Management and Storage Alternatives for Corn Silage
by Nathan A. Pyatt and Dr. Larry L. Berger
Whole-plant corn silage (Zea mays L.) continues to be a major forage
and energy source in the North American cattle industry (16). In 2000, roughly
7.5% of the US corn acreage was harvest as silage, totaling just less than
2.4 million hectares (36). Corn silage is a palatable, consistent, yet versatile
wet forage crop that requires less labor in production when compared to hay.
Feed quality characteristics become dependant on a number of environmental
and managerial factors, such as climate, harvesting DM content, maturity,
mechanical processing, storage system, storage length, fill and feed-out rates,
surface area exposure, additives used, etc. The purpose of this review is
to summarize literature focusing on harvest, storage, and feed-out practices
required to maximize the value of corn silage.
A harvest management plan should be in place to ensure that the silage is
harvested at the appropriate time to prevent unnecessary losses in DM and
nutrient quality (17). Corn silage harvest can be completed in approximately
two weeks, however feeding of the stored silage can take place for the remaining
50 weeks of the year. Therefore, poor planning at harvest and in storage
may take a serious toll on productivity, animal health, and profitability
all season long. Corn should be harvested for silage after the corn is well
dented, but before the leaves turn brown and dry. Bal et al. (3) reported
optimum maturity stage for corn harvest was 2/3 milk line (ML), with some
flexibility between 1/4 and 2/3 ML. Quality and quantity measures are at their
peak during this stage of development. Bal et al. (3) reported lowest
digestibility of DM (58.0 vs. >64.9% DDM), organic matter (OM),
protein, acid detergent fiber (ADF), and starch for corn silage harvested
at black layer maturity, when compared to 2/3 ML. Corn silage cut late has
brown/dead leaves and stalks, which in turn will sharply decrease total production
per hectare and animal performance. Field losses from dry leaves can be as
high as 30%, resulting in a 10% reduction in total DM stored. For each advancing
day of forage maturity beyond 2/3 ML, approximately 1% more concentrate was
needed to maintain milk production (8). Cleale and Bull (8) reported a 19
day delay in harvest resulted in a 40% reduction in the rate of disappearance
of the potentially DDM.
Climatic factors may also contribute to maturity and storage concerns. Warm
weather accelerates drying, thus narrowing the optimum maturity window. McGechan
(22) reported that weather conditions alter grass sugar content up to 1%,
changing buffering capacity and ensiling characteristics drastically. Additionally,
Weinberg et al. (38) reported corn silage stored at elevated (37-41oC)
ambient temperatures resulted in unfavorable ensiling characteristics (higher
pH and DM losses, and less lactic acid) and less aerobic stability when compared
to silage stored at room temperature (< 33oC). Likewise,
Ruppel et al. (31) found that an elevated temperature at filling was
correlated to greater DM loss, change in non-starch carbohydrate (NSC), and
greater pH and ammonia concentration.
Among environmental and management factors worthy of consideration in maintaining
corn silage quality, Bolsen et al. (5) recognized the degree of anaerobiosis
as the leading concern. Wet crops, stored directly or with minimal drying,
can be preserved under anaerobic conditions in a silo (storage structure),
through a four-phase process known as ensiling or anaerobic fermentation (27).
In the pre-seal phase (day 1), fresh chopped forage cells continue to respire
and aerobic/epiphytic bacteria (naturally occurring in forage) begin to breakdown
water soluble carbohydrate (WSC) to yield carbon dioxide, water, and heat.
Plant DM and WSC losses persist until anaerobic conditions are established.
Respiration losses are considered to be unavoidable (27). Plant enzymes hydrolyze
starch and hemicellulose to monosaccharides yielding extra sugar substrate
for both aerobic and anaerobic bacteria. Plant enzymes also reduce feed value
by metabolizing protein to non-protein nitrogen (NPN) under aerobic conditions
As the oxygen supply trapped among the forage particles is metabolized, phase
two, active fermentation begins (day 2). Active fermentation utilizes desirable
bacteria to eliminate oxygen, reduce pH, and inhibit undesirable bacteria
growth. Lactic acid producing bacteria (LAB) and acetic acid producing bacteria
feed on WSC yielding their respective organic acids (day 3). LAB proliferation
(day 4-7) yield adequate amounts of lactic acid to inevitably reduce the original
pH (6.0) of the forage mass to a stable pH range of 3.8-4.5 (day 8-21) (4).
Quantitatively, the amount of acid required to drop the original pH 6 to a
stable pH 4 is dependant on the silages DM content and storage system. Once
the stable phase is reached, pH inhibits all bacterial action (> day 21)
and preservation of the forage occurs until the feed out phase.
