Introduction to Roughages
In the U.S., roughages comprise over 50% of all feedstuffs fed to livestock
animals. Roughages are plant-based feedstuffs. Technically, forage and
herbage are defined as plant materials available for consumption by an
animal. Technically, roughage refers to a feedstuff with a higher fiber
content forages. Practically speaking, the terms are used interchangeably.
The National Research Council classifies a roughage as a feedstuff with
a minimum crude fiber content of 18% and a maximum content of total digestible
nutrients (TDN) of 70%. Roughages provide a range of nutrients to animals.
Roughages also function to maintain and optimize the efficiency of the
GI tract for selected species. For selected species, fibrous carbohydrates
function to maintain structure, activity, and microbial population of
the GI tract, essential for optimal function of the GI tract. Roughages
are a link to the efficient utilization of earth’s resources. Roughages
alone are of minimal value to humans. However, roughages consumed by selected
species provide a means for conversion of relatively low-quality raw materials
to relatively high-quality products such as food and fiber that may be
used to fulfill human needs. Roughages may be fed either in a fresh, dried,
or ensiled state. Types of roughages used as feedstuffs include grazed
roughages (e.g. pasture and range), preserved roughages (e.g. hay and
silage), and crop residues and by-products (e.g. straw, stover, and hulls).
The following is a general introduction to roughages. It is important
to note significant exceptions to the generalizations do exist.
Roughages are the bulkier feeds in the ration; feedstuffs with less mass
per unit volume. Generally, the digestible energy contents of roughages
are low. The digestibility of other nutrients, such as protein, are also
relatively low. Roughages are high in fibrous carbohydrates such as hemicellulose
and cellulose. Fibrous carbohydrates are primarily present in the cell
wall of the plant cell. As fibrous carbohydrates are associated with the
structural components of plants, fibrous carbohydrates are often referred
to as structural carbohydrates. Roughages may also contain relatively
high amounts of lignin. Lignin content increases with plant maturity.
In a nutrition analysis, the fiber components of roughages may be expressed
as crude fiber, acid detergent fiber (ADF), and/or neutral detergent fiber
(NDF). Crude fiber contains cellulose and a portion of the lignin. ADF
contains cellulose and lignin. NDF contains hemicellulose, cellulose,
Figure 8.1 illustrates the relationship between the components of the
plant cell wall and the ADF and NDF analyses.
The plant cell contents also contribute to the roughage. The cell contents
include such components as nonfibrous carbohydrates, proteins, and lipids.
The nonfibrous carbohydrate content is comprised of simple sugars (i.e.
fructose, glucose, and sucrose), starches, and/or fructosans. The protein
component in forages is comprised of both true protein and nonprotein
nitrogen compounds. Protein content varies by roughage; from 2% up to
30% on a dry matter basis. In general, the protein content of legumes
is greater than the content of grasses. The mineral content of roughages
is influenced by roughage and mineral content of the soil. In general,
compared to concentrates, roughages are higher in calcium, potassium,
and microminerals and moderate to low in phosphorus. Legumes have a higher
calcium and magnesium contents compared to grasses. Regarding vitamins,
compared to concentrates, roughages are higher in fat-soluble vitamins.
Roughages are also a good source of the B-complex vitamins. Roughages
may contain one or more antinutritional factors such as alkaloids, cyanogenic
glycosides, toxic amino acids, and/or mycotoxins.
The nutritional value of roughages varies. In addition to other factors
such as plant species, the nutritional value of roughages depends on the
proportion of cell contents to cell wall components and on the extent
of cell wall lignification. Most roughages can be effectively incorporated
into at least one type of ration. Effective use of a roughage requires
matching nutrient requirements of an animal with the nutritional value
of a roughage. Effective use of a roughage also requires appropriate processing
As stated earlier, in ruminants, enzymes from rumen microorganisms are
required for the digestion of roughages. As the population of rumen microorganisms
is dependent upon the feedstuffs consumed, the composition of the diet
influences the extent and rate of digestion of roughages. Feeding of high-energy
feedstuffs has a negative associative effect on the degree of utilization
of a roughage. A negative associative effect occurs when the addition
of one feedstuff negatively influences the utilization of another feedstuff.
One of the primary species responsible for the digestion of roughages
is cellulolytic. The primary end-product of digestion is the acetate.
Acetate is a relatively weak acid. The primary end-product of fermentation
of high-energy feedstuffs is propionate. Propionate is a relatively strong
acid. An additional end-product of microbial fermentation of high-energy
feedstuffs is lactate. Lactate is also a strong acid. Compared to roughages,
the digestion rate and extent are higher and the resultant pH of the rumen
is lower for high-energy feedstuffs. The lower pH has a negative effect
on the microorganisms responsible for digestion of roughages; the cellulolytic
microbes are inhibited by a pH of 6.0 or lower. Therefore, the incorporation
of high-energy, high-nonfibrous carbohydrate feedstuffs decreases the
utilization of roughages. Management strategies to increase the utilization
of roughages include: 1) addition of buffers, such as bicarbonate, to
the diet; 2) increasing particle size of roughage to increase the production
of bicarbonate in the animal; and/or 3) reducing the rate of fermentation
of high-energy feedstuffs either by substitution with another feedstuff
or applying an alternative method of processing.
As with other feedstuffs, addition of roughages to rations is dependent
on the GI tract. As roughages are high in fibrous carbohydrates and microbial
enzymes are required for digestion of fibrous carbohydrates, utilization
efficiency of roughages is dependent on the site and extent of microbial
fermentation in the GI tract. Roughages are primarily added to the rations
of herbivores. The proportion of forage in the ration varies with species
and class of animal and also cost of feedstuffs. Based on the relatively
high utilization efficiency of roughages in the GI tract and roughages
are a source of fibrous carbohydrates to maintain optimal functioning
of the GI tract, generally, roughages are added to ruminant rations. Although
the utilization efficiency is less, roughages are also used in the rations
of horses. In the horse, the cecum is the primary site of microbial fermentation.
As the cecum is located posterior to the primary site of absorption, horses
may practice coprophagy or consumption of feces to increase efficiency
of utilization. For monogastrics such as swine and poultry, the low utilization
efficiency limits the use of roughages in rations. Roughages can be added
to the ration of swine with low nutrient requirements.