Dietary Energy and Energentics

feedstuff energy

used to denote the value of feed for its primary function - to furnish energy for body processes and to form the non-nitrogenous, organic matter of tissues and secretions, functions in which all organic nutrients can take part.

measures of feed energy value:

Total digestible nutrients (TDN), digestible energy, metabolizable energy, net energy, roughage/concentrate ratio

Total digestible nutrients (TDN) =

digestible crude protein + digestible crude fiber + digestible nitrogen-free extract (starch and sugars) + 2.25*Digestible ether extract (fat) 

TDN is a general measure of 

the nutritive value (energy) of a feed 

the principal limitation for TDN is that

it does not account for other important losses, such as urine, combustible gases in the case of herbivores, and, most importantly, heat loss

Heat loss in TDN are relative larger for

roughages than for concentrates

a pound of TDN in roughage has considerably less value for productive purposes than 

a pound of concentrates

TDN is expressed as

a percentage of the ration of in units of weight (lbs or kg) and not as an actual caloric figure 

Energy is the amount of

heat produced when a compound is completely oxidized in the body, or as the lass of energy from the body.

Different units are used to measure energy:

calories, british themal units (BTUs), joules

in the U.S., the calorie (cal), kilocalorie (kcal) and megacalorie (Mcal) are commonly used but european countries use

the joule 

calorie is the amount of heat required to

raise 1 g of water from 14.5 degrees Celsius to 15.5 degress celsius

kilocalorie (kcal) =

1000 calories

megacalorie =

1000 kcal (1,000,000 calories)

gross energy

the total potential energy of a feedstuff consumed

gross energy is the amount of heat produced when

a feed is completely oxidized (burnt)

a known amount of substance is sealed in the bomb compartment, and20-30 atmospheres of oxygen are added. contents are ignighted with an electrical spark, and then 

the temperature rise of water bathing the bomb is measured 

Digestible energy equation

gross energy - fecal energy

digestible energy is a measure of the amount of energy

absorbed from a feed after it has been consumed and digested

digestible energy takes account of digestible losses and is subject to the same variables that

affect digestion and the same additional losses in metabolism as have been mentioned for TDN

Metabolizable energy is equal to digestible energy without

urinary energy and gaseous products of digestion, sometimes referred to as the usable portion of the ingested energy

net energy is equal to

metabolizable energy - heat increment

includes the amount of energy used either for maintenance only of

form maintenance plus production

NE_m+P =

maintenance plus production

NE_m =

maintenance only

NE_p =

production only (NE_g, NE_I, etc.)

Net energy - maintenance is a fraction of total NE expended to keep the animal 

in energy equilibrium, no net gain or loss of energy in the body tissue 

Net energy - maintenance may include:

Basal metabolism (BM), Energy of voluntary activity (VAE), heat to keep body warm (HBW), Heat to keep body cool (HBC)

Basal metabolism (BM)

chemical change that takes place in the cells of an animal in the fasting and resting state when it uses just enough energy to mantain vital cellular activity, respiration, and circulation (BMR)

Energy of Voluntary Activity (VAE)

amount of energy needed in getting up, standing, moving about to obtain food, grazing, drinking, lying down, etc. 

Heat to keep body warm (HBW)

additional heat needed to keep the animal’s body warm when the temperature of the enviroment is below critical temperature

Heat to keep body cool (HBC)

energy expended when the temperature of the environment is above themal neutrality; panting, respiration rate, heart rate, etc. 

Net energy - production is fraction of the total NE required (in addtioin to that needed for body maintenance) for

involuntary work, for tissue gain (growth or fat production), or the synthesis of a fetus, milk, eggs, wool, fur, feathers, etc. 

Fecal Energy (FE)

readily determined by collection of the solid fecal residue and use of a bomb calorimeter to measure gross energy of the feces; indigested feed residues, dead/sloughed cells of the intestinal tract, microflora residue, etc. 

Gaseous products of digestion (GPD)

these include combustible gases that escape the body during the digestion and absorption process; mainly consists of methane with traces of H, CO, H2S, acetone and ethane (difficult and expensive to measure)

Urinary energy (UE)

those portions of the absorbed nutrients that have served their useful purpose must be discarded, typically through the urine. some of this material is directly of feed orgin, and some of it is of endogenous orgin, arising from the catabolism of some of the tissues in the body.

Heat Increment (HI)

the increase in heat production following consumption of feed when the animal is in a thermally neutral environment; includes heats of fermentation (HF) and heats of nutrient metabolism (HNM)