Physical characteristics of corn silage can lead to poor fermentation characteristics.
If insufficient amounts of lactic acid are produced, butyric acid production
becomes evident with a foul odor, usually associated with spoilage (24).
Poorly fermented silage appears dark green, with a strong odor, slimy soft
tissues, and will commonly have a pH > 5. Overheated silage appears
brown/black, with a caramel odor or slightly burned sugar odor due to Maillard
reaction products. Properly heated silage appears light green or yellow,
with a slight vinegar odor (acetic acid), firm plant tissues and a pH <
Temperature of the silage mass throughout ensiling becomes dependent on the
efficiency of transition between aerobic to anaerobic environment. As cells
respire and aerobic bacteria conduct proteolysis, temperatures of the mass
steadily increase above ambient temperature. Silage mass temperatures above
the normal range (10-40°C) have elevated proteolysis
rates, resulting in increased NPN production (27). Bates (4) reported temperatures
in the pre-seal phase increased from 21°C to 35°C within the first two days
of ensiling. However, as anaerobic conditions developed, temperatures stabilize
between 26.5-29.5°C by days 4-7.
Storage Dry Matter
Next to the rate of oxygen exclusion, forage characteristics (e.g. DM content,
maturity, WSC content, forage type) at the time of ensiling are likely to
be the predominant factor that dictates the final quality of corn silage.
Recommendations for harvesting DM of whole plant corn silage range between
30-40% DM. Fermentative efficiency is greater for silages with low DM contents
(20-29% DM), however effluent losses are substantial. Corn silage with moisture
contents greater than 70% are prone to excessive effluent or seepage losses
due to the hydraulic pressure occurring in most storage systems. In the case
of excessive moisture, seepage losses will peak by day 4 of ensiling. Effluent
poses two problems, silage nutrient losses and pollution effects (20). Mahanna
et al. (20) reported nutrient composition in lost effluent as 20% nitrogenous,
55% non-nitrogenous organic matter, and 25% mineral. Gordon (11) noted a
6.8-13% DM loss in seepage from silage ensiled at 77-82% moisture. Similarly,
McGechan (22) indicated the DM loss associated with seepage averaged 5.8%
(range 3.2-8.8%) in grass silage. However, corn silage produces less effluent
than grass at similar DM, therefore seepage losses would likely be lower than
this value. Mahanna et al. (20) also reported that silage seepage
has 180 times greater biological oxygen demand (BOD) than that of domestic
sewage. One thousand tons of silage reportedly has an equivalent BOD of a
city with 250,000 people.
Silages with relatively high DM (> 40%) at ensiling have no effluent loss,
but have less efficient rates of fermentation. High DM silages are often
difficult to pack, resulting in trapped oxygen pockets within the mass, allowing
greater plant cells respiration. A reduction in fermentation end products
results with increasing silage DM level (6).
The ensiling process also emits several gaseous end products of fermentation.
Gas concentrations, including NO2, N2O4,
NO, CO2, NH4, and CH4, are highest 12-72
hours after ensiling, but can persist up to 10 days post-fill. These emissions
are often overlooked in terms of total storage losses. Air tight or lowly
ventilated storage systems present a concern for worker safety due to dangerous
gas concentrations at ensiling.
Density and Particle Size
Silage mass density varies with storage system, but is often directly related
to fermentative efficiency and total DM losses. Ruppel et al. (31)
reported that optimal density and sealing reduces porosity or air infiltration,
increases storage capacity and reduces capital investment. For optimal packing
and ensiling characteristics, a bunker silo at 30-35% DM forage, requires
a minimum density of 225-kg of DM/m3. Darby and Jofriet (9) estimate
upright tower silos to have 10% greater density when compared to horizontal
silos, due to the hydraulic pressure of the silage mass. A study published
by Ruppel et al. (31) indicated density of silage mass was directly
correlated to DM loss. Silage packed at 160-kg DM/m3 incurred
a 20.2% DM loss after 180d storage, while 225, 260, 290, and 350-kg DM/m3
sustained 16.8%, 15.1%, 13.4%, and 10.0%, respectively. The savings of increased
DM recovery is worth the cost of extra packing time.
