Composition of Herbage
The chemical composition and subsequent nutritional value of a forage
is complex and is influenced by a number of factors. The primary factors
are plant species, environmental and soil conditions, and/or plant maturity.
The following section will briefly discuss, in general terms, the chemical
composition of forages.
The nitrogen component is expressed as crude protein content. Crude protein
measures both protein and nonprotein nitrogen compounds in forages. Crude
protein content is influenced by plant species, environmental and soil
conditions, and stage of maturity. In general, the crude protein content
of legumes is higher than the crude protein content of grasses. For legumes,
in general, the range of crude protein is 15-30% on a dry-matter basis.
For cool-season grasses, the content is lower at 10-20% on a dry-matter
basis. For both legumes and grasses in specific situations, crude protein
content can either be lower or higher than the given range. For grasses,
crude protein content in the forage is dependent upon nitrogen fertilization.
In general, the true protein accounts of 75-85% of the nitrogen. As the
plant matures the portion of nitrogen as true protein increases. In excess
of 200 NPN compounds have been identified across forages. Factors influencing
the content of NPN compounds include environmental and soil conditions
and plant species. Legumes contain higher contents of nonprotein amino
acids compared to grasses. For nonruminants, the nutritional value of
nonprotein amino acids is approximately half of the nutritional value
of protein amino acids. In contrast, for ruminants, nonprotein amino acids
are similar in nutritional value to amino acids present in protein molecules.
The next component is the carbohydrate component. The carbohydrate component
may be divided into nonstructural or structural carbohydrates.
Nonstructural carbohydrates are the readily available carbohydrates in
the forage. In forages, the concentrations of mono and disaccharides are
relatively low; generally, less than 8% on a dry-matter basis. The primary
mono and disaccharides in forages are glucose, fructose, and sucrose.
The primary nonstructural polysaccharides in forages are starch and fructosans.
Polysaccharides are the stored energy for the plant. In general, the primary
nonstructural carbohydrates in legumes are sucrose and starch. For warm-season
grasses, the primary nonstructural carbohydrate is starch. And for cool-season
grasses, the primary nonstructural carbohydrate is fructosans. If harvested
at similar stages of growth, total nonstructural carbohydrate content
will be similar between cool-season legumes and grasses. The total nonstructural
carbohydrate content varies by plant species, environmental conditions,
and soil fertility. As a correction to the text, nonstructural carbohydrate
content in forages is highest at the end of the day (i.e. following the
photosynthetic daylight hours). Therefore, to increase the nonstructural
carbohydrate content at harvest, forages are harvested following the daylight
period.
The structural carbohydrates are comprised of polysaccharides. The three
classes of structural carbohydrates in forages are pectic substances,
noncellulosic polymers such as hemicellulose, and cellulose. Noncellulosic
polymers and cellulose are digestible only by microbial enzymes. The structural
carbohydrate content varies by species, and environmental conditions.
As legumes mature, the most significant change in structural carbohydrates
is the increase in stem cellulose. For grasses, with maturity, stem cellulose
and hemicellulose increase.
The next chemical component of forages is lipids. In the leaf tissue,
the lipid content ranges from 3-10%. The lipid content decreases with
maturity. The majority of the lipids are galactolipids and phospholipids.
Regarding fatty acids, 60-75% is linolenic acid. Upon disruption of the
outer surface of the plant, unsaturated lipids are susceptible to oxidative
processes.
The next component is organic acids. In general, the organic acid content
of forages is not reported specifically on the analysis. In general, organic
acid content is included with additional chemical components on an analysis.
Organic acid content is included in the nitrogen-free extract portion
of the proximate analysis and the cell contents or solubles portion of
the detergent fiber system. Most of the organic acids in forages are nonvolatile.
The organic acid content in forages ranges from 2-8% on a dry-matter basis.
Organic acid content varies by plant species, maturity, and fertilization.
Organic acids play a role in the palatability of a feedstuff.
Table 7-7 on page 126 of the text describes the range and typical mineral
concentration for grasses and alfalfa. Note: the information on the table
does not include all macro and microminerals provided in forages. Both
grasses and legumes are high in potassium. In comparison to grasses, legumes
contain a higher content of calcium, magnesium, sulfur, and copper. Grasses
contain higher contents of manganese and zinc. Both grasses and legumes
are moderate to low in phosphorus. The mineral content of forages is variable.
Factors influencing the mineral content of forage include plant species
and soil and environmental conditions.
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