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The Babcock Institute © Michel Wattiaux at University of Wisconsin

The Babcock InstituteUniversity of Wisconsin

Dairy UpdatesFeeding No. 502 Author: Dr. Michel Wattiaux

IntroductionSilage-making is a management tool that

allows producers to match feed resources(forages, crop residues, agro-industrial by-products, etc.) with feed demand for a dairyherd. The basic function of silage-making is tostore and preserve feed for later use withminimal loss of nutritional qualities.

The need to store feed for use by cattle is nota novelty. The Naples museum in Italy displaysEgyptian paintings dated 1000-1500 BCshowing storage of fodder in what look likestone silos (Vanbelle, 1985).

In modern animal agriculture, hay-making ofexcess pasture preceded silage-making as theprimary method of preservation on the farm;however, silage-making has progressivelyreplaced hay making as the technique of choicein some parts of the world. Silage making isless dependent than hay-making on goodweather conditions and can be extended to agreat variety of forage crops (corn, sorghum,immature cereal grains, etc.) and locallyavailable agro-industrial by products (sugar beetpulp, brewers grain, etc.). Actually, the practiceof silage making evolved in parallel with thesuccess of corn as a high yielding crop that ispreserved extremely easily in a silo. Difficultiesarose when silage making was extended to otherforage crops that are less easily preserved assilage, in particular legumes.

Silage making has become an important toolfor producers to manage crop production anddairy herd feeding programs in many productionsystems around the world. However, silagemaking requires considerable capital and laborinvestments on the farm; it also demands a fairlyhigh level of technical expertise. Theunderstanding of how ensiling works to preservea crop by fermentation is important. Thisknowledge is key to making the bestmanagement decisions for minimizing the

inevitable losses that occur when fresh feedresources are ensiled and preserved for longperiods of time in a silo.

Thus, the objectives of this presentation are:(a) to briefly discuss how silage making fits invarious dairy production systems, (b) to explainsome of the biological principles of silagepreservation, and (c) to provide a set of practicalrecommendations for the preparation of highquality silage.

The Role of Silage in VariousProduction Systems

One of the main features that distinguishesdairy production systems from each other is thestrategy used by producers to supply feed to thedairy herd. For simplification, let us look at twosystems that are commonly found in variousparts of the world:1) In grazing systems, the strategy is to rely on

minimum inputs. Cows utilize natural orimproved pasture directly. These systemsare found in parts of the world whereclimatic conditions allow pasture to growduring most of the year;

2) In some land-based, capital-intensivesystems, the strategy is to use the land togrow crops for forageproduction and store theforage with the objectiveof balancing dairy cowrations and maximizingmilk production per cowthroughout the year.

The need to preservepasture or forage for dairycattle varies considerablybetween the grazing and thecapital-intensive productionsystems. Nevertheless,

Introductionto Silage-Making

In thisDairy Update

1Role of Silage in

ProductonSystems

3Principles of

SilagePreservation

9Summary

Introduction to Silage-Making

2 Dairy Updates

silage making plays an important role andcontributes significantly to the efficiency ofboth systems.

uuuu Role of silage-making incapital intensive systems

Forage preservation as silage is a keycomponent of high input (zero-grazing) systems.It has allowed producers to intensify theproductivity of the land and the productivity ofthe cows independently from each other. Assilage making allows storage and preservationof feed resources for months, if not years,producers can focus on two separate objectives:1) To maximize yield of digestible nutrients

(energy, protein, etc.) per hectare of land;2) to maximize milk production per cow

throughout the year.

Although separate, these two objectives remaininterrelated as the production, harvest andpreservation of high quality forages are keyelements of designing a feeding program thatimproves cow performance.

Thus silage making gives producers a feedinventory that can be used to plan a detailedfeeding program for the herd. If feed analysesare performed, diets can be formulatedspecifically to meet cow requirements andimprove nutritional status at all stages oflactation. Thus, despite the fact that silagemaking represents high costs and/or capitalinvestments, benefits accrue from higher cowproductivity.

