B₁₂, pantothenic acid, or choline; content of biotin was increased slightly (van den Berg, 1989, 1991; Kirchgessner et al., 1991b).

Milk concentrations of nutritionally important mineral elements (calcium, phosphorus, sodium, iron, copper, and manganese) were not affected by bST (Eppard et al., 1985, 1991; Annexstad et al., 1990; van den Berg, 1991). Other studies have also observed normal milk concentrations of ash, calcium, and phosphorus throughout lactation in bST-supplemented cows (Hard et al., 1988; Bauman et al., 1989b; Oldenbroek et al., 1989a,b; Pikus et al., 1989; Hartnell et al., 1991).

Production responses to bST have been obtained under a wide variety of feeding programs including feeding total mixed rations, feeding grain and forage separately, and pasture-fed cows. A particularly important adaptation is that cows typically adjust their voluntary feed intake upward within a few weeks after initiation of bST treatment (Peel and Bauman, 1987; Chalupa and Galligan, 1989; Chilliard, 1989). Bauman et al. (1985) suggested that feed intake regulation was more likely associated with tissue metabolism and use of nutrients than with bST per se. Indeed, covariate adjustment for the increased production of fat-corrected milk showed that most of the increased consumption was accounted for by higher milk yields (Marsh et al., 1988). In addition, energy status of the animal, level of nutrient intake at the onset of bST treatment, and the magnitude and pattern of the milk response all are important factors driving dry-matter intake. Current equations used to estimate feed intake can be applied to cows supplemented with bST (National Research Council, 1987, 1988b).

In some long-term bST studies, dry-matter intake (energy intake) was reported to be similar to that of control cows. However, in some of these studies, bST was administered once every 28 days and dry-matter intake was numerically increased in a dose-dependent manner (Leonard et al., 1990a; McGuffey et al., 1991a,b). Because voluntary intake did not increase to an extent that matched the increased milk yield, body weight gain was significantly less and body condition scores were substantially lower (approximately 0.5 points) at the end of lactation in the bST-treated cow. Only during the second lactation (Leonard et al., 1990a) was the increase in dry-matter intake significant (P < 0.05) and body weight gain and body condition scores similar among treatment groups. Leonard et al. (1990a) postulated that when cows are treated with a prolonged-release formulation of bST, during which milk production returns to baseline and remains at that level for a period of time before bST readministration, weaker signals are sent to drive voluntary intake.

Attention to management factors that affect dry-matter intake will become important in maximizing the milk response to bST. Excellent quality forage is a critical component in obtaining high levels of voluntary intake. Other important factors include adequate bunk space, ease of access to the feed bunk, ad libitum and frequent offerings of feed, unlimited access to clean water, nutritionally balanced diet, adequate dietary protein, proper levels of effective as well as digestible fiber, and control of temperature and humidity. Even though cows adjust their feed intake upward within a few weeks after initiation of treatment, the magnitude of response to bST likely will depend on nutrients provided by feeding programs. If cows consume an insufficient quantity of nutrients or are fed diets with inadequate nutrient balances, the response to bST will undoubtedly decrease according to the extent of the inadequacy (Bauman, 1992).

More nutrients are needed for the increased synthesis of milk and milk components that occurs with bST supplementation of lactating dairy cows. Initially, body stores of protein and fat may provide additional nutrients, but nutrients for sustained increases in production are derived from changes in voluntary intake (see previous section) and the coordinated changes in the metabolism of body tissues so that more nutrients can be used for milk synthesis (Bauman and McCutcheon, 1986; Bauman et al., 1989a; Vernon, 1989; Breier et al., 1991). The mechanism of action for bST and effects on specific metabolic processes are detailed in Chapter 2 (see Table 2-4). Overall, it is these orchestrated changes that allow the animal to achieve an increased milk yield while remaining normal and healthy. In most regards, the bST-treated cow is similar to the genetically superior cow with a comparable milk production (Table 3-1). The following sections will address the effect of bST on requirements for specific nutrients and on recommendations for diet formulation and feeding programs.

A lactating cow requires a substantial quantity of energy to meet its requirements for maintenance and milk production. Therefore, formulating diets that allow for an adequate energy intake (rumen digestible carbohydrate) is a major consideration. Body fat reserves can provide a temporary supply of energy and these are typically important in early lactation. In a high producing dairy cow, the use of body energy reserves in the first month of lactation can be energetically equivalent to about one-third of the milk produced (Bauman and Currie, 1980). However, over a lactation cycle these body fat reserves must be replenished in preparation for the next lactation.

The effects of bST on dimensions of energy metabolism of dairy cows have been examined. Studies have consistently

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)