produced from the degraded protein and NPN in these feeds may be enough to produce 20 kg milk/day (Conrad and Hibbs, 1968; Tamminga and von Hellemond, 1977). As milk production increases, a substantial amount of additional dietary protein from protein supplements must escape rumen fermentation to meet the cow's requirement for protein. The current recommendations for protein requirements of lactating cows include both a percentage of rumen degraded protein, to allow for maximum microbial growth and digestion of fiber, and a percentage of undegradable protein that will escape ruminal degradation and augment the supply of essential amino acids that can be absorbed from the small intestine (National Research Council, 1988b).

Long-term studies (one or more lactations) have demonstrated that by following current recommendations, bST-supplemented cows produce milk with normal protein content and composition (see previous sections). In addition, changes in protein supply to bST-treated cows alter protein balance and milk protein content in an identical manner as occurs in untreated cows, as previously discussed. Therefore, the results from these extensive studies are consistent with the protein requirements for maintenance and milk production being the same for cows receiving bST supplementation. The possibility of an increase in biological value, such as that which occurs with ST treatment of growing pigs (Chapter 5) and growing ruminants (Chapter 4), has not been examined for lactating cows. If such an effect occurred it would represent a subtle decrease in the protein requirements. However, with current feeding systems for dairy cows, the precision of estimates of the supply and requirement for protein (amino acids) makes it difficult to experimentally detect or commercially implement subtle differences in biological value of absorbed amino acids (Clark et al., 1992).

Several studies have examined aspects of protein metabolism in cows treated with bST. Overall, the digestibility of dietary protein is not altered in cows treated with bST (Peel et al., 1981, 1985; Tyrrell et al., 1988; Sechen et al., 1989a; Kim et al., 1991; Lynch et al., 1991; Robinson et al., 1991; Winsryg et al., 1991a,b).

de Boer and Kennelly (1989) found an increased milk yield response to bST when cows were fed diets containing 16 versus 11 percent crude protein. A subsequent report by de Boer et al. (1991) observed no difference in bST response for cows fed diets containing 17 versus 24 percent crude protein, although results were confounded by the fact that bST-treated cows produced no more milk than untreated cows. McGuffey et al. (1990) examined protein level and undegradability of protein for bST-treated cows and found that both higher protein level and increased undegradability of protein enhanced response to supplemental bST. However, these results were confounded by the energy sources of the diets, so that response could have been caused by energy (carbohydrate) source rather than protein. Performance differences may also be related to dietary differences in the quantity and quality of absorbed amino acids. Other studies comparing protein sources of different degradabilities indicated that level of rumen undegradable protein had no effect on the response to supplemental bST (Lormore et al., 1990; Hof et al., 1991; Winsryg et al., 1991b; Calsamiglia et al., 1992). In addition, postruminal infusion of casein (Peel et al., 1982) or essential amino acids (Aldrich et al., 1990; Lynch et al., 1991) did not alter response of cows to exogenous bST.

Data currently available indicate that dietary recommendations for quantity of protein and proportions of rumen degradable and undegradable protein (National Research Council, 1988b) are adequate to meet the needs of cows supplemented with bST. Therefore, protein requirements of the bST-treated cows are identical to the untreated cow producing the same quantity of milk.

There have been no studies that specifically examined the effects of bST supplementation on the vitamin and mineral requirements of lactating cows. Studies involving bST supplementation for weeks or months as well as those involving treatment for one or more lactations have simply followed current dietary recommendations for vitamins and minerals. If these recommendations were inadequate, one should see abnormalities in health and performance and classical subclinical and clinical symptoms of vitamin and mineral deficiencies or excesses. No study has reported observing such abnormalities or symptoms. Rather, studies with bST-supplemented cows have consistently documented increases in milk yield and treatment has had no effect on milk content of vitamins and minerals (see previous sections). Even when pharmacologic doses of bST were used (up to 3 g/14 days), milk concentrations of ash, calcium, phosphorus, magnesium, and zinc remained normal and there were no bone abnormalities as evidenced by a range of radiological, macroscopic, and microscopic indices (Eppard et al., 1991). This total pattern of mineral homeostasis is consistent with the concept that bST coordinates mineral partitioning (Peel and Bauman, 1987; Bauman et al., 1989a). Overall, results demonstrate that mineral and vitamin requirements must be similar between bST-supplemented cows and untreated cows of comparable production and indicate that requirements are adequately met by following current recommendations.

In general, the fiber content of the diet of dairy cattle is inversely related to its net energy content. Nevertheless, a minimum amount of fiber of the proper quality and physical form is necessary to obtain maximum dry-matter and energy

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)