Capital intensive production systems aremost suited to parts of the world where thegrowing season is short, either because of longwinters (northern latitudes of Europe and NorthAmerica), or extended periods of drought (sub-tropical latitudes).

uuuu Role of silage-making ingrazing systems

Grazing systems are more suited to regions ofthe world with clement climates in whichpasture growth is almost uninterruptedthroughout the year. These climates can befound in parts of New Zealand, Argentina, andIreland.

In grazing systems, the primary objective isusually to maximize milk yield per hectare ofpasture at minimal cost. As grazing systemsintensify, producers adopt low-cost strategiessuch that pasture growth (kg dry matter/

hectare/year) meets dairy herd requirements (kgdry matter/hectare/year) as closely as possible.Although these systems tend to minimize capitalinvestment, they also tend to require high levelsof management and decision-making skills tooptimize both pasture growth and cow nutrition.The challenge for the producer is to avoid bothexcess and deficit of pasture cover (kg of drymatter available per hectare), as pasture growth(yield and quality) and cow requirementschange continuously and simultaneouslythroughout the year. Managing grazing systemsis thus particularly challenging as pastureproduction is inherently variable and dependsheavily on weather conditions (drought). Thecommon management tools used to manageintensive rotational grazing include:• Manipulation of stocking rate (cows/

hectare) by purchasing or selling animals;• Manipulation of grazing area by renting

pastures from or to neighbors;• Altering the sequence of grazing by various

animal groups (lactating cows, dry cows,heifers);

• Adjusting calving dates to match cowrequirement curves with pasture growthcurves (seasonal calving), etc.

An additional tool used to maintain adequatenutrition of grazing cows is the purchase oflocally available feed (concentrates or agro-industrial by-products). However, silagemaking is also an alternative that can be used toimprove grazing management and control risksand variability associated with pasture growth.Silage making permits transfer of pasture fromthe period of excess cover in early spring to theperiod of deficit cover in fall and winter.

uuuu What does silage-making allowa producer to do?

Silage making is a tool for producers toachieve whole farm management goals. Silagemaking has some distinct advantages comparedto grazing or hay-making. For example, silagemaking allows:• Intensification of forage production (i.e.,

increased yield of forage per hectare);• Minimization of risk factors associated with

weather conditions (rainfall losses) whentrying to harvest high quality forages. Forexample, compared to hay-making, silagemaking shortens the time between cutting


Feeding No. 502 3

and storage—completion is possible in awider range of weather conditions, and riskof dry matter losses due to rainfall isminimized;

• Improvement of the producer’s control overcutting dates and optimal stage of maturityat harvest;

• Minimization of loss of leaves and othersmall plant parts of high quality in the field(compared to hay-making);

• Storage of non-forage feeds that cannot bepreserved as hay, such as agro-industrial by-products (brewers’ grains, sugar beet pulp,etc.);

• Storage of an inventory of forage of constantnutritive value, which makes it possible tobalance rations of dairy cows accurately (asopposed to trying to balance rations ofgrazing cows ingesting pasture that hasvariable nutritive value).

uuuu What are the difficulties anddrawbacks of silage making?

Silage making also has some limitations ordrawbacks that need to be accounted for whenassessing the costs of adopting this technology:• Silage making requires high capital

investment (or direct cost for customharvest) and heavy reliance on fossil fuel.Harvesters are needed to chop the forage,tractors or other heavy equipment are usedto pack the silage, storage facilities (silostructures) may be expensive, and additionalequipment may be required to remove silagefrom a silo;

• The management of silos is sometimesdifficult on the farm because once a silo isopened, silage should be removed on a dailybasis (to minimize loss of nutritive value).Adjusting the number of silos and theirdimension to the expected feed out rate for agiven herd size is difficult. Usually, onlylarge herds can afford to feed out ofdifferent silos of varying forage qualities fordifferent groups of animals on the farm(heifers, dry cows, lactating cows, etc.);

