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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. The improvement in carcass quality of growing animals and the increases in efficiency of growth and lactation that occur with the use of metabolic modifiers are unprecedented. For a successful animal production enterprise, however, sound management practices must be followed. This requirement does not change when metabolic modifiers are used. Indeed, there are several studies in which no response was observed with ST treatment of growing pigs or lactating cows because of limitations in the management program. One aspect of the management program that is critically important for successful implementation of these new technologies is nutrition. The primary objective of this report is to discuss the impact of metabolic modifiers on the nutrient requirements of food-producing animals. These effects differ somewhat between growing and lactating animals. In the former, the use of metabolic modifiers results in a major shift in the type of growth (fat versus lean tissue), thereby requiring matching changes in dietary formulations. In contrast, use of metabolic modifiers for lactating dairy cows does not alter the specific nutrient requirements per unit of milk because the composition of milk is not affected. To fully 
FIGURE 1-1 Accretion rates for protein and fat in pigs over the body weight range from 45 kg to 100 kg (market weight). Adapted from Boyd and Bauman (1989) where daily dose of pST was 120 µg/kg body weight. 
FIGURE 1-2 Effects of a maximally effective dose of pST (rpGH) on nutrient partitioning in growing pigs. Pork loins are from pigs treated for 77 days with either excipient (control) or a daily dose of 140 µg pST/kg body weight (Evock et al., 1988). Loin-eye area of the loin from the control pig is 27.2 cm2; from the treated pig, 51.7 cm2. understand and appreciate the effects that metabolic modifiers have on nutrient requirements, it is necessary to consider the biology of their action. Thus, a second objective of this report is to discuss the current understanding of the mechanisms by which these metabolic modifiers alter nutrient partitioning and productive efficiency. The extensive research conducted with metabolic modifiers have demonstrated that these technologies when appropriately used are effective in improving food animal production and are safe for the target animal and consumer. Following a protracted and thorough review of the use of bovine somatotropin in dairy production, the Food and Drug Administration approved bST for commercial use in the United States in November 1993 (waiting for exact citation). Evidence that supports this approval action can be found in several sources (U.S. Congress, Office of Technology Assessment, 1991; Bauman, 1992; Executive Branch of the Federal Government, 1994). Likewise, extensive research has shown that the other metabolic modifiers under commercial development are safe (reviewed in Etherton, 1991; Etherton et al., 1993). 
FIGURE 1-3 The effect of bST on the quantity of energy used for milk production and maintenance in lactating cows (from Bauman, 1987).
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