Agriculture Reference
In-Depth Information
changing diets gradually; and avoiding environmental
stress. Feeding glucogenic compounds like sodium propio-
nate or oral doses of propylene glycol will reduce NEFA
formation at kidding and ketone body formation after
kidding and may alleviate ketosis. Injection of glucose
or glucose-forming compounds will temporarily elevate
insulin to suppress fat mobilization and ketone body
formation.
maintenance, growth, and production. Energy loss occurs
at each stage of digestion and metabolism. Readers are
referred to NRC (1981a) for more detailed information on
nutritional energetics. Gross energy is measured as the
amount of heat released when 1 gram of feed is oxidized
to carbon dioxide and water in bomb calorimeter. The GE
value of feeds is proportional to the carbon and hydrogen
contents of the feed's organic matter, carbohydrates, pro-
teins, and lipids. Because all GE is not available to animals,
GE is not precisely related to usefulness of a feed to an
animal.
Digestible energy (DE) is an index of the amount of feed
energy value presumably available for meeting an animal's
requirement for energy. Within the GI tract, available GE
is digested while waste is removed from the system as
feces (fecal energy [FE]). Apparent digestible energy is the
difference in energy between GE and FE. The term “appar-
ent ” refl ects the fact that some energy in feces does not
come from feed but instead represents inherent loss of
enzymes, sloughing tissues, and microbial cells that col-
lectively are called “metabolic fecal matter.” Depending
on the nature of feeds, DE can be as high as 80% for con-
centrate diets and as low as 50% for forage diets. Straw
may be even lower (45% DE).
Total digestible nutrients (TDN) is another index of
available feed energy that accounts for the higher energy
content of lipids. TDN is the sum of digestible nitrogen
free extract (NFE) (carbohydrates), digestible crude protein
(CP), digestible crude fi ber (CF), and 2.25 times the digest-
ible ether extract (lipids) for animals fed at an energy
intake equal to maintenance. TDN can be approximated
from DE, with 1 kg TDN being about 4.4 Mcal DE, or by
empirical equations using feed composition data such as
CP, neutral detergent fi ber (NDF), and acid detergent fi ber
(ADF) together with their predicted digestibility. The TDN
value of a feed may be lower when feed intake is above
maintenance due to reduced time for digestion of nutrients
by the GI tract. Total digestible nutrient values also appear
to overestimate the energy availability from diets rich in
fi ber relative to high concentrate diets (Moore et al., 1953).
Metabolizable energy (ME) deducts energy lost in urine
(UE) and energy lost in rumen gases (GasE) or methane
from DE. Compared with DE or TDN, ME more precisely
estimates the usable energy available to support tissue
accretion, milk, and conceptus. The energy loss in urine
(4-5% of GE) and heat of gases loss in fermentation
(4-5% of GE) can be considerable. The UE losses are
higher for ruminants than nonruminants because microbial
nucleic acid by-products as well as urea are excreted in
urine. Gaseous energy loss increases as the level of dietary
REQUIRED NUTRIENTS
Many nutrients are required in the goat's diet for metabo-
lism, both for maintenance of body functions, and for
production that includes tissue accretion (growth), repro-
duction, and production of meat, milk, and fi ber. The
National Research Council (2007) published an extensive
review of nutrient requirements of goats based on current
scientifi c information on goats. Where information about
goats was lacking, information from cattle and sheep was
used. The nutrient requirements for different classes of
goats are presented in Appendix B. For more detailed
information on nutrient requirements of goats, the reader
should refer to NRC (2007) and Sahlu et al. (2004).
Specifi c classes of nutrients include carbohydrates and
lipids that provide energy; protein or nonprotein nitrogen
that provide amino acids and energy; vitamins; minerals;
and water. Though often ignored, water is classifi ed as a
nutrient that is necessary for digestion, metabolism, and
products.
Energy
Energy, as fuel for the body, is defi ned as the potential to
do work. The international unit of energy is the joule (J);
however, the calorie (cal) is used most often in the United
States. One calorie, equal to 4.184 J, is defi ned as the amount
of energy required to raise the temperature of one gram
of water from 15.5-16.5°C at one atmospheric pressure.
This is the calorie, sometimes called a small calorie. One
calorie or joule is a small amount of energy; therefore, feed
or body energy utilization typically is expressed in terms
of kilo (1,000; kcal or kJ) or mega (1,000,000; Mcal or
MJ) calories or joules. All functions of the body including
prehension, digestion, and metabolism require energy.
Energy in the body is in constant fl ux. Amounts of energy
required will vary with breed, sex, age, climatic conditions,
and activity. Energy is expended for maintenance, growth,
reproduction, and formation of products.
F LOW OF E NERGY IN THE B ODY
Ingested energy (gross energy [GE]) represents input while
its destination is expressed as net energy (NE) for body
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