unit and as a starting point for most systems based on the net energy (NE) concept.

The value of feed energy for the promotion of energy retention is measured by determining the RE at two or more amounts of intake energy (IE). The NE of a feed or diet has classically been illustrated by the equation:

Determination of NE by this method assumes the relationship between RE and feed intake is linear. Actually the relationship is curvilinear and shows a diminishing return effect (Garrett and Johnson, 1983). The relationship is conventionally approximated by two straight lines. The intersection of the two lines is the point at which RE=0 and is defined as maintenance (M). Conversely, when RE=0, ME=HE. The relationship between feed intake and body tissue loss (negative RE) comprises one portion of the curve and the relationship between body tissue gain (positive RE) comprises a second portion of the curve. The heat production at zero feed intake (HeE) is equivalent to the animal’s NE requirement for maintenance. The ability of the food consumed to meet the NE required for maintenance is expressed as NEm and is represented by the following expression:

where Im is the amount of feed consumed at RE=0. Similarly, the value of feed consumed to promote energy retention is represented by the expression NEr and is determined as

where (I-Im) represents the amount of feed consumed in excess of maintenance requirements.

The relationship ME=RE+HE can be rewritten in terms of NE. Thus, HE can be partitioned into HeE, HjE and HiE (heat increment of intake energy) as

Because in practical situations the heat of activity associated with obtaining feed (HjE) is often included with HeE, the expression becomes

The NEr used in this expression does not distinguish among different forms in which energy may be retained, such as body tissue (TE), milk, (LE) or tissues of the conceptus (YE). Thus, the former expression might be expanded such that in a pregnant lactating heifer it becomes:

where NEr, NEl, and NEg are equivalent to RE, LE, YE, and TE, respectively.

Thus,

In this expression, a portion of the heat increment (HiE) is associated with the feed consumed for maintenance and each of the productive functions.

The primary advantages of an NE system are that animal requirements stated as net energy are independent of the diet, and the energy value of feeds for different physiological functions are estimated separately—for example, NEm, NEg, NEl, NEy. This requires, however, that each feed must be assigned multiple NE values because the value varies with the function for which energy is used by the animal. Alternatively, the animal’s energy requirement for various physiological functions may be expressed in terms of a single NE value, provided the relationships among efficiencies of utilization of ME for different functions are known.

Relationships for converting ME values to NEm and NEg (Mcal/kg DM) have been reported by Garrett (1980) and are

The NEm and NEg values used in the derivation of these equations were based on comparative slaughter studies involving 2,766 animals fed complete, mixed diets at or near ad libitum intake for 100 to 200 days. Digestion trials were conducted on most diets fed at about 1.1 times the maintenance amount. The ME values were estimated as DE * 0.82. Data were not uniformly distributed across the range of ME concentrations encountered in practical situations (1 percent, <1.9 Mcal/kg; 22 percent, 1.9–2.6 Mcal/kg; 65 percent, 2.6–2.9 Mcal/kg; 12 percent, >2.9 Mcal/kg). Caution should be exercised in use of these equations for predicting NEm or NEg values for individual feed ingredients or for feeds outside the ranges indicated above. The relationship between DE and ME can vary considerably among feed ingredients or diets as a result of differences in intake, rate of digestion and passage, and composition (for example, fiber vs starch vs fat). In addition, conversion of ME to NEm or NEg may vary beyond that associated with variation in dietary ME in part because of differences in composition of absorbed nutrients.

Available data, as discussed in subsequent sections, indicate efficiencies of ME use for lactation and maintenance are similar in beef cattle; thus, energy requirements for lactation have been expressed in NEm units. Efficiency of utilization of ME for accretion of energy in gravid uterine tissues is, likewise, discussed in a subsequent section. Some evidence is available to indicate that the efficiency of utilization of ME for maintenance (km) and pregnancy (ky) vary similarly with changes in ME concentration in the diet (Robinson et al., 1980). For convenience, estimates of



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