Digestible Energy

Digestible energy is defined as intake energy minus fecal energy (DE = IE FE). The word ''apparent'' is often associated with this definition, which acknowledges that FE includes materials of metabolic (FmE) as well as of food (FiE) origin. There are additional losses of energy associated with the digestion of food that are not included in the conventional determination of DE. These losses include methane (GE), heat of fermentation (HfE), and heat of digestion and absorption (HdE). ''True'' digestibility, which attempts to account for those additional losses, is greater than estimated by apparent digestibility for most feeds.

Fecal energy represents a major loss of dietary energy. In herbivores that consume a wide variety of diets, FE varies from 15 20% of dietary intake on a high grain diet to about 80% of intake on a low-quality forage diet. The primary determinants of FE, hence DE, are the physical and chemical characteristics of the food consumed, both of which may be influenced extensively by processing. These characteristics influence the site of digestion, rate and extent of digestion, and the amounts and proportions of products available for absorption. Total tract digestibility, as measured in most digestion studies, considers only total disappearance, and may not reflect the nature of the products absorbed from the gut. Site and rate of digestion, as well as total tract digestibility of dietary energy may differ among animals due to species, age, breed, intake level, and temperature. For species of animals or feeding situations in which a relatively few dietary ingredients are used and the animals' dietary requirements have been determined using similar feeds, levels of intake and animal production, DE may be a satisfactory method of assessing energy available to the animal. Digestible energy has been used extensively as the basis to formulate diets and estimate the energy requirements of swine, as well as other species. However, for ruminants, a major weakness is that DE overvalues high-fiber (hays, straws) in relation to low-fiber (grains) feedstuffs. A substantial portion of this discrepancy may be attributable to energy losses unaccounted for by apparent digestibility and to differences in digestive end-products.

METABOLIZABLE ENERGY

Metabolizable energy (ME) is an estimate of the dietary energy available for metabolism by the tissues of the animal and is defined by the relationship: ME=IE (FE+UE+GE). Metabolizable energy is an improvement over DE for use as a measure of energy available to the animal and animal requirements because it incorporates urinary and gaseous losses, but has some of the same limitations as DE. The total digestible nutrient (TDN) system of feed evaluation accounts for the higher energy concentration of lipids by multiplying digested lipid by 2.25, but does not incorporate a similar correction for protein. As a result, TDN is a hybrid measure (not precisely DE or ME) that maintains many of the limitations of DE. Physiological fuel values (PFV) are essentially ME values derived by the use of average heats of combustion and average digestion coefficients for protein, fat, and carbohydrates, with values for protein corrected for urinary nitrogen loss. The physiological fuel values commonly quoted (4, 4, and 9 kcal/g of protein, carbohydrate, and fat) may give reasonable estimates of ME and have long served as the basis to formulate diets and express requirements of humans. Metabolizable energy has major significance as a reference unit and as a starting point for nearly all systems that are based on the net energy concept.

NET ENERGY

The equations and energy balance identity indicate that ME can appear in only two forms: heat (HE) or in formed products (RE). Thus, ME = HE + RE. Total heat production (HE) includes all energy that is transferred from the animal to the environment in a form other than combustible energy. Recovered energy (RE) is the heat of combustion of all animal products (TE, LE, YE, OE, VE, etc.) that may be produced or lost with a given energy intake not accounted for in any other category. It is evident that if two of the three terms in the previous equation are measured, the third may be obtained by difference. Thus, by measurement of ME and HE (as done in indirect or direct calorimetry) RE may be determined, or by measurement of ME and RE (as done by comparative slaughter approaches) HE can be determined.[3-5]

The general relationships between RE and HE to ME intake are shown in Fig. 2. At zero food intake, body tissues are metabolized to provide energy for necessary body functions. Fasting heat production (FHP) is the heat produced at zero food or ME intake and is about

70 kcal/kg . /d. Estimates of FHP vary with animal age or maturity, physiological state, prior nutritional status, environmental adaptation, breed, gender, etc. As ME intake increases, dietary energy provides increasingly higher proportion of the required energy supply, and body tissues provide a lower proportion. The ME intake at which net recovered energy is zero, or HE is equal to ME intake, is termed maintenance. Maintenance is often considered to be a constant, but in actuality may vary substantially for numerous reasons. It is also important to note that maintenance refers to the whole system, whereas components of the system may not equal zero. For example, a lactating animal may have a net loss of body tissue in combination with milk production, with the sum of those processes resulting in zero RE.

Although the relationship between RE and ME intake is nonlinear over the full range of intake (as shown in Fig. 2), the relationship is usually approximated by two linear relationships, which intersect at maintenance. Efficiency of dietary energy use below maintenance is high, in part because its efficiency of use is relative to efficiency of use of body tissue energy. Estimates of the efficiency of use of body tissues for maintenance or milk production are typically 0.80 to 0.90, whereas typical estimates of efficiency of ME use below maintenance (km) are about 0.70, but vary depending on dietary source, environmental temperature, etc. Efficiency of ME use above maintenance for milk production (kl) is similar to km, whereas efficiencies of use for accretion of body tissue (kg; 0.40), conceptus development (kp; 0.13), etc., are generally lower. Dietary differences, contributing to differences in metabolites available, and variation in functions for which those metabolites are used contribute

300 250 200 150 100 50 0 -50 -100

HE

FHP —

Maintenance *

50 100 150 200 250 300 Metabolizable Energy Intake

300 250

S P4

50 100 150 200 250 300 Metabolizable Energy Intake

Fig. 2 Relation of heat production and retained energy to metabolizable energy intake.

to variation in efficiencies of ME use by the animal. Some of those issues have been reviewed.[5'6]

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