Packing is dependent on a number of managerial practices. First, forage
delivery rate must be slow enough to ensure proper packing time. A delivery
rate of 30-T/hr allows 1 to 4-minutes/T for packing, which is the minimum
time to achieve the necessary packing density. Delivery rates exceeding 60-T/hr
allow less than one minute per ton for packing silage, which is inadequate
to meet minimum packing density. Secondly, heavy single wheeled tractors
apply the greatest force to a given area to reduce trapped air pockets in
horizontal silos. Consideration of an additional packing tractor may be necessary
depending on bunker size and forage delivery rate. Another factor affecting
packing density is layer thickness and packing method. Experts recommend
thin even layers, 15 to 30-cm in depth, be frequently incorporated into the
silage mass. Frequent packing ensures consistent fermentation throughout
the storage structure. Ruppel (31) reported a 3% reduction in ADF and an
8% increase in nonstructural carbohydrate (NSC) when using the progressive
wedge packing method. Finally, a greater silage depth can aid in packing
density, as added gravitational force causes cell collapsing.
Greater packing intensity was associated with larger temperature rises above
ambient temperature within the top surface layer of a horizontal silo (31).
However, packing intensity was also positively correlated to better aerobic
stability at the working face of the feed out phase. Similarly, McGuffey
and Owens (24) noted that temperature was a good indicator of compaction.
They reported lower temperatures in silage located at the bottom of the silo,
indicating good compaction, while higher temperatures near the top surface
indicating less compaction of the silage mass.
Jones et al. (17) recommend corn silage particle size to be chopped
at 1 to 2-cm in theoretical length. Proper chop length is necessary to obtain
uniform breakage of cobs and kernels with conventional harvesters. It is
often necessary to chop finer than we would like and still maintain effective
fiber. However un-cracked kernels tend to pass undigested through the GIT,
and large pieces of cobs are prone to sorting in the feed bunk. Mechanical
processing, such as chopping, bruising, rolling and kernel processing, help
breakdown cell wall structure, aid in starch and fiber digestibility and fermentative
efficiency (16, 22). Similarly, Bal et al. (2) reported that processing
corn silage through a 1-mm roller clearance numerically increased dry matter
intake (25.9 vs. 25.3-kg/d), milk production (46.0 vs. 44.8-kg/d), milk fat
concentration (1.42 vs. 1.35-kg/d), and lowered GIT starch digestibility (99.3
vs. 95.1% DDM) over control silage.
Nutrient losses are often worse than DM losses indicate, as WSC is metabolized.
Ruppel et al. (31) reported DM losses ranging from 3-25%, but noted
potentially 70% loss of digestible carbohydrate and up to 50% of soluble protein.
Ruppel et al. (31) blamed these losses on surface area exposure allowing
oxygen penetration, ineffective sealing of horizontal bunkers, and improper
management at fill and feed-out phases. Pre-seal losses of 1-3% DM were reported,
citing cell respiration and gaseous emissions as primary sources of loss.
Total nutrient losses throughout the storage phase vary with storage system.
Comparison of storage losses between storage system will be discussed later
in this paper. However, differences in DM recovery exist in physical location
within the forage mass. McLaughlin et al. (23) reported DM losses of
60% in the top 25-cm and 22% at 25 to 50-cm in horizontal silos. Ashbell
and Kashanci (1) found DM losses at the surface and near walls of sealed bunker
silos to be highest (76%), but lower in the center (16%).
Most producers dont understand that 2.5-cm of black forage may have been
5 to 8-cm of green high quality feed when placed into storage (14). This
represents a 50-65% loss in DM. Additionally, there is often a transition
zone (30 to 60-cm) of brown-gray forage below the black layer where a 20-30%
of DM loss occurs.
Delays in Fill
Silo filling rate affects the establishment of anaerobic conditions, growth
of LAB, substrate availability, DM losses and acid detergent insoluble fiber
(ADIN) (31). Delays in silo fill postpones pH decline and lengthen microorganism
activity, causing a decline in the relative feed value (RFV) of the silage
at feed-out (27). In fact, slow fill rates extend the duration plant cells
utilize WSC for energy, reducing available substrate for LAB metabolism.
Extreme delays may leave WSC so low that lactic acid production is inadequate
to attain the necessary pH drop required for preservation. Woolford (40)
noted that >50% of the WSC can be lost within 24 hours of filling if the
silo is slowly filled or inadequately sealed. Miller et al. (25) reported
less DM (5.8% less) and protein loss and less NFE and ash content in rapidly
filled silos. Additionally, prolonged cell respiration yields excessive heat,
which may lead to Maillard product accumulation (ADIN) (27).
Other climate factors, such as rain during silo fill, can cause extended
plant respiration, leaching of WSC, altered DM, and lower feed value. Addition
of precipitation lowers silage buffering capacity and quantitatively increases
the amount of organic acids needed to reach the stable pH phase.
