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Silage Harvesting, Storing, and Feeding1
A. T. Adesogan and Y. C. Newman
2
Silage
can be a convenient and economical feed for the cattle industry. Some
producers routinely produce silage, but others only produce silage when
field drying is difficult or impossible. Crops such as corn and sorghum
are not ideal for hay production because they contain considerable
moisture at the optimal harvest time, and their thick stalks delay
drying. Warm-season grasses like bermudagrass and bahiagrass can be
harvested as hay, but, in Florida, frequent rains often delay harvesting
and curing and can lead to extensive losses of dry matter (DM) from the
hay. Harvesting such grasses as silage may reduce harvesting losses and
allow a more timely harvest, thereby improving forage quality.
Silage
is high moisture forage, stored in the absence of oxygen and preserved
by acids produced during the fermentation. During ensiling, bacteria
ferment sugars in the plant to organic acids that lower the pH of the
silage to levels that inhibit the growth of undesirable organisms. The
silage remains preserved as long as air is kept out because
spoilage-causing yeasts in silages remain dormant in the absence of
oxygen. Entry of oxygen into the silo revives the yeasts and may cause
spoilage.
This publication discusses the advantages,
disadvantages, and phases of silage fermentation and the factors
affecting silage quality.
Properly made silage has several advantages over hay, including the following:
• Lower probability of weather-related damage or delays during harvest
• Lower field, harvest, and storage losses
• Greater flexibility and fit for many livestock feeding programs
Disadvantages include:
• Higher moisture content results in heavier forage that is less economic to haul
• Requires specialized equipment for harvesting, storing, and feeding operations
• High loss potential if silage is not made well
• Less marketable if the silage is not fed on the farm
• Shorter shelf life after the silo is opened
Crops for Silage
Many crops grown in Florida can be preserved as silage or haylage
(round-bale silage; discussed later). The type of livestock, available
machinery, soil type, rainfall, availability of irrigation, and
potential yield are important considerations in deciding which crops to
plant and store as silage.
Corn silage is usually considered the
best silage because of its high energy concentration, which can be used
to optimize animal performance. The best time to harvest corn silage was
previously thought to be when the grain is denting and the milk line
has moved 1/2 to 2/3 of the way down the kernel (Table 1). However, the
kernel milk line should no longer be the only index used to predict
harvest dates. This index can give misleading predictions of optimal
harvest time due to the presence of new attributes in modern corn
hybrids like high stay-green. Decisions on time of harvest should be
based on oven dry matter (DM) measurement, and corn should be harvested
for silage when the DM concentration is between 30% and 35%.
Alternative, less accurate methods for measuring DM concentration
include using a Koster moisture tester or a microwave oven.
Good
silage can also be made from forage sorghum, sorghum-sudan hybrids,
soybeans, and other warm-season annuals, but they are lower in energy
than corn silage. Forage sorghum is usually direct cut, but the moisture
content at optimal maturity for harvest is often too high for proper
ensiling. Therefore, choose hybrids with low moisture ratings when
possible, and aim to harvest when the DM concentration is 30%–35%.
Sorghum-sudan hybrids, soybeans, and cowpeas usually have high moisture
concentrations at optimal maturities. Therefore, wilting is often needed
to ensure good silage quality.
Excellent silage can also be made
from cool-season annuals, such as small grain cereals and ryegrass when
they are harvested at optimal maturities. Wilting is advisable if the DM
at harvest is less than 30%. Silage production from bermudagrass,
stargrass, limpograss, perennial peanut, and alfalfa is also feasible.
However, low sugar contents and high buffering capacities (resistance to
change in pH) make it more difficult to ensile these crops.
Consequently, these crops should be wilted to 35%–45% DM to concentrate
the sugars they contain before they are ensiled.
Phases of Silage Fermentation
The ensiling process takes several days and can be divided into five
phases. Each of the five phases is characterized by different changes in
the forage.
•
Phase 1: This phase begins from the time of
harvesting the crop to the time oxygen is depleted from the sealed silo.