• Once silage is removed from a silo, itbecomes unstable (because of exposure tooxygen) and tends to spoil within a day ortwo (especially in warm weather conditionswith silages that are well-preserved);

• Silage cannot be marketed easily (difficult totransport long distances compared to hay);

• Loss of nutrients during storage in a silo isunavoidable and may be high if the silage isnot prepared properly. The major lossesassociated with silage making include:1) The potential loss of highly digestible

dry matter due to soluble sugars in theeffluents (“juices”) when silage has alow dry matter content;

2) The inevitable loss of protein quality(especially with certain legumes such asalfalfa) as protein peptides and aminoacids are converted to soluble nitrogen(e.g., amino acids and ammonia) duringthe fermentation process;

3) The inevitable loss of energy as sugarsare converted into organic acids, carbondioxide (and other gases) and heat.

Principles of SilagePreservation

uuuu What is silage?As a forage crop is cut, harvested and stored,

loss of dry matter (quantity) and nutritionalquality inevitably occur. These losses are due toenzymes1 that degrade the plant after it has beencut. Enzymes may originate from the dyingplant itself or from bacteria and other micro-organisms. Thus the goal in silage making is tostop enzymatic reactions and minimize loss ofenergy, protein and other nutrients. Thus silage-making can be defined simply as a method offorage preservation in which most of the energy,protein, and other nutrients that were in theoriginal plant remain in a form that can beefficiently utilized by cows. A more technicaldefinition follows in the gray box below.

As a bunker silo is filled, each load of freshlychopped forage is packed to expulse as much airas possible from the growing mass of silage.The absence of oxygen allows lactic acidbacteria to grow by converting sugars (simplesugars and starch) into lactic acid, a strongorganic acid.2 As lactic acid bacteria grow,lactic acid accumulates in the ensiled mass andthe acidity increases, that is, pH3 drops. As pHdeclines, the degrading actions of plant enzymesand undesirable bacteria (clostridia andenterobacteria), yeast and molds are slowed.When pH is sufficiently low (pH of 3.8-4.2 incorn silage and pH of 4.2-4.7 in alfalfa silage)


4 Dairy Updates

most degrading enzymes are inhibited and thegrowth of lactic acid bacteria is also inhibited.

Thus the goal in silage-making is to excludeoxygen from the forage mass in order topromote the fermentation of sugars by lacticacid bacteria and drop the pH as rapidly aspossible to stop all forms of degradationactivity.

uuuu The “aptitude” of a forage tobe preserved as silage

The chemical composition of a forage crop oragro-industrial by-product plays an importantrole in determining the ease with which lacticacid fermentation can take place; and thus theease with which a particular feed can bepreserved as silage. It is easier to ensile foragesthat have:• A high level of fermentable sugar;• A low level of protein;• A low buffering capacity;4

• An “ideal” dry matter content at ensilingtime (see “wilting green forage” below).

As indicated in Table 1, corn is preserved assilage more easily than grasses or alfalfabecause of its high sugar content, low proteincontent and low buffering capacity. In contrast,alfalfa is more difficult to ensile well because ofits low sugar content and high bufferingcapacity, which is due in part to its high proteincontent. Thus the higher the quality of alfalfa,the more challenging it is to ensile successfully.

uuuu Silage fermentation: A battlebetween “good” and “bad”bacteria

As indicated in the above definition, silage isessentially a battle between “good” and “bad”micro-organisms. The outcome of the battle,that is, whether a silage is well preserved orpoorly preserved, depends not only on foragecharacteristics (Table 1), but also in great parton the practices followed during the ensilingprocess. Before discussing these practices, wewill describe the fermentation process that takesplace in a silo. A better understanding of theprinciples of silage fermentation will helpproducers make the best decisions in developinga silage-making strategy adapted to theirparticular situations. The desirable processesthat take place in a silo may be described by asequence of four phases:

Phase 1: Respiration,5 which degrades plantnutrients in the presence of oxygen(1 to 2 days);

