Plants serve as the major source of feed for
livestock. Nutrients required for maintenance and
production are provided by various plants and plant
derived feeds along with small amounts from non-plant
sources. The major constituents of plants are water,
carbohydrates, protein, fat, minerals and vitamins. Both
plants and animals contain these nutrients, but the
relative proportions vary more in plants.
Water content causes great variation in economic value
of feed because it dilutes the nutrients. A good example
is the value of alfalfa silage compared to alfalfa hay.
Wilted alfalfa silage contains about 65 percent moisture
while alfalfa hay contains about 12 percent moisture. Chopped alfalfa hay is worth $45 per ton, wilted alfalfa
silage is worth about $17.90.
Another frequently encountered problem is comparing
the value of dry vs high moisture corn, for example 14
percent moisture corn and 20 percent moisture corn. If 14
percent moisture corn is worth $2.50 per bu, then 20
percent moisture corn would be worth $2.33 per bu minus
any additional cost incurred due to its high water
content.
Comparisons of feeds should be on the basis of
nutrients contained per unit of dry matter, thereby
removing the influence of water. On the other hand, high
moisture feeds may be valuable in reducing dustiness in
some rations.
Proteins are complex compounds composed of amino
acids. These amino acids contain nitrogen in addition to
the carbon, hydrogen and oxygen, which constitute
carbohydrates and fats. When all the different proteins
are considered, their average nitrogen content is about
16 percent. For simplicity in evaluating feeds, the
nitrogen is measured and then converted to protein by
multiplying by 6.25 (100 divided by 16). In using this
method of estimating crude protein content in feeds,
non-protein nitrogen compounds such as nitrates and urea
are also credited toward the protein value.
Digestible protein is reported in some feed
composition tables. However, many requirement tables now
list only crude protein because it is more accurate for
calculating quantity of protein required in mixed
rations. It is also much simpler to use crude protein in
using individual feed analyses in ration formulation.
Analyses can be made that will help in determining the
presence of unavailable (indigestible) protein.
Feedstuffs, especially medium moisture silages, often
undergo heating during storage which causes some proteins
to become bound to carbohydrates. This protein is not
digestible. Analysis for acid detergent fiber nitrogen
can be used to adjust forage protein values for this
unavailable protein.
Microorganisms in the rumen assist in providing the
total protein and individual amino acid requirements of
ruminants. Microorganisms are able to synthesize protein
and amino acids from nonprotein nitrogen (NPN) compounds,
such as urea and ammonia. When the digestible energy
content of the ration is high enough, 1/3 or more of the
total protein needs of many ruminant rations may be
supplied by nitrogen from nonprotein nitrogen sources.
Growing and finishing cattle weighing over 550 lbs can
effectively use nonprotein nitrogen.
Young cattle and dairy cattle, however, have limited
ability to use NPN, apparently because the microbial
protein formed in the rumen is inadequate in quantity or
quality, or both.
When cattle over 550-600 lbs are fed high energy
growing or finishing rations, nonprotein nitrogen can
supply all the needed supplemental protein equivalent
needed without appreciably affecting rate of gain or
efficiency of feed utilization.
In low energy calf growing rations and beef cow
maintenance rations, urea is not very effective as a
protein substitute, especially when it is fed once daily
or less often. For dairy cow rations, some urea can be
fed in complete rations designed for medium and low
producers. Urea does not appear to be effective in
rations designed for high producers.
The concept of feeding higher levels of protein which
escape rumen degradation is relatively new and needs
further refinement and simplification before becoming
practical for ruminant ration formulation.
The protein content of roughages is an indicator of
digestible energy content. Roughage with lower than
average protein (for its kind) will usually be more
mature, higher in fiber and lower in total digestible
energy and vice versa.
Energy in feedstuffs is carried primarily in the
carbohydrate and fat fractions. Proteins also supply
energy when fed in excess of protein needs.
Carbohydrates
Carbohydrates supply most of the energy needed by
cattle. Carbohydrates make up 65 to 75 percent of the dry
weight of most grains, forages, and roughages. They
include sugars, starch, cellulose, hemicellulose and
lignin.
The two carbohydrate fractions commonly used in
evaluating the carbohydrate content of feed are crude
fiber and nitrogen-free extract (NFE). As crude fiber
increases, digestible energy usually decreases. In most
concentrate feeds, crude fiber is the less digestible
portion and NFE the most digestible portion of the
carbohydrates. However, in some roughages, the crude
fiber fraction is as digestible as the NFE. In fact, the
indigestible lignin of plants appears largely as NFE.
Although crude fiber is a poor absolute measure of feed
value of a roughage, it is related to digestibility of
roughages and therefore is of some useful value.
New methods of analyzing for fiber have been
developed. Neutral and acid detergent fiber clearly
differentiate between actual fiber (cellulose,
hemicellulose and lignin) and non-fibrous parts of the
plant. These methods are not widely used by the feed or
livestock industries, which still rely on the use of
crude fiber and NFE. They will be used increasingly as
more data are generated on forages.
Microorganisms in the rumen use fibrous materials such
as cellulose and hemicellulose as energy sources. Because
of bacterial fermentation of cellulose and hemicellulose
to fatty acids, ruminants can utilize roughages and
forages as sources of energy better than non-ruminants.
Feeds high in cellulose can furnish most of the
ruminant's energy needs when only small amounts of energy
are needed above maintenance.
In the past, a minimum amount of crude fiber (about
6.0 percent) was thought to be needed in ruminant rations
for proper rumen and gut function. Recent success with
high or all concentrate, low fiber fattening rations
points to the need for reappraisal of the fiber needs of
beef cattle.
Fat
Fat is a good source of energy and is analyzed in
complete feed analysis as ether extract. Fats in the
ration aid in the absorption and transportation of
fat-soluble vitamins (A, D, E, K). In concentrates, the
ether extract consists largely of glycerides of fatty
acids which provide about 2.25 times as much energy per
unit of weight as carbohydrates. However, in many species
of forbs and shrubs, such as sagebrush, various oils and
resins are present in considerable amounts, and in all
green forages considerable ether soluble pigments are
present. These are extracted by ether but do not furnish
available energy to the animal.
Fat is generally low in forages and most roughages,
but it can be an important source of energy in
concentrates. When animal fats are low in price, adding
fat up to 5 percent of the total ration may supply a
practical source of energy. The fat content of range
cubes may be important because of its effect on
palatability of the cube. Solvent processed meals,
without added fat, often produce a hard, unpalatable cube
which may be refused by livestock. Certain fats and fatty
acids have been effective in improving intake of energy
and milk yield of dairy cows.
Energy Values
Energy values are usually expressed in feed
composition tables as total digestible nutrients,
metabolizable energy, net energy for maintenance and net
energy for gain or lactation. These values are based on
digestion or balance trials for certain feedstuffs. For
others, values are calculated by formulas derived from
basic digestion or energy balance data. Because of this
and the fact that many feeds deviate from averages,
energy values should not be considered absolute. When a
feed appears to deviate substantially from normal,
adjusted energy values should be used in formulating
rations. The best estimate of the energy value of a feed
is that derived from determining in vitro dry
matter digestibility where this analytical service is
available. Energy values estimated from protein and crude
fiber are useful if appropriate formulas are used in
their calculation. The additional cost of a proximate
analyses does not appear justified for calculating the
energy value of a feedstuff.
Minerals remain as ash after the feed has been
completely burned. Ash shows the total mineral content of
the feed. It is used in determining the proximate
analysis of feeds and can be used to estimate
contamination of a feedstuff with dirt.
Minerals are needed to form skeletal structures, for
digestion, and in metabolic processes within the body.
Seven major and six trace minerals are known to be
essential for livestock, and several additional trace
minerals are believed essential. Although trace minerals
are required in very small amounts, adequate levels are
just as important as adequate amounts of the major
minerals.
Excesses of minerals may be harmful and cause lowered
production or even death (selenium, molybdenum and
fluoride are examples). Sodium and chlorine (both
supplied in common salt), calcium and phosphorus are
minerals most likely to be deficient in cattle diets. In
addition, cobalt, copper and zinc may often be deficient
in high concentrate rations if not supplemented.
Vitamins are needed by livestock for various body
processes in very minute amounts. About 14 different
vitamins are recognized to have specific functions in
animal metabolism. Under normal conditions vitamin A is
of major concern for producing cattle. Vitamin D
supplementation will likely be needed in housed animals
that get very little sunlight. The others are either
produced in the healthy ruminant within the body, by
rumen bacteria, or are normally present in natural
feedstuffs.
Properly harvested current year's forage contains
carotene, which is converted to vitamin A by the normal
body processes. Cattle fed liberal amounts of good
quality current year's roughage usually require no
supplemental vitamin A. Prolonged storage of feeds or
excessive bleaching during curing destroys carotene.
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