Due to continued plant respiration, plant enzymes and aerobic bacteria
cause nutrient losses by degrading plant proteins and converting sugars
into carbon dioxide and water, and generating heat. The heat increases
the silage temperature by 15 to 20°F or more, depending on the amount of
air available. This phase progresses until the oxygen is depleted. It
takes a few hours ideally but can last for up to 48 hours in poorly made
silages. Harvesting at the correct plant maturity, chopping to the
right particle length, packing the silo tightly, and sealing within 12
hours of harvesting are key to minimizing nutrient losses during this
phase.
•
Phase 2: This phase starts after the oxygen is
used up, when anaerobic bacteria begin to ferment plant sugars into
organic acids, alcohols, carbon dioxide, and nitrogenous compounds. The
organic acids lower silage pH from above 6 to about 5. If silage pH
drops slowly and the moisture concentration is high due to harvesting
too early, clostridial bacteria may grow. These bacteria degrade sugars
and convert lactic acid to butyric acid, releasing strong offensive
odors. They also break down protein to nonprotein nitrogen and
undesirable end products like amines. These changes lead to increased
dry matter loss and reduced palatability and quality of the silage.
•
Phase 3:
Once the pH falls below 5, the lactic-acid-producing bacteria (LAB)
dominate the fermentation and reduce the pH to about 4 or 4.5 in
well-made silages and haylages (round-bale silage), respectively. Lactic
acid is more effective than other organic acids at reducing the pH.
Therefore, LAB that ferment sugars to lactic acid alone (homolactic LAB)
are more efficient at causing a fast pH drop and preserving nutrients
than others (heterolactic LAB) that produce lactic acid and other
products. This phase can last for three days to four weeks, and it ends
when fermentable sugars are depleted. This phase is often limited in
warm-season grasses and other forages with low sugar concentrations
(less than 5%–8% of dry matter) because of inadequate amounts of sugars
for the fermentation.
•
Phase 4: Once the pH drops to or
below 4.0 (4.5 in haylages), the silage becomes stable, and the growth
of undesirable microbes is prevented. The quality of the silage can be
maintained for the rest of the storage duration if the silo remains
sealed, and air does not penetrate the silo.
•
Phase 5:
This phase begins after aerobic conditions are restored once the silo is
opened during feedout. After air penetration occurs, yeasts and molds
that were dormant during the fermentation are revived. These fungi use
sugars, lactic acid, and other nutrients for growth and produce carbon
dioxide and heat as byproducts. Excessive heat accumulation denatures
proteins and other nutrients in the silage. Collectively, these changes
increase DM losses and reduce silage quality. Molds on the silage may
also produce mycotoxins that when consumed reduce animal performance and
cause various diseases. To prevent these problems, excellent management
is necessary, particularly in bunker or drive-over pile silos. The
silage should be fed out at a rate of at least 8–12 inches per day, and a
straight face should be maintained with shavers. Additives or
inoculants that hinder the growth of spoilage yeasts and molds will
increase bunk life and preserve the quality of the silage.
Factors Affecting Silage Quality
Several factors influence the fermentation, preservation, and quality
of silage. These include sugar concentration, buffering capacity, DM
concentration, chop length and fineness, temperature during ensiling and
storage, rate of harvest, packing density, and air exposure during
harvest, storage, and feeding.
Sugar Concentration and Buffering
Water-soluble carbohydrates (mostly sugars) are used up during plant
respiration until the oxygen that remains in the forage mass is
depleted. These sugars are the primary carbohydrates fermented to lactic
and acetic acids by bacteria to produce a low pH and stable silage. In
general, forages with less than 5%–8% water-soluble carbohydrates in the
DM may not reach a pH low enough to produce stable, high-moisture
silage. Corn, sorghum, sorghum-sudan hybrids, and cool-season annual
grasses usually have sugar concentrations above 5% dry matter (Table 1),
and a good, stable silage is often achieved. Forage crops, such as
warm-season perennial grasses and legumes, have low sugar
concentrations, and the high protein concentration of legumes buffers
(slows) the pH decline from 5.5 to 4.5 during ensiling. Consequently,
these forages are more difficult to ensile and should be wilted to
35%–45% DM before ensiling, which can be challenging if bunkers are
used. Additive or inoculant application may also aid the fermentation of
such forages.