Phase 2: Early fermentation, which producesacetic acid, formic acid and otherorganic acids as a result of thegrowth of facultative aerobic bacteriasuch as enterobacteria, which canlive in the presence (aerobe) orabsence (anaerobe) of oxygen (1 to 2days);

Phase 3: Lactic acid fermentation by lacticacid bacteria that are strictlyanaerobic, that is, they can only grow

Silage is a method of feed resource preservation...... which is based on the removal of air (oxygen) from a mass of feed...... to promote the fermentation of sugars into lactic acid by lactic-acid bacteria...... causing an increase in acidity (a reduction in pH)...... which inhibits further silage degradation by:

- plant enzymes (primarily protein degrading enzymes);- undesirable bacterial species (enterobacteria, clostridia), yeast and molds;- and the lactic acid bacteria themselves.

Table 1: Chemical composition and buffering capacity of typical forage cropsWater solublecarbohydrate

(g/kg DM)Crude protein

(% DM)Ratio wsc/cp

Bufferingcapacity

(mEq/kg DM)“Aptitude” for silage

preservationCorn 80 - 300 80 - 100 1.0 – 3.0 150 - 300 HighGrasses 35 - 300 100 - 160 0.4 – 1.8 250 - 550 IntermediateAlfalfa 20 - 150 140 - 200 0.1 – 0.75 350 - 650 Low


Feeding No. 502 5

and multiply rapidly in the absenceof oxygen (14 days);

Phase 4: Stabilization phase due to thepresence of lactic acid, whichinhibits further degradation(indefinite period).

Two other undesirable phases can also takeplace and cause important loss of dry matter andforage quality in a silo:1) Butyric acid fermentation by clostridia (or

butyric acid bacteria), which may occur iflactic acid fermentation (Phase 3) fails toproduce enough lactic acid to stabilize thesilage (Phase 4);

2) Aerobic deterioration caused by molds andyeast that develop rapidly when a wellpreserved silage is exposed to oxygen afteropening a silo.

Phase 1 - RespirationOnce a plant is cut and cells lose their

structures, they continue to consume oxygenaccording to the equation at the bottom of thepage.

This equation indicates that respirationconverts sugar into carbon dioxide (a gas), water(a liquid), and heat. Thus respiration results in aloss of dry matter and available energy. Inaddition, the heat released by respiration raisesthe temperature of the forage. Temperaturesgreater than 26-32°C may cause significant lossof nutrients. Research indicates that the rise intemperature is not as high in well packed silosas it is in poorly packed silos (Pitt, 1983). Therapid expulsion of oxygen is desirable because itdecreases both the length of the respirationphase and the associated nutrient losses.

Normally, respiration continues for one totwo days, but takes place only as long as oxygenis present in the silage. Thus, compacting silageto remove as much air as possible as rapidly aspossible will curtail respiration losses.

Phase 2 - EnterobacteriafermentationThis phase has sometimes been described as

the lag or dormancy phase by some authors.Whatever it is called, the fate of silage dependsa great deal on the outcome of this criticalfermentation phase.

There are some 107 to 1010 micro-organismsper gram of forage freshly harvested, and mostof them are undesirable for the process of silagepreservation. Most of these organisms requireoxygen to grow (strict aerobic bacteria). Thus adecrease in oxygen in the silage as it is packedresults in a “natural selection” and a decrease inthe type of bacteria that require oxygen to grow.

As oxygen is removed and fermentationbegins, the bacteria that become predominantare those that have the ability to live either inthe presence or in the absence of air (facultativeaerobic bacteria). This group includes theenterobacteria, which convert sugars into avariety of organic acids (formic acid, aceticacid, lactic acid and sometimes butyric acid),carbon dioxide (CO2) and hydrogen (H2). Theseacids are responsible for the early decrease inpH in the silo.

As fermentation proceeds, enterobacteriabecome less competitive because they areparticularly sensitive to decreasing pH. Thegrowth of enterobacteria is inhibited when thepH falls below 4.5, which usually occurs withina few days of ensiling. However, enterobacteriatend to persist for longer periods in silage wherepH decreases slowly, as is the case with wiltedsilage (see below).