Storage System Comparisons
Several factors, including herd size, capital investment, labor and feeding
situation, access to equipment, amount of forage, and plans for future expansion,
must be considered when selecting a silage storage system. Storage options
for corn silage include oxygen limiting tower, concrete tower, horizontal
pit, and silage bags. Capital cost is inversely related to DM losses for stored
forage systems. Each option offers various advantages and disadvantages for
a given scenario.
Upright tower silos typically reduced surface area expose to oxygen infiltration,
thus reducing DM and nutrient losses over other alternatives. Towers are
mechanized, easily accessible for feed out in good and bad weather conditions,
and require very little land space, but they do require high initial investments
and general maintenance. Towers silos are capable of ensiling forages 40-60%
DM, with expected losses of 5-17% (21). Horizontal bunkers are economically
attractive (25-50% of upright silo cost) and advantageous for storage of large
amounts of ensiled feed, quick filling capacity with conventional equipment,
and less energy for feed removal. McGuffey and Owens (24) reported that the
quantity of organic acids in bunker silos was similar to that of gas tight
silos, indicating typical fermentation occurred in both silos. However, bunker
silos are prone to incur greater storage losses (15-30% DM loss) without proper
management. Management options for reducing storage and feed-out losses will
be discussed in a subsequent section. Bagged silage is very economical, allows
great flexibility for expanding operations, and is easily fed out with modern
equipment. Bags require greater space allotment for storage, as well as proper
plastic disposal through the feeding period. Bags typically incur 17% DM
loss, while round bale silage sustains approximately a 30% DM loss (21).
Keller et al. (18) reported improved DM recovery and reduced mold with
round bale silage as the number of layers of plastic increased.
Bunker Silo Management
While harvesting at the proper DM and meeting minimum bunker packing density
are necessary in obtaining quality silage, they alone are not enough to ensure
optimum recovery. It has been well documented that covering bunker silos
with a plastic cover immediately after ensiling will reduce DM spoilage up
to 30% in the uppermost 1-m. Oelberg et al. (28) compared fermentative
conditions (pH 4.9 vs. 6.8) and DM recovery (total 96.2% vs. 68.0% DM) in
the top (95.8% vs. 49.2% DM) and bottom (96.6% vs. 86.8% DM) portions of covered
and uncovered bunker silos with alfalfa silage.
Plastics can vary in thickness, permeability, and anchoring/sealing technique
(34). Savoie et al. (33) determined that plastic thickness of 0.01,
0.015, and 0.02-cm provide optimum storage protection for 3, 7, and 12 months,
respectively. The cost of plastic sheeting increases linearly with thickness.
Economic investment for plastic covering varies with color. Black plastic
costs $0.0023/m2 or $0.11/T, while white plastic (radiates more
heat and is more UV resistant) costs $0.003/m2 or $0.16/T. Regardless
of price, the cost of losses and spoilage are 20 times greater than the cost
of covering the bunker. According to Bolsen et al. (5) losses on a
12.2 x 30.5-m bunker exceed $2000, while losses on a 30.5 x 76.2-m bunker
Gordon (11) noted that sealing technique, as well as proper weighting of
the plastic is critical in improving covering effectiveness. Adding weights
to the plastic, such as tire centers (20-25/100ft2), sawdust, sandbags,
soil, or limestone, provides additional weight for packing surface material
and also secures plastic into place. Mechanical failure of plastic coverings
due to handling, wind, hail, rodents or birds can be costly. Thicker plastic
is easier to handle and more resistant to tears and to oxygen infiltration
(15). A puncture 1-cm in diameter will incur a monthly spoilage loss of 0.8%,
0.7%, or 0.6% DM for 0.01, 0.015, or 0.02-cm plastic, respectively. Specially
designed tape is useful to repair punctures after the plastic is installed
Many producers leave bunkers uncovered because of the belief that awkward
plastic, tires, and labor intense covering arent worth the savings in spoilage.
DM losses in the top 30 to 90-cm can exceed 60-70% and may comprise 15-25%
of the corn silage in the bunker. Bolsen et al. (6) reported even
a short (7 day) delay in sealing horizontal silos can reduce DM recovery and
fermentative conditions. Sealed bunkers had greater DM and OM recovery in
the top 67-cm, while unsealed was lowest, and delayed seal was intermediate.
During the feed-out phase, silage becomes re-exposed to ambient air. While
pH inhibits bacterial activity within anaerobic conditions, 15-30% of the
bacteria remains present and active. Oxygen infiltration allows undesirable
aerobic bacteria to proliferate if management strategies are not employed.
Some silages begin to heat within hours of aerobic exposure, however some
remain stable for several weeks (21). Cereal grains are less aerobically
stable than legume silages because of their 10-fold greater concentration
of WSC. Primary losses at feed-out result from yeast and other aerobic bacteria
capable of metabolizing lactic acid, as well as molds, which feed on available
WSC. Harrison (12) reported acetic acid, bacteria and yeast as most likely
to cause aerobic deterioration in alfalfa silages at feed-out.
Pitt and Muck (30) determined the DM loss during feed-out of bunker silos
as a function of removal rate. Bunker size should be designed to maintain
a removal rate of 15.24 to 30.5-cm of silage across the entire face of exposed
silage. However, during warm, humid weather removal of 45.8-cm may be necessary,
especially corn silage, sorghum, or wheat silages. DM losses of 3% were incurred
at the recommended 15.24-cm/d for 35% DM silage stored at a density of 225-kg/m3
(30). DM losses decreased to less than 3% as silage density increased. Holmes
and Muck (15) reported differences in feed-out losses with bunker flooring
and management. Feed-out losses with good management ranged from 3-5% on
a concrete floor, 4-6% on asphalt, 6-8% on macadam, and 8-20% with earthen
floors, assuming good face management. With less than good management, losses
increased an additional 7%, regardless of floor type. Additionally, any loose
silage could start to heat as its exposed to oxygen, and should be fed with
that day's ration. Experts recommend maintaining a smooth, clean and tightly
packed face, perpendicular to the floor and sides. Ruppel et al. (31)
assigned face scores to several commercial bunker silos. Bunkers with lower
face scores had greater heat damage, and up to 10% greater concentrations
of ADIN, due to oxygen penetration.
Aerobic stability is important because many (dairy) producers contract silage
delivery for 2-4 days worth of feed (32). Silage removed, but not fed immediately,
is exposed to air for an extended time period (15). This is of special concern
in warm weather, as aerobic deterioration can occur rapidly during exposed
conditions. Ohyama et al. (29) noted pH and temperature can be good
indicators of deterioration in exposed silage. Spoiled silage should not
be fed, due to the negative effects on animal performance. Molds produce
toxins that may reduce cattle intake and negatively affect animal immune response.
Spoiled silage creates an imbalance of nutrients within ingredients used to
balance rations. A study conducted at Kansas State University indicated a
linear decrease in DMI, CP digestibility and NDF digestibility in cattle consuming
diets with increasing levels of spoiled corn silage (39).
Covering bunker silos with plastic, while vastly improved over uncovered,
is not 100% effective in reducing aerobic spoilage. The covering process
requires much hand labor. Visual observation of the silo top frequently reveals
a 5 to 20-cm layer of spoiled (black) feed (13). Producers consider this
thin layer as a small loss, which can be sacrificed so as to avoid the labor
of covering the bunker silo with plastic. Producers have sought less time
consuming and less difficult alternatives. Research has shown that covering
silage with a roof (32.6% DM loss), sawdust (30.0%), soil (25.1%), or ground
limestone (23.6%) may provide some protection compared to no cover (34.2%)
at all (13, 26). Other covering research such as candy, molasses, "nutri-shield",
small grain sod, manure solids, small grain straw, or corn fodder, has shown
no benefit over uncovered silage (13, 14). Watson (37) applied soybean soapstock
(60-70% DM) to bunker silos at a rate of 30 to 40-kg/m2 in an attempt
to reduce spoilage and add nutritional value ($0.02/cm2). Soapstock
applied 1.3 to 2.5-cm in thickness showed excellent sealing properties, reducing
spoilage from 42.4-T to 10.6-T in a 30.5 x 12.2-m bunker.
Common salt has been used in preservation of foods by inhibiting growth of
harmful bacteria (12). Some strains of LAB are salt tolerant (7, 10, 35).
Shockey and Borger (35) added NaCl, at a rate of 4-g/100-g to alfalfa silage
and observed a reduction in the total number of clostridium bacteria. Cai
et al. (7) added NaCl, at a rate of 40-g/kg to wet silage and observed
a reduction in DM loss (7%), pH, total gas production and NH3 levels.
The addition of salt increased lactic acid and WSC concentrations, and also
inhibited aerobic bacteria (clostridia) growth. Erickson (10) conducted a
study in which they top-dressed 22.5-kg of NaCl/121-m2 prior to
sealing the bunker silo with 6-mil plastic. Salt-treated sections had reduced
total aerobic bacteria counts, improved fermentation (lower pH), and improved
feed quality (NDF). Un-salted sections had two times greater mycotoxin zearlenone
(mold) content, in addition to greater clostridia counts.
The advantage of feeding these alternative covers is the goal producers would
prefer over disposing of plastic covering. Several spray-on products have
been developed and tested, but to date nothing has emerged as a successful
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