depends on the energy density of gain allowable by the dose of a particular metabolic modifier. For example, at a dose that maximizes protein accretion, pST reduced caloric gain in growing swine from 3.30 to 1.75 Mcal/day during the 50 to 100 kg phase (see Figure 2-2). Equally striking are the dynamics of pST dose in relation to both intake and protein accretion (Figure 2-2). Thus, voluntary intake must be documented in relation to dose of the particular metabolic modifier when administered under ad libitum feeding conditions; subtle differences exist for pigs on energy-restricted regimens.

Also, the energy intake versus pST dose relationship may vary with phase of growth and, to a lesser extent, by genotype and gender. For example, pST reduces feed intake of ad libitum fed pigs less (-8 percent) when administered during the early growth phase (20 to 50 kg body weight) than when administered during the finishing phase (-23 percent; Table 5-1). The biological basis for this is not clear but appears to be a reflection of the relative changes in protein and fat accretion induced by pST. The impact on intake may also differ with metabolic modifier. Table 5-3 shows that the β-agonist ractopamine causes intake to decrease only slightly in comparison to pST.

The tissue requirement for amino acid deposition is a function of both the need for deposition in tissues and for maintenance. The dietary requirement, however, is a function of the extent to which dietary protein is digested and amino acids absorbed as well as the efficiency with which absorbed amino acids are used for protein deposition. In principle, the effects of ST on energy and protein utilization in farm animals appear to be principally associated with the use of absorbed nutrients (Boyd and Bauman, 1989). It is conceivable, however, that digestion is altered indirectly. For example, a reduction in feed intake could result in a slower rate of passage, which in turn may lead to increased digestibility. This probably accounts for the small improvements in nitrogen and energy digestion observed with pST treatment (Wray-Cahen at al., 1991). The relative advantage observed for digestibility of nitrogen in pigs administered pST compared to control counterparts (87 versus 84 percent for controls), with each group fed ad libitum, was predictable from the reduction in intake (Haydon et al., 1983; see also Verstegen et al., 1990). Thus, little or no difference is expected for pigs already fed via a restricted intake regimen.

pST has been shown to alter the maintenance requirement for both energy and amino acids. The higher maintenance energy requirement for pST-treated pigs (Campbell et al., 1988; Verstegen et al., 1989) appears to be an inevitable consequence of increased protein mass (Dickerson, 1985; Campbell and Taverner, 1988). Therefore, when the maintenance cost is constant per unit of lean tissue, the pST-treated pig will have a greater maintenance requirement because it has a greater proportion of lean tissue at any given body weight. However, an increase of 10 percent of the

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)