tables can only be used as a guide. Only computer-simulated models can integrate requisite animal, environmental, and dietary factors to arrive at biological and economic optimums for dietary nutrients. A number of such models have been developed for growing pigs (Black et al., 1986; Whittemore, 1986; Moughan, 1991).

It is also important to appreciate that nutrient recommendations need to be amenable to "whole-model" production economics. Ultimately, the most appropriate requirement in practice is that which accommodates an economic optimum. It is arrived at by relating predicted responses from nutrient input and associated costs to predicted return. This approach is based on factorial methodology and best accommodates the needs of animal producers.

β-adrenergic agonists (β-agonists) cause reciprocal shifts in protein and lipid accretion, but more quantitative information is needed to determine whether nutritional requirements are changed. Several important differences exist that are examples of factors that must be considered in evaluating the impact of new technologies on nutrient requirements. First, while it is convenient to discuss β-agonists in general terms, one must appreciate that each differs structurally. As a result of differences in mode of action, effects on promoting lean deposition and ancillary actions can be expected (see Chapter 2). Second, at least one compound has been shown to elicit a transient response in gain and efficiency of gain. This has not been adequately documented in studies with other β-agonists, yet it is a fundamental consideration. Third, β-agonists appear to cause a differential growth in that increases in protein accretion are only observed in specific carcass components. This is in contrast to ST, which causes a generalized growth of proteinaceous tissues. This would have a bearing on the pattern of amino acids needed for tissue growth.

Unlike ST, β-agonists are orally active and effective at relatively low concentrations (0.5 to 20 mg/kg diet). Their general effect is to increase the rate of skeletal muscle growth, concurrent with a reduction in lipid accretion. Rate and efficiency of growth are generally improved, but differences have been variable and possibly related to differences in effectiveness between compounds, phase of growth, and nutrient provision. Results from 12 studies involving ractopamine hydrochloride fed to castrate male and female pigs have been summarized by Veenhuizen and Anderson (1990) and appear in Table 5-3.

Proper characterization of the response to metabolic modifiers also involves definition of the temporal pattern of growth (i.e., pattern of nutrient deposition). The importance of this is illustrated in Figure 5-9 with the β-agonist L-644,969. The data show a declining response in growing swine to this particular β-agonist throughout the treatment period (Wallace et al., 1987). A similar response pattern has been observed for growing cattle (Moloney et al., 1990). The difficulty that this poses when either attempting to define required nutrient input or when measuring metabolic or hormone patterns is that the tissue requirement is also in a dynamic flux.

The relative importance of dietary lysine is expected to increase in relation to other indispensable amino acids because intestinal growth is not adversely affected by treatment or by the rerouting of nutrients from adipose to muscle tissue coincident with enhanced muscle growth with β-agonist administration (Reeds and Mersmann, 1991). This appears to be a common feature for this class of metabolic modifiers and results in a greater proportion of lysine deposition relative to other amino acids. Thus, lysine would appear to be first limiting and, as such, should be the focus of initial efforts in the evaluation of amino acid requirements. The differential pattern of tissue(s) growth may also confer an advantage in amino acid use relative to ST in that the greater relative allocation of amino acids to muscle confers an efficiency that may affect the amino acid requirement. However, such differential effects on visceral and carcass components may imply a specificity for β-agonist effects during the finishing phase of growth when the relative rate of visceral tissue accretion is diminished. This may partially account for the positive response of growth in pigs fed marginal protein diets (12 percent protein; Mitchell et al., 1991).

Although the efficacy and relative responsiveness to the β-agonist ractopamine has been established for growing swine (Table 5-3), quantitative information is generally lacking

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)