Dry-Matter Concentration
Harvesting should be planned for dry days because small amounts of
rainfall can reduce silage quality. Forages that have excess (>70%)
or inadequate moisture (<45%) may not ensile well for different
reasons. Higher moisture concentrations can result in greater seepage
losses and possible pollution of nearby water bodies. Such high-moisture
silages are also more likely to undergo a clostridial fermentation,
which leads to high DM losses, protein degradation, high butyric acid
concentrations, and reduced palatability. Wilting high-moisture forage
to at least 35% dry matter is a good practice that reduces clostridial
fermentations. Wilting usually results in good silage particularly when
sugar concentration is low, and buffering against pH decline is high.
Wilting is usually necessary before ensiling bermudagrass, legumes,
sorghum-sudan, and millet forages because these forages are often only
20%–25% dry matter at the time of cutting.
Forages harvested late
or those with low moisture concentrations often present packing
problems. In such cases, air pockets trapped within the forage mass are
more difficult to exclude because the dry stems hinder packing. Yeast
and mold growth in the air pockets can lead to increased temperatures
and poorer fermentation. Therefore, forages should be harvested for
silage at recommended moisture concentrations and maturities (Table 1).
Water should be added to high DM silages (>55%) to provide adequate
moisture for the bacteria and to improve packing.
Chop Length and Processing
Precision-chop forage harvesters with sharp knives should be used to
achieve a chop length of 3/8 inch for unprocessed corn silage and 3/4
inch for processed corn silage. Processing helps ensure proper
utilization of the energy in the corn kernel. Processing is advised for
flint corn or hybrids with high dry-down rates, high stay-green
rankings, high vitreousness, or hard kernels. Processing can increase
starch digestibility by about 5 percentage units, which can lead to over
1 lb of extra milk produced per day. The roll clearance of processors
should be set to 1–3 mm because inadequate processing may not
sufficiently damage kernels and release the energy-dense starch, whereas
excessive processing can reduce fiber digestibility and predispose cows
to acidosis. Processing forage sorghum hybrids before ensiling has not
shown consistent benefits in research trials.
Packing Density
Silage shrinkage (DM losses) increases as packing density decreases,
and poor packing density can also reduce the effectiveness of silage
inoculants. A target packing density of 15 lb of DM per ft
3 (43 lb of fresh forage per ft
3
if silage is 35% DM) is required to minimize shrinkage. Kansas State
University research reported that the optimum packing density can be
achieved by aiming for a packing time of 1–4 minutes per ton and using
delivery rates of about 30 tons/hour (wet weight). Delivery rates of
over 60 tons/hour will lead to packing times less than 1 minute/ton,
which can reduce packing density. High delivery rates that leave
unpacked silage overnight should be avoided. A spreadsheet for properly
managing bunker filling is available at
www.uwex.edu/ces/crops/uwforage/storage.htm.
Temperature
The optimal internal temperature during fermentation is below 100°F.
Higher temperatures often result in poorer-quality silage. Temperatures
above 100°F could reduce the fermentation quality, enhance protein
degradation, and reduce the rapid pH decline necessary for an efficient
fermentation. Excessively heated or heat-damaged silages have a brown to
dark brown color with a tobacco-type smell. Part of the protein in
"heat-damaged" silages is complexed with carbohydrates and is less
digestible. The concentration of heat-damaged protein depends on both
the temperature and the length of time the temperature is elevated.
"Heat-damaged" silage may be palatable, but part of the protein and some
of the energy it contains will be unavailable to livestock.
Air Exposure
Minimizing air (oxygen) infiltration during ensiling is essential for
making good quality silage. The presence of air in the forage mass
after harvest allows the respiration process to continue, and this
depletes sugars essential for the fermentation. Air exposure during
storage leads to yeast and mold growth on and beneath exposed surfaces.
Air exposure at feeding also results in rapid yeast and mold growth,
heating, and reduced palatability. Bunkers and bags should be sealed on
the day of harvest to prevent subsequent spoilage and quality losses.
After sealing, bag and bunker plastic integrity should be examined
frequently, and any holes or splits should be immediately sealed with
proper waterproof silage tape. Tire sidewalls (that are touching) should
be used to weigh down the plastic and exclude air from bunker or
drive-over-pile silos.