Phase 3 - Lactic acid bacteriafermentationLactic acid bacteria begin to dominate the

fermentation process after silage pH drops to 5.5– 5.7 (from 6.5 - 6.7 at ensiling time). A fewspecies of lactic acid bacteria can live in thepresence of oxygen, but most are strictlyanaerobic, meaning that oxygen is toxic to them.The reaction describing lactic fermentation issimple; one unit (molecule) of sugar is brokendown into two units (molecules) of lactic acid:

Some species of lactic acid bacteria produceonly lactic acid (as indicated in the aboveequation), they are called “homofermentative”bacteria. However, other species of lactic acidbacteria, called “heterofermentative” bacteria,produce lactic acid and other end products such

C6H12O6 + 6 O2 à 6 CO2 + 6 H2O + 3.8 Mcal/kg sugarsugar + oxygenà carbon dioxide + water + heat

C6H12O6 à 2 C3 H6O3sugar à lactic acid


6 Dairy Updates

as acetic acid, alcohol (ethanol) and carbondioxide. Homofermentative species arepreferable in silage because they produce morelactic acid, which is stronger and reduces pHmore than acetic acid. Actually, as pH drops,lactic acid becomes the predominant endproduct of fermentation. Proper lactic acidproduction depends on the following threefactors:• The number of lactic acid bacteria present at

the time of ensiling;• The presence of a sufficient amount of

fermentable sugars;• The absence of oxygen in the silage.

The number of lactic acid bacteria present atensiling time can vary from less that 1,000 toabout 20,000,000 per gram of fresh forage(Muck, 1988) and cannot be controlled easilywith management decisions. Nevertheless, thecount of lactic acid bacteria tends to increasewhen wilting time is between 24 and 48 hours,as compared to less than 24 hours, and whentemperatures during wilting range from 22-25°C, as compared to 18-22°C.

Research has focused on inoculation of silagewith strains of lactic acid bacteria. Whenapplied, a silage inoculant should provide atleast 100,000 bacteria/gram of ensiled alfalfa tobe effective. The best chance for benefitingfrom this investment may be with first and lastcuttings of alfalfa, since cooler and shorterwilting conditions (of early spring and earlyautumn) lower the naturally occurring lacticacid bacteria (Satter et al., 1988).

Phase 4 – Stable silageAfter about 14 days of fermentation, a well

preserved grass silage contains 1.5 to 2% lacticacid, and pH may range from 3.5 to 4.2. Inlegumes such as alfalfa, however, the pH rarelydrops below 4.5, even under the best conditions.The strong acidity created during Phase 3 leadsto a sort of “semi-sterilization” of the silagemass in the sense that all bacterial growth isparalyzed, and eventually the growth of thelactic acid bacteria themselves is inhibited. This

stable phase may last for months (if not years)as long as the silo remains closed and protectedfrom oxygen. Thus, it is important to cover asilo with a good plastic sheet that is well sealedand has a low level of aire permeation. Largedry matter losses can occur due to a poorcovering (see aerobic deterioration below).

Undesirable fermentation due toClostridia (butyric acid bacteria)These bacteria grow in the absence of oxygen

(anaerobic) and are normally found in soil andmanure. The fact that clostridia live in theabsence of oxygen and resist pH as low as 4.2allows them to compete with lactic acid bacteriaeven after the pH drops below 5.0. Essentially,clostridia dominates the fermentation whenlactic acid bacteria do not produce enough lacticacid to drop pH to a stabilization value fastenough. When silage is contaminated with dirt(e.g., from a soiled tractor wheel) or manure, therisk of clostridial fermentation increases.Clostridia tend to growth faster at a temperatureof about 35°C (a higher optimal temperaturethan most previously discussed bacteria). Thusthis type of undesirable fermentation happenswhen extensive respiration and enterobacterialfermentation occur and silage temperatures risein the early phases of the fermentation process.Some species of clostridia ferment sugars andchange the lactic acid produced by lactic acidbacteria into butyric acid, carbon dioxide andhydrogen (H2) (see box at bottom of page).

