the diet has been shown to impair ruminal fermentation, decrease fiber digestibility, and lower milk content of fat and protein. However, these effects are related to the level and type of fat supplement. Animal fats, blended animal-vegetable fats, or oilseeds are preferred over vegetable oils (National Research Council, 1988b). The level of added fat typically should not exceed 0.5 to 0.7 kg/day or 4 to 5 percent of the total mixed diet on a dry-matter basis; however, the use of ruminally inert fats may allow inclusion of slightly higher levels in the diet.

A few investigators have examined the effect of adding supplemental fat to the diet of cows treated with bST. Their studies demonstrated that supplemental fat in the corn-based and barley-based diets, both ruminally active fat (Lough et al., 1988) and ruminally inert fat (Lormore et al., 1990; Marty and Block, 1990), had no effect on milk response of bST-treated cows over and above that of bST-treated cows fed a diet without added fat. In a study by Schneider et al. (1990), there was a tendency for ruminally inert fat to enhance the galactopoietic effect of bST, but results were not significant. Therefore, results indicate the energy density required in diets for cows of a given milk yield should be similar, independent of bST administration.

Although body stores of protein and fat may be needed to provide additional nutrients until feed intake is adjusted upward, Peel et al. (1989) reported that body condition score at the start of bST supplementation was not associated with the magnitude of response. In contrast, Crooker and Otterby (1991) reported a major effect of body condition prior to start of bST supplementation on milk response to bST. Achieving proper body condition prior to calving should be an important management strategy.

Lower body fat in cows supplemented with bST (Bauman et al., 1988; Soderholm et al., 1988; Brown et al., 1989; Chilliard et al., 1991; McGuffey et al., 1991b) is the result of partitioning calories to milk production at the expense of body fat. It therefore is important to monitor body condition so that body reserves can be replenished during late lactation or the dry period. Impact of bST on body composition and body condition score depends on the magnitude of the milk response to bST and the level of intake and nutrient density of the diet.

Movement of cows to feeding programs with lower nutrient densities should be on the basis of milk yield and body condition. Restoration of body condition is more efficient in late lactation than during the dry period; however, if cows are not in proper body condition at dry-off, then dry cow management should be aimed at replenishing reserves prior to calving. Economics and management strategies may indicate that with higher levels of milk production and changes in persistency obtained with the use of bST, longer calving intervals may be appropriate. This strategy would provide the additional time in late lactation needed to replenish body reserves for subsequent lactations (Crooker and Otterby, 1991; Bauman, 1992; Patton and Heald, 1992).

The physiology and metabolism of bST-treated cows are like those of genetically superior cows at the same level of production. Substantial responses in milk yield occur when bST is administered over the last three-fourths of lactation. Lactation curves are shifted upward and are generally more persistent. Dairy cattle of all breeds and all parities respond to exogenous bST administration. Milk composition remains normal in bST-supplemented cows and factors that affect milk composition, such as nutritional status, genetics, and stage of lactation, cause the same variation as observed for untreated cows. St administration does not alter digestibility of the dietary nutrients, maintenance requirements, or the partial efficiency of milk synthesis. Thus, current NRC recommendations for nutrient requirements should be followed for bST-supplemented cows. Similarly, the NRC recommendations for diet formulation and feeding programs are the same as for cows administered bST. These include such considerations as dietary levels of fiber, energy density, and degradable and undegradable protein as well as nutritional considerations in regard to maintaining adequate body reserves. Overall, the nutritional needs and feeding strategies for cows supplemented with bST are identical to current recommendations for untreated cows of comparable milk production.

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)