been demonstrated for growing pigs treated with pST (Chapter 5).

There are relatively few data on which to base sound conclusions or predictions of the nutrient requirements of growing ruminants receiving β-adrenergic agonists. Systematic investigations into the protein and energy requirements of growing or finishing cattle or sheep have not been conducted. These important data are also limited for swine (Chapter 5) and poultry (Chapter 6). Defining these requirements requires (1) applying current knowledge regarding mechanism of action, growth performance, and composition of gain results determined over a range of protein intake when energy is not limiting and (2) determining whether altering the balance of available amino acids influences the efficacy of the β-adrenergic agonists for their effects on protein deposition rate. A detailed discussion of the biological basis of amino acid and energy requirements in growing animals has recently been published (Reeds and Mersmann, 1991) in which the authors also address the important issues regarding assessment of nutrient requirements in animals fed β-adrenergic agonists. Use of a wide range of protein intakes and the break-point analysis, which has been employed to define the nutrient requirements of swine receiving pST (Chapter 5), has likewise been applied to growing swine fed β-agonists (Dunshea, 1991). Although appropriate, studies designed to investigate the effects of postruminal protein infusion on protein deposition response in ruminants fed β-agonists have not been reported.

Extrapolation from results of ST studies in growing ruminants to the β-agonist-treated ruminant is logical and may be appropriate, with one exception: the significant increase in weight of the liver and kidneys caused by ST treatment is not observed with β-agonist administration, although increased energy expenditure has been observed [see reviews by Reeds and Mersmann (1991) and Beermann (1993)]. As is the case for administration of ST, composition of gain and tissue requirements of gain must be known across the range of genotypes, stages of growth, gender, and dose when feedlot diets are used for cattle and lambs. These are not currently available. Also, the quantity and balance of amino acids provided by rumen fermentation must be understood and accurately predicted across a wide range of diet formulations currently used in commercial beef and lamb production before diet alterations can be recommended. Although models are being developed, these models must first be validated for the range of commercial situations to which they will be applied.

Another important consideration for defining nutrient requirements in ruminants fed β-adrenergic agonists that alter composition is that they do not chronically alter feed intake, as is the case with ST administration to lactating dairy cattle (increase) or finishing steers (decrease). In most studies, feed intake is not altered, although rate and efficiency of gain are improved, albeit in a transient manner in some instances (Beermann et al., 1986a; see reviews by Boyd et al., 1991; Beermann, 1993).

The efficiency with which energy and protein are used for protein accretion declines with increasing age or weight of the growing ruminant (Black and Griffiths, 1975) and is influenced by sex, genotype, and environmental differences. Dietary modifications used to study these effects in growing lambs administered ST or GRF have also been investigated using β-adrenergic agonists. Addition of fishmeal to the diet of lambs treated with oST improved feed conversion efficiency in an additive manner with ST when compared with the conventional diet containing soy protein as the predominant protein source (Beermann et al., 1990). Similar effects on feed efficiency were not observed in growing-finishing lambs fed cimaterol (Beermann et al., 1986a). However, fishmeal increased the weight of individual skeletal muscles in the hind leg by 15 to 19 percent over a 10-week growing period and the effects were additive with the 20 percent increase caused by cimaterol. Hind leg muscles from lambs that received both fishmeal and cimaterol were 40 to 45 percent heavier than the same muscles in lambs that received neither. These data suggest that providing adequate quantity and/or quality of absorbed nitrogen may also be important prerequisites to achieving maximum response with β-agonists in ruminants.

Front Matter (R1-R12)

Executive Summary (1-2)

1. Introduction (3-4)

2. Mechanisms of Action of Metabolic Modifiers (5-22)

3. Effect of Somatotropin on Nutrient Requirements of Dairy Cattle (23-29)

4. Effect of Metabolic Modifiers on Nutrient Requirements of Growing Ruminants (30-37)

5. Effect of Metabolic Modifiers on Nutrient Requirements of Growing Swine (38-51)

6. Effect of Metabolic Modifiers on Nutrient Requirements of Poultry (52-58)

References (59-73)

Authors (74-74)

Index (75-82)