Production of carbon dioxide and hydrogengas indicates a loss of digestible energy. Thebreakdown of lactic acid into butyric acid,which is a weaker acid, means that the pH ofsilage going through clostridial fermentationwill tend to rise. Butyric acid has a strong,repulsive smell. Trace amounts of butyric acidsuffice to decrease voluntary intake by cows.Also, some species of clostridia ferment aminoacids, leading to the formation of toxicsubstances such as cadaverine and putrescine.A silage spoiled by clostridia is easilyrecognizable due to its strong odor, a pH above5.0, ammonia nitrogen greater than 10% of totalnitrogen, and more butyric acid than lactic acid.

C6H12O6 à C4H8O2 + 2 CO2 + 2 H2sugar à butyric acid + carbon dioxide + hydrogen

2 C3H6O3 à C4H8O2 + 2 CO2 + 2 H2lactic acid à butyric acid + carbon dioxide + hydrogen


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Fortunately, many clostridia are moresensitive to high acidity and high osmoticpressure6 (high levels of dry matter content)than lactic acid bacteria. Thus clostridialfermentation can be avoided by:• Ensiling at more than 30% dry matter

content;• Packing the silo as densely as possible to

reduce increases in temperature due torespiration;

• Avoiding soil contamination;• Ensiling forages with the highest possible

sugar content;• Using proper ensiling techniques that favor

maximum fermentation to the lowestpossible pH as soon as possible.

Undesirable aerobic deteriorationAerobic deterioration in bunker silos occurs

when oxygen is allowed to penetrate the mass ofpacked silage. This may occur (a) on the topsurface of a silo that is not sealed with a plasticsheet; (b) locally, around a whole in the plasticcovering, or (c) when less than 10-15 cm ofsilage is removed daily from the vertical front ofa well preserved silo. Losses caused by notprotecting a silo from oxygen at ensiling time(Table 2) are due to continued respiration andundesirable bacterial fermentation. However,aerobic deterioration can also result from thedevelopment of other microorganisms includingmolds and yeast.

Molds and yeast (and some aerobic bacteria)resist pH as low as 2.0, but remain dormant instable silage at a pH of around 4.0-4.5. Moldsrequire sugar and oxygen to grow and canresume their development rapidly once oxygenis present. Yeast, on the other hand, may growwith or without oxygen and can produce alcoholin silage that is rich in sugars, such as corn.More than 60 species of molds have beenisolated from silage, but the risk of aerobicdeterioration depends upon the type of forageensiled. Paradoxically, the risk of developmentof these undesirable micro-organisms increases

with the quality of silage preservation. Theresidual sugar in a well preserved silage is theideal energy source for molds and yeast. Inaddition, a well preserved silage has little or nobutyric acid, which is a strong inhibitor of moldand yeast growth. Thus, after a silo has beenopened, a well preserved silage has a greaterchance of mold and yeast deterioration thanpoorly preserved silage.

uuuu Wilting green forage - Why is itimportant to ensile at the“right” dry matter content?

The dry matter content of silage stronglyinfluences the type of fermentation that takesplace in a silo. Losses of dry matter in effluentsoccur when silage dry matter content is less than25%. Dry matter losses decrease from 7.2 to 1.6and 0.4% from grass silage ensiled at 15, 20 and25% dry matter respectively (Harrison andFransen, 1991).

On the other hand, too much dry mattermakes it difficult to pack and expulse theoxygen from the silage mass. The “ideal” drymatter content of the forage depends on the typeof silo (which influences the mode and level oflevel of compaction that can be achieved). Therecommended levels of dry matter in forages atensiling are:• 30 to 40% for bunker silos;• 35 to 50% for tower silos;• 40 to 50% for wrapped round bales.

Often, wilting a green forage for 24 – 48hours to increase dry matter is highly desirable.Why ? Essentially because wilted silage requiresless lactic acid production and will stabilize at ahigher pH level than silage that has a lower drymatter content (Table 3). This is the casebecause higher dry matter content increases theconcentration of soluble dry matter in the silage,and the resulting increase in osmotic pressure(see endnotes) inhibits bacterial growth. Thusfermentation stops in wilted silage because ofthe high acidity, high osmotic pressure

Table 2: Average losses and quality changes in covered and uncovered bunker silos (Oelberg et al. 1983)

Temp.(°C)

DMa Losses(%)

pH Lactic acid(% DM)

CPa

(% DM)ADFa

(% DM)SPa

(% CP)

ADIP(% CP)

Covered 36.7° 4 4.9 3.2 21 39 20 28Uncovered 53.9° 32 6.8 1.7 22 47 16 37

a DM = dry matter; CP = crude protein; ADF = acid detergent fiber; SP = soluble protein; ADIP = acid detergent insolubleprotein.7


8 Dairy Updates

combination (rather than high acidity alone insilage with lower dry matter content). Thuswilting has a “sparing” effect on the level ofsugar and the level of fermentation needed tostabilize the silage. As a result, wilting isparticularly important when the ensiled forage isa legume, which has a relatively low level offermentable sugars as compared to grass orcorn.

Wilting has additional advantages as well:• Wilting tends to increase the number of

lactic acid bacteria present at ensiling. Thisimproves the likelihood of an early start oflactic acid fermentation and reduces theneed for a commercial lactic acid bacteriainoculant;

• Wilted silage is usually more palatable, inpart because of lower acid content. A 500kg cows will ingest one additional kg ofsilage for every 5% increment in dry mattercontent above 20%.

uuuu Change in forage compositionin a silo

Loss of soluble carbohydrates andproteinsAs dry matter is lost during silage-making,

forage composition also changes. The changesin dry matter composition are due primarily tothe fact that the most valuable nutrients (solublecarbohydrates and proteins) are also the first tobe lost during respiration and fermentation andin the effluents (juices). Fiber on the other handremains essentially unaffected by the naturallyoccurring fermentation process in a silo. Thusthe overall effect of these losses is to increasethe proportion of fiber; the percentage of boththe acid detergent fiber (ADF8) and neutral

detergent fiber (NDF9) tend to be higher insilage than in freshly cut forage.

Changes in the protein (nitrogen)fractionsSilage-making change the protein fraction of

forages. Respiration is responsible for proteinbreakdown. As plant cells die after cutting,proteolytic enzymes break down large proteinsinto smaller soluble compounds including:peptides, amino acids (the building blocks ofproteins) and ammonia. In addition,enterobacteria have proteolytic enzymes thatremain active even after the pH has dropped to5.0. Thus most of the protein degradation thattakes place in a silo occurs within the first 24 to72 hours (Phase 1 and Phase 2 of silagefermentation). By the time pH is about 4.0,proteolytic enzymes have lost 65 to 85% of theiractivity. Consequently, a rapid drop in pH isdesirable to reduce the amount of proteinbreakdown in a silo. Nevertheless, recentresearch indicates that as much as half of thetotal nitrogen in alfalfa silage may be in theform of non-protein nitrogen. The extensivebreakdown of alfalfa protein in a silo may be afactor in limiting milk yield in high producingcows (Broderick, 1995). Some scientists haveproposed the use of ammonia content as oneindicator of adequate silage fermentation. Forexample, a grass silage containing less than 5%of total nitrogen in the form of ammonia mightbe classified as excellent. In contrast, a silagethat undergoes clostridial fermentation mightcontain more than 35% nitrogen in the form ofammonia and thus would be classified as poor(Vanbelle, 1985).

Although limiting protein solubilization andammonia production in a silo is desirable, oneshould realize that this process is relativelysimilar to what happens in the rumen of a cowafter ingesting silage. Not all soluble proteinand ammonia produced in the silo arenecessarily lost. