absorptive nutrient changes, should be followed. According to phase II, if a metabolic modifier alters composition and rate of gain, then the nutrient-calorie ratio and the daily intake of specific nutrients should be altered as appropriate.
This flow chart (Figure 4-1) can be effectively applied to somatotropin, a 190- or 191-amino acid homeorhetic molecule that coordinates metabolism and controls nutrient flow to support a specific physiologic state (McGuire and Bauman, 1997). Somatotropin increases efficiency of growing and lactating animals through multiple sites of action. In a lactating animal, somatotropin affects a wide variety of tissues to coordinate metabolism to support increases in milk production. In a growing animal, somatotropin supports nutrient flow to lean muscle accretion and bone mass. Because lean mass is increased in growing animals, the nutrient requirements are altered. This is not the case in the lactating dairy cow where additional nutrients required are determined by the size of the increase in milk yield. There is no net change in lean body mass.