Metabolic Modifiers: Effects on the Nutrient Requirements of Food-Producing Animals (1994)

This chapter examines the importance of nutrition in dairy cows supplemented with bovine somatotropin (bST). For background, the production response to bST is also reviewed. Evidence is presented to show that (1) nutrients required for maintenance and per unit of milk are not changed, (2) cows supplemented with bST are like genetically superior cows at the same level of milk production, and (3) cows supplemented with bST should be fed according to level of production using current National Research Council (NRC) recommendations (National Research Council, 1988b).

Somatotropin supplementation modifies the shape of the lactation curve (Bauman et al., 1985; Peel and Bauman, 1987). The first modification that occurs is a shift to a higher level of milk production. The second modification that can also occur is an improvement in persistency. Although responses of milk yield are fairly uniform, there is variation for individual cows within experiments and variation for groups of cows between experiments. Variation of responses may be caused by breed, parity, level of milk production, stage of lactation, and management factors such as environment, herd health, and nutritional program.

Effects of bST on milk yield have been reviewed (Peel and Bauman, 1987; McBride et al., 1988; Chalupa and Galligan, 1989; Chilliard, 1989; Peel et al., 1989; Crooker and Otterby, 1991; Hartnell et al., 1991; McGuffey et al., 1991a). Examples of milk response to varying doses of bST are summarized in Chapter 2 (see Table 2-3). Similar milk responses to bST have been obtained when milking frequency was increased from 2 to 3 or 4 times per day (Armstrong et al., 1990b; Jordan et al., 1991) or when bST was administered under commercial conditions (Chalupa et al., 1988; Armstrong et al., 1990a,b; Bath et al., 1990; Duque et al., 1990; Thomas et al., 1991).

Administration of bST has involved daily injections as well as prolonged-release formulations injected at 2- to 4-week intervals. Milk response to bST varies according to stage of lactation. In general, response has been minimal when bST is administered early in the lactation cycle, prior to peak yield. Therefore, commercial use would likely involve bST treatment over the last two-thirds to three-fourths of a lactation cycle with the increase in milk yield persisting throughout this interval. Researchers have administered bST treatments for a period of a few weeks, a single lactation, or multilactations. Somatotropin has been administered for as long as eight successive lactations (see Muller, 1992).

Milk-yield responses to bST have been reported in all dairy breeds examined including African, Asian, Australian, European, North American, and South American breeds as well as Murrah buffalos. Most research has used Holstein cows, but responses of a similar magnitude have also been reported for other dairy breeds (Oldenbroek et al., 1989a,b; Duque et al., 1990; West et al., 1990a; Schams et al., 1991; Jenny et al., 1992; Pell et al., 1992).

Parity can affect the magnitude of the milk response to bST. Several researchers have demonstrated higher levels of response in multiparous cows as compared to primiparous cows when administered the same amount of bST (Baird et al., 1986; Chalupa et al., 1986; Huber et al., 1988; Palmquist, 1988; Soderholm et al., 1988; Crooker and Otterby, 1991). In contrast, other researchers have observed no difference between primiparous cows and multiparous cows in the milk response to the same dose of bST (Chalupa et al., 1988; Hard et al., 1988; Lamb et al., 1988; Samuels et al.,

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

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

demonstrated that bST treatment does not alter the digestibility of dietary dry matter, energy, or carbon (Peel et al., 1981, 1985; Tyrrell et al., 1988; Sechen et al., 1989a; Kim et al., 1991; Kirchgessner et al., 1991a; Lynch et al., 1991; Robinson et al., 1991).

Bioenergy studies demonstrate that the energy requirements for maintenance and milk production are not altered in bST-treated cows (Tyrrell et al., 1988; Sechen et al., 1989a; Kirchgessner et al., 1991a). These calorimetry measurements have involved bST administration for short-term and long-term periods and included animals in both positive and negative energy and nitrogen balances. Results indicate that heat loss in bST-treated cows was predictable from changes in milk yield and nutritional status. When cows were in negative energy balance, bST caused an increased heat energy loss equal to that predicted for the increased milk yield (Tyrrell et al., 1988; Kirchgessner et al., 1991a). In contrast, when energy balance was positive, supplementation with bST had no effect on energy lost as heat because the increase in heat associated with the extra milk matched the decrease in heat associated with the reduction in synthesis of body fat (Sechen et al., 1989a).

For bST-treated cows, theoretical calculations of energy metabolism are also in agreement with observed changes. This close agreement would only occur if digestibilities and the energy requirements for maintenance and milk are unchanged. These comparisons have included studies in which changes in body composition and energy balance were compared to theoretical estimates (e.g., Soderholm et al., 1988; Brown et al., 1989; Chilliard et al., 1991; McGuffey et al., 1991b; McGuffey and Wilkinson, 1991), studies in which kinetic measurements of metabolite turnover were compared to energy balance (Bauman et al., 1988), and long-term studies in which theoretical and observed estimates of energy efficiency were compared (e.g., Bauman et al., 1985, 1989b; Soderholm et al., 1988).

Overall, results demonstrate that the current NRC requirements (National Research Council, 1988b) for maintenance and milk production are appropriate for cows receiving bST (Table 3-1). The bST-treated cows have a greater total energy requirement because they are producing more milk. However, they have greater productive efficiency (milk/unit of feed; Chapter 2) because maintenance is unchanged and a greater proportion of total nutrient intake is being used for milk synthesis (Table 3-1). This is frequently referred to as a dilution of maintenance effect and is also the basis for gains in efficiency achieved with other dairy technologies (Bauman, 1992).

Absorption of essential amino acids from digested protein is vital for maintenance, growth, reproduction, and lactation of dairy cattle. These amino acids are derived either from microbial protein produced during fermentation of feed in the rumen or from dietary protein that escapes rumen fermentation. An additional short-term supply of amino acids may be derived from the mobilization of labile tissue protein, and use of these reserves would typically occur in early lactation. Estimates of the protein reserves available to support milk synthesis indicate they are quantitatively limited (National Research Council, 1988b).

The ruminal production of bacterial and protozoal crude protein is a function of energy intake (rumen digestible carbohydrate) (National Research Council, 1985). In addition, an adequate supply of nitrogen from degradable protein and nonprotein nitrogen (NPN) is essential to maximize feed intake, ruminal digestibility, and microbial protein yield (National Research Council, 1988b).

The protein in practical dairy forage and concentrate sources supplies some dietary protein that escapes rumen fermentation, and this protein plus the microbial protein

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

intake, maintain adequate ruminal fermentation, and allow for normal performance and health of the lactating cow. The amount of fiber to be included in the diet of dairy cattle is influenced by a number of variables such as the animal's level of production, the type of fiber, the particle size and distribution of the fiber, buffering capacity of the forage, and frequency of feeding. Animals producing large amounts of milk should receive more energy and less fiber than cows producing smaller amounts (National Research Council, 1988b).

Current NRC recommendations are that a minimum of 21 percent acid detergent fiber (ADF) and 28 percent neutral detergent fiber (NDF) be provided to cows during the first 3 weeks of lactation (National Research Council, 1988b). During times of high milk production, however, ADF and NDF contents of the diet may be reduced to 19 and 25 percent, respectively, so that adequate energy can be included to meet the cow's requirements. Virtually all the long-term studies have followed these recommendations for both control and bST-treatment groups and none have highlighted any unusual observations that might be related to rumen fermentation. Therefore, it appears that current recommendations apply for dietary fiber, even if cows are receiving supplemental bST. This is not surprising because the biological effects of bST are associated with the utilization of absorbed nutrients rather than digestive processes (Chapter 2).

As the level of milk production and genetic merit for milk yield increase, the energy intake of dairy cows must also increase. Energy intake is affected by energy density of the diet and factors affecting dry-matter intake. Three ways to increase the energy density of the diet are (1) increase the proportion of concentrate versus forage in the diet, (2) select concentrate ingredients of higher energy content (i.e., shelled corn versus ear corn or oats; soy hulls versus oat hulls), or (3) add fat sources such as animal fat, vegetable oils, oil seeds, or inert/bypass fat sources. The maximum amount of concentrate that can be used without causing milk fat depression and disturbances in digestion, metabolism, and production depends on the type of forage used as well as its physical form. Generally, higher proportions of concentrate can be used without the need for dietary buffers to maintain milk fat content when the major forage is alfalfa or grass hay, haylage, or silage as compared to corn or small grain silages (Erdman, 1988; National Research Council, 1988b). However, minimum levels of fiber should be maintained (National Research Council, 1988b). Under some circumstances, supplementation with dietary buffers has increased dry-matter intake and milk yield. The most significant improvements from feeding buffers have been obtained in early stages of lactation when corn silage was the major forage fed (National Research Council, 1988b). Chalupa et al. (1984, 1985), feeding corn silage as the only forage (40 percent of dietary dry matter), reported milk yield responses of bST-treated cows to be additive to responses with sodium bicarbonate.

Altering the forage-concentrate ratio affects the lactational performance of dairy cows. Because of gut-fill limitations with high forage diets, increasing the proportion of concentrates results in a greater energy intake thereby allowing for a higher level of milk production. Recently, Tessmann et al. (1991b) reexamined forage-concentrate ratios using alfalfa silage as the forage source and cows of high genetic potential for milk yield. Consistent with earlier work (National Research Council, 1987), they observed that a modest decrease in milk yield occurred as the proportion of forage in the diet was increased. Tessmann et al. (1991b) also demonstrated that replenishment of body reserves was less adequate as the dietary ratio of forage-concentrate was increased; these results are consistent with earlier work and the fact that their design based the dietary shifts on stage of lactation rather than level of milk yield or body condition scores.

Tessmann et al. (1991a) also examined the effects of bST treatment over the interval of 13 to 43 weeks postpartum using two of these dietary groups. Although milk yield was lower in the control group receiving the higher proportion of forage as compared to the low-forage control group, cows on both diets responded in 3.5 percent fat-corrected milk to a similar extent with bST treatment. Consistent with their design and the work of others, effects on replenishment of body reserves were as expected, regardless of whether animals received bST.

The impact of variations in the forage-concentrate ratio on milk response to bST has also been examined by others. Forage-concentrate ratios over the range of 60:40 to 40:60 had no effect on the lactational response to bST (Hemken et al., 1988; McGuffey et al., 1991b). In fact, typical increases in milk yield have also been observed even when pasture was the only source of nutrients (Peel et al., 1985; McCutcheon et al., 1989).

Selecting fiber and carbohydrate sources with higher energy densities is another way of increasing energy intake. Beet and citrus pulp and soy hulls are highly digestible fiber sources containing twice as much energy as oat hulls and 10 times as much energy as rice hulls (National Research Council, 1988b). Corn grain contains more energy than ear corn, oats, or milo. Although corn contains more energy than barley, the barley starch has been reported to be more digestible. Eisenbeisz et al. (1990) reported that cows fed corn-based diets achieved greater milk production than cows fed barley-based diets; however, the milk response to bST was similar.

Substitution of supplemented fat for a portion of the starch and cereal grains in the diets of high-producing cows is a way of maintaining high energy intakes and high fiber intakes (see reviews by Palmquist and Jenkins, 1980; Coppock and Wilks, 1991). In some situations, addition of fat to

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.