Crop residues
In various regions of the world, large quantities of crop residues are
available as animal feedstuffs. The specific type of residue varies by
location and examples of crop residues include straws, stover, hulls,
and various standing feeds. The decision to feed crop residues requires
consideration for factors such as nutritional value, collection, storage,
treatment, transportation, feeding, availability, and economics. As a
result of the low nutritional value of crop residues, the preparation
and feeding of the residues must be relatively low-cost for the feeding
of crop residues to be economical. In various regions, crop residues have
the potential to be a significant alternative feedstuff. Crop residues
may be grazed, processed as a dry feed, or ensiled and fed as silage.
In addition to use as a feedstuff, crop residues may also be used as bedding
and a soil amendment.
In general, crop residues are low-quality roughages. Specifically, the
general nutritional characteristics in crop residues include: low digestibility,
low crude protein content; low digestible energy content; high structural
carbohydrate content; high lignin content; low mineral content; lower
digestibility of minerals; and low vitamin content. Table 7-38 on page
160 of the text describes the nutrient content of number of low-quality
roughages. In addition, Table 7-38 compares the nutrient content of the
low-quality roughages to a few high-quality roughages such as alfalfa
hay and vegetative grass pasture. In addition, generally, crop residues
are unpalatable and are bulky (low mass per unit volume).
Use of low-quality roughages as feedstuffs is limited to mature ruminants
and horses with low nutrient requirements. The limited nutrient yield
from the digestion of the roughages limits the use for species and classes
with higher nutrient requirements. The basis of the limitation is a result
of the contents of structural carbohydrates, lignin, and crude protein.
The digestibilities of structural carbohydrates and lignin are low. Low
digestibility decreases rate of digestion and therefore passage rate.
In turn, low passage rate decreases consumption. Table 7-39 on page 160
of the text describes the effect of roughage quality on consumption, digestibility,
and rumen turnover rate. Rumen turnover rate is directly related to passage
rate. In addition, low digestibility also decreases the total nutrients
derived from the roughage. An additional factor limiting the digestibility
of low-quality roughages is the low crude protein quantity and quality
of the roughages. To digest the structural carbohydrates in the roughages,
the rumen microorganisms require a nitrogen source. Limitation of nitrogen
will limit metabolism and growth of the microbial population of the rumen,
reducing the digestion of the structural carbohydrates from roughages
and therefore production of energy-yielding compounds from the roughages.
In general, the minimum crude protein level required to maintain rumen
functioning is 8%. In times of inadequate dietary nitrogen, for a short
period of time, the ruminant has physiological mechanisms to recycle nitrogen
in the body and fulfill nitrogen requirements.
In general, supplementation is required to effectively use low-quality
roughages as a feedstuff. In general, low-quality roughages are supplemented
with protein, energy, minerals, and vitamins. Crude protein or nitrogen
supplementation promotes microbial activity. Factors to consider when
selecting a protein supplement include protein content, solubility, and
digestibility. True protein sources are preferred over nonprotein nitrogen
sources.
Low-quality feedstuffs may be processed or treated to improve digestibility,
passage rate, and therefore consumption. In addition, increasing digestibility
of nutrients increases the nutritional value of the roughage. The following
is a general discussion of the physical and chemical methods used to increase
the digestibility of crop residues.
Physical processing methods for low-quality roughages include methods
such as pelleting, chopping, and grinding. In addition, high pressure
steam applied to low-quality roughages increases the digestibility of
the roughage. Two of the primary chemical treatment methods for low-quality
roughages include treatment with sodium hydroxide (NaOH) and ammonia.
Treatment of crop residues with alkaline compounds increases the digestibility
of the roughage. Alkali dissolves the lignin. As a result, the structural
carbohydrates such as cellulose are available to be digested by microorganisms.
Sodium hydroxide (NaOH) increases the digestibility of the roughage. Sodium
hydroxide can be applied to dry roughages or to roughages prior to ensiling.
A few of the disadvantages of use of NaOH are: 1) generally, not economical;
2) increases sodium provided to the animal; 3) sodium contamination to
soil; and 4) concentrated NaOH is a hazardous chemical. Another treatment
to improve the nutritional value of poor quality roughages is the use
of ammonia. Ammonia is also an alkaline or basic compound. One method
is the use of anhydrous ammonia. Anhydrous ammonia is applied to stacked,
covered roughages. Several weeks are required from treatment to feeding.
The higher the ambient temperature and the moisture content of the roughage,
the more rapid the rate of the reaction. To achieve the maximum benefit
of the treatment, provision of an energy source to the animal is required.
Anhydrous ammonia should not be applied to high-quality roughages with
high nonstructural carbohydrate contents. In comparison to sodium hydroxide,
anhydrous ammonia is less hazardous and more economical to use and provides
a source of nitrogen. In comparison to use of ammonia, in general, use
of NaOH provides a greater increase in digestibility.
As a feedstuff, low-quality roughages may be fed to mature ruminants and
horses and in a few additional situations. Given the ration is formulated
to fulfill the nutrient requirements of the specific animal, low-quality
roughages may be added in lesser amounts in some rations. Low-quality
roughages may also be added to high-concentrate rations. The low-quality
roughage will provide a cost-effective source of structural carbohydrates
to maintain functioning of the gastrointestinal tracts of ruminants and
horses.
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