1988; Bauman et al., 1989b; Franson et al., 1989; Remond et al., 1991). Hartnell et al. (1991) reported approximately 0.5 kg/day lower milk-yield response but a similar percent response to various doses of bST in primiparous cows as compared to multiparous cows. On the basis of 15 on-farm trials in the United States, Thomas et al. (1991) reported that responses in milk production for multiparous cows were higher than responses of primiparous cows at all stages of the lactation cycle. The observed variation in milk response between primiparous and multiparous cows is likely related to differences in level of production, differences in the shape of the lactation curve, and/or differences in the extent to which primiparous cows need to divert nutrients for growth in order to achieve mature size.

Quality of management will be the major factor affecting the magnitude of milk response to bST (Bauman, 1987). Responses do vary on a per-herd basis according to average pretreatment production when pretreatment differences are related to quality of management (Chilliard, 1989; Peel et al., 1989; Crooker and Otterby, 1991; Bauman, 1992). Several studies have examined the relationship between genotype and milk response to bST. With one exception (Michel et al., 1990), results demonstrate that there is no evidence of a genotype-response interaction in bST-supplemented cows (McDaniel and Hayes, 1988; Gravert, 1989; Leitch et al., 1990; Nytes et al., 1990).

Within a herd, where differences in quality of management are less of a factor, studies with bST have consistently shown that the variation within bST-supplemented groups is similar to that of untreated groups (Peel et al., 1989; Bauman, 1992). In addition, the level of response to bST appears to be similar for individual cows regardless of whether they were high- or low-level milk producers in the herd prior to initiation of bST supplementation (Peel et al., 1989; Thomas et al., 1991). Thus, to a large extent, all cows in a herd respond to bST in a fairly similar manner.

Cows exposed to hot or cold environments have production responses to bST similar to cows in a thermoneutral environment. In the Climatology Laboratory at the University of Missouri, cows were subjected to thermoneutral (15 to 22°C), hot (25 to 35°C), and cold (-5 to 5°C) daily cycles of temperature (Becker et al., 1990; Johnson et al., 1991; Manalu et al., 1991). Under all cycles, cows supplemented with bST produced more heat, as expected from the higher milk yield; but dissipation of heat also increased so that no adverse heat balance problems occurred. In other short-term studies in Arizona (Armstrong et al., 1990b), Florida (Elvinger et al., 1988, 1992; Staples et al., 1988; Zoa-Mboe et al., 1989), Missouri (Mohammed and Johnson, 1985), and Southern California (Chalupa et al., 1988) and in long-term studies in Arizona (Huber et al., 1990) and Georgia (West et al., 1990b, 1991), heat-stressed cows responded positively to exogenous bST. However, magnitude of milk responses to bST under differing environmental conditions may depend largely on the effect of environment on feed intake. Similar to untreated cows, nutritional interventions such as additional water, dietary potassium, and the use of low-heat increment feeds like fat should be considered during periods of high temperature and humidity (Beede and Collier, 1986).

Concentration of fat and protein in milk is normally variable because of factors such as genetics, stage of lactation, age, diet composition, nutritional status, environment, and season (Linn, 1988; Sutton, 1989). These factors also affect the milk composition of bST-supplemented cows, and the variations in content of fat and protein are of the same magnitude as that usually observed in dairy herds. The effect of bST on milk fat and protein composition depends on the nutritional status of the cows both before and during bST treatment (Peel and Bauman, 1987; McBride et al., 1988; Bauman et al., 1989a; Chalupa and Galligan, 1989; van den Berg, 1989, 1991; Dell'Orto and Savoini, 1991; Barbano et al., 1992; Laurent et al., 1992; Lynch et al., 1992). In the early stages of bST treatment, increases in milk fat and decreases in milk protein may occur whenever milk-yield increases cause changes in energy and protein balance in the cow such that body fat and protein stores are mobilized to meet the increased nutrient demands. These changes in energy balance are similar to, but smaller than, changes that normally occur at the onset of lactation. With prolonged bST administration, cows adjust their voluntary feed intake to meet their increased nutrient requirements, and nutrient balance is restored. In general, the percentages of milk fat and protein were not different for bST and control cows when bST was administered over a full lactation (see reviews cited above). Overall, the results demonstrate that nutritional status affects the fat and protein composition of milk and that this relationship is not altered with bST supplementation.

Generally, the proportion of total milk protein represented by whey proteins and casein, and the composition of casein (α-casein, β-casein, κ-casein) are not altered by bST supplementation (Baer et al., 1989; Leonard et al., 1990b; Austin et al., 1991; van den Berg, 1991; Barbano et al., 1992; Laurent et al., 1992). Because protein and nonprotein nitrogen (NPN) content of milk reflect protein adequacy of diets (Refsdale et al., 1985), variations caused by bST supplementation will depend on protein nutritional status regardless of whether the animal receives exogenous bST.

Milk from cows supplemented with bST did not differ in content of vitamin A, thiamin, riboflavin, pyridoxine, vitamin

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)