1
Introduction
It has become increasingly evident that technological developments in a variety of scientific and engineering disciplines will be needed to support the growing world population, which is expected to double in the next 40 years. Thus, the supply of food required to adequately meet human nutritional needs over the next 40 years is quantitatively equal to the amount of food previously produced throughout the entire history of humankind (Bauman, 1992). The need for technological innovations in agriculture production systems is obvious. The success of agriculture scientists in developing new technologies to enhance food production is perhaps best exemplified by the 45 percent annual return on investment in agriculture research in the United States (Fox et al., 1987).
Food products from animals have always been a mainstay of the American diet, and between 33 and 100 percent of the major dietary nutrients are derived from animal products (National Research Council, 1988a). For example, animal products account for 69 percent of the dietary protein intake. To meet the challenge of world food needs, animal scientists must develop new technologies to increase productive efficiency (i.e., yield of milk/feed input; yield of muscle/feed input), produce leaner animals, and provide increased economic return on investment to the producer.
Technologies that lower the quantity of feed consumed per unit of output (such as meat or milk) will be of benefit to both the producer and consumer because feed constitutes a major component (about 70 percent) of farm expenditures. These technologies also represent an advantage in reducing environmental pollution (National Research Council, 1989; Bauman, 1992; Johnson et al., 1991). A reduction of the quantity of feed required to produce a unit of meat or milk would be expected to reduce fertilizer and other inputs associated with growing, harvesting, processing, and storing animal feed. Likewise, reduction in animal excreta, including methane production (predominantly from ruminants), occurs when productive efficiency is increased.
There is also a real need to reduce fat content of both meat and dairy products. Consumers are concerned about the saturated fat content of animal products because of numerous studies that show saturated fatty acid intake is a major risk factor for coronary disease (Kris-Etherton et al., 1988). Because animal products provide approximately 60 percent of the total saturated fatty acids consumed in the United States (National Research Council, 1988a), it is prudent and timely that new technologies be developed that are more cost effective and efficient in reducing carcass fat than the current practice of trimming the excess fat.
Recent developments have identified several new technologies that will improve the nutritional attributes of animal products and increase productive efficiency (U.S. Congress, Office of Technology Assessment, 1992; Bauman, 1992; Etherton, 1992). Foremost of these new technologies is a group of compounds that modify animal metabolism, referred to as "metabolic modifiers." Metabolic modifiers covered in this report include somatotropin (ST), growth hormone-releasing factor, and β-agonists. All of these metabolic modifiers affect how animals use absorbed nutrients. This is illustrated by considering ST.
Administration of porcine ST (pST) to growing pigs results in more nutrients being directed to lean tissue growth (protein) and less to fat accretion in adipose tissue (Figures 1-1 and 1-2). This shift in nutrient partitioning results in a substantial improvement in feed efficiency, and the quantity of feed per unit of gain can be reduced by up to 35 percent when pST is used (see Chapter 2). Treatment of dairy cows with bovine ST (bST) causes more nutrients to be directed for milk synthesis. Dairy cows are typically managed so that excessive fattening does not occur. In this case, bST treatment has no effect on body composition or milk composition, but milk yield is increased (see Chapter 3). Thus, the proportion of nutrients used for maintenance decreases resulting in a marked improvement in the amount of milk produced per unit of feed consumed (Figure 1-3).
The improvement in carcass quality of growing animals and the increases in efficiency of growth and lactation that occur with the use of metabolic modifiers are unprecedented. For a successful animal production enterprise, however, sound management practices must be followed. This requirement does not change when metabolic modifiers are used. Indeed, there are several studies in which no response was observed with ST treatment of growing pigs or lactating cows because of limitations in the management program. One aspect of the management program that is critically important for successful implementation of these new technologies is nutrition. The primary objective of this report is to discuss the impact of metabolic modifiers on the nutrient requirements of food-producing animals. These effects differ somewhat between growing and lactating animals. In the former, the use of metabolic modifiers results in a major shift in the type of growth (fat versus lean tissue), thereby requiring matching changes in dietary formulations. In contrast, use of metabolic modifiers for lactating dairy cows does not alter the specific nutrient requirements per unit of milk because the composition of milk is not affected. To fully
FIGURE 1-1
Accretion rates for protein and fat in pigs over the body weight range from 45 kg to 100 kg (market weight). Adapted from Boyd and Bauman (1989) where daily dose of pST was 120 µg/kg body weight.
FIGURE 1-2
Effects of a maximally effective dose of pST (rpGH) on nutrient partitioning in growing pigs. Pork loins are from pigs treated for 77 days with either excipient (control) or a daily dose of 140 µg pST/kg body weight (Evock et al., 1988). Loin-eye area of the loin from the control pig is 27.2 cm2; from the treated pig, 51.7 cm2.
understand and appreciate the effects that metabolic modifiers have on nutrient requirements, it is necessary to consider the biology of their action. Thus, a second objective of this report is to discuss the current understanding of the mechanisms by which these metabolic modifiers alter nutrient partitioning and productive efficiency.
The extensive research conducted with metabolic modifiers have demonstrated that these technologies when appropriately used are effective in improving food animal production and are safe for the target animal and consumer. Following a protracted and thorough review of the use of bovine somatotropin in dairy production, the Food and Drug Administration approved bST for commercial use in the United States in November 1993 (waiting for exact citation). Evidence that supports this approval action can be found in several sources (U.S. Congress, Office of Technology Assessment, 1991; Bauman, 1992; Executive Branch of the Federal Government, 1994). Likewise, extensive research has shown that the other metabolic modifiers under commercial development are safe (reviewed in Etherton, 1991; Etherton et al., 1993).
FIGURE 1-3
The effect of bST on the quantity of energy used for milk production and maintenance in lactating cows (from Bauman, 1987).