After opening a silo, silage at the silo
face should be fed within 24–48 hours. Higher temperatures during the
summer increase aerobic spoilage and reduce bunk life of the silage.
During feeding, silage should be covered or left tightly packed until
fed, and at least 8–12 inches of the exposed surfaces should be removed
daily.
Additives
Many different additives can be used to improve silage fermentation
or provide supplementary nutrients to cattle. Forages such as corn or
sorghum usually do not need additives to improve the fermentation
provided they are harvested at the correct maturity stage and properly
ensiled. However, additives are usually necessary to enhance the aerobic
stability (bunk life) of such forages. Additives are also important for
improving the fermentation and aerobic stability of forages that are
difficult to ensile such as warm-season grasses and legumes.
Carbohydrate Sources
Molasses can be used to add fermentable sugars to forages low in
sugars, such as warm-season grasses and legumes. Adding 40–100 lb of
molasses/ton of forage at ensiling can increase the fermentation rate by
increasing organic acid production and lowering the pH. Other
high-energy ingredients, such as ground corn and citrus pulp, may be
added to increase the dry matter concentration in wet, warm-season
grasses or legumes, and they often increase the energy value of the
silage. However, molasses and other sugar or high-energy sources should
not be added to corn, sorghum, or cool-season grasses at ensiling
because the excess sugar availability will likely stimulate the growth
of yeasts and the incidence of spoilage.
Bacterial Inoculants
Forages naturally contain up to 100,000 lactic acid bacteria/gram of
plant. However, various types are present, and their ability to
efficiently ferment sugars into lactic acid and rapidly drop the pH is
usually low. Therefore, adding commercially-available silage inoculants
containing selected strains of homolactic bacteria that enhance acidity
can improve the fermentation and minimize nutrient and DM losses. Ideal
homolactic inoculants should supply at least 100,000 live bacteria/gram
of silage. Beneficial effects of such inoculants are often more common
in forages with low sugar and DM concentrations, such as legume and
grass silages, than in corn silage.
One of the limitations of
using homolactic inoculants is that they may not reduce the incidence of
aerobic spoilage (heating) when the silo is opened. Heterolactic
inoculants containing
Lactobacillus buchneri bacteria enhance
the production of the antifungal agent, acetic acid. Such inoculants are
therefore usually effective at reducing the growth of yeasts and molds
and increasing aerobic stability of the silage. 'Combo' inoculants,
which contain both homolactic inoculants and heterolactic
L. buchneri
bacteria, aim to enhance both the fermentation process and aerobic
stability. Recent studies have confirmed their effectiveness on corn
silage and bermudagrass silage.
Acids
Propionic acid reduces molding, heating, and aerobic deterioration
and is an effective forage preservation agent. Adding acids such as
propionic, sulfuric, or formic acid decreases the pH rapidly and
improves the preservation, but the corrosiveness of these acids has
discouraged widespread use. Several buffered propionic acid additives
are commercially available that are less corrosive than the pure acids.
Enzymes
Forages with marginal concentrations of sugars may benefit from
addition of enzymes that can break down complex plant carbohydrates to
simple sugars, which then can be fermented to lactic acid. Enzymes such
as amylases, cellulases, and pectinases can break down starch,
cellulose, and pectin, respectively, in the forages. Most commercially
available enzymes are mixtures of several enzymes produced from Bacillus
and Aspergillus organisms. Although adding enzymes to forages that are
difficult to ensile holds promise, enzyme addition has not consistently
improved the ensiling quality of forages in research trials.
Nitrogen Sources
Ammonia and urea are sources of nonprotein nitrogen used to increase
the crude protein concentrations of corn, forage sorghum, and other
silages that are low in protein. Adding 5–10 lb of anhydrous ammonia/ton
or 10–20 lb of urea/ton can increase crude protein concentration by 3–7
percentage units in the silage DM. Higher rates of ammonia application
should not be applied to silage because of the risk of formation of a
compound that is toxic to cattle, especially very young livestock.
Ammonia also inhibits growth of molds, therefore, ammonia-treated
silages are less prone to heating and have a longer bunk life. Ammonia
can be added at the chopper, blower, or bagger depending on the
situation. Major drawbacks of ammonia are its volatility and
corrosiveness, which pose risks to operators handling this chemical.
