requirements for beef cows were converted to NEm equivalents. Conversion of requirements for lactation and pregnancy to NEm equivalents allow the energy value of feedstuffs to be adequately described by only two NE values (NEm and NEg).
The maintenance requirement for energy has been defined as the amount of feed energy intake that will result in no net loss or gain of energy from the tissues of the animal body. Processes or functions comprising maintenance energy requirements include body temperature regulation, essential metabolic processes, and physical activity. Energy maintenance does not necessarily equate to maintenance of body fat, body protein, or body weight. Although for many practical situations maintenance may be considered a theoretical condition, it is useful and appropriate to consider maintenance energy requirements separate from energy requirements for “production.” ME required for maintenance functions represents approximately 70 percent of the total ME required by mature, producing beef cows (Ferrell and Jenkins, 1987) and more than 90 percent of the energy required by breeding bulls. The fraction of total ME intake that growing cattle use for maintenance functions is rarely less than 0.40, even at maximum intake. Successful management of beef cattle, whether for survival and production in poor nutritive environments or for maximal production, depends on knowledge of and understanding their maintenance requirements.
Basically, three methods have been used to measure maintenance energy requirements. These include the use of
long-term feeding trials to determine the quantity of feed required to maintain body weight or, conversely, determine body weight maintained after feeding a predetermined amount of feed for an extended period of time (Taylor et al., 1981, 1986);
calorimetric methods (Agricultural Research Council, 1965, 1980); or
comparative slaughter (Lofgreen, 1965; Lofgreen and Garrett, 1968).
Each approach has advantages as well as limitations.
Estimates of feed required for maintenance of body weight, usually measured in long-term feeding trials, are obtainable with relative ease and can be determined with large numbers of cattle. Values obtained generally correlate well with energy maintenance in mature, nonpregnant, nonlactating cattle (Jenkins and Ferrell, 1983; Ferrell and Jenkins, 1985a; Laurenz et al., 1991; Solis et al., 1988). Changes in body composition and composition of weight change in growing, pregnant, or lactating cattle are problematic with this approach. Expression of the results in terms of ME or NE requirements depends on use of information from other approaches.
The energy feeding systems of the Agricultural Research Council (ARC) (1965, 1980), Ministry of Agriculture, Fisheries, and Food (MAFF) (1976, 1984), Commonwealth Scientific and Industrial Research Organization (CSIRO) (1990), and Agricultural and Food Research Council (AFRC) (1993), and the energy requirements of dairy cows (National Research Council, 1989) are primarily based on calorimetric methods. Fasting heat production (FHP) measured by calorimetry plus urinary energy lost during the same period provide measures of fasting metabolism (FM), which by definition, equates to net energy required for maintenance (NEm). Measurement conditions are standardized such that animals are fed a specified diet at approximately maintenance for 3 weeks prior to measurement. Animals are trained to the calorimeter and kept in a thermoneutral environment. Measurements are usually made during the third and fourth day after withdrawal of feed. For practical use, FM values are adjusted for the difference between fasted weight of an animal and its liveweight when fed. In addition, recognizing that fasted animals are less physically active than fed animals, ARC (1980) adjusts FM by adding an activity allowance of 1 kcal/kg liveweight for cattle. CSIRO (1990) has incorporated additional corrections for breed, sex, proportional contribution of milk to the diet, energy intake, grazing activity, and cold stress.
Because of the complexity and cost of measurements, numbers of animals that can be used is limited. With this approach, measurements are basically acute in that they are made over one or at most a few days. Practical limitations of these systems stem largely from difficulties in adjusting data obtained in well-controlled laboratory environments to the practical feeding situation.
The California Net Energy System, proposed by Lofgreen and Garrett (1968) and adopted in the two preceding editions of this volume (National Research Council, 1976, 1984), is based on comparative slaughter methods. In contrast to calorimetry, in which ME intake and HE are measured and RE is determined by difference, comparative slaughter procedures measure ME and RE directly and HE by difference. RE is measured as the change in body energy content of animals fed at two or more levels of intake (one of which approximates maintenance) during a feeding period. RE equates, by definition, to NEg in a growing animal. The slope of the linear regression of RE on ME intake provides an estimate of efficiency of utilization of ME for RE and in growing animals equates to kg. The ME intake at which RE=0 provides an estimate of ME required for maintenance (MEm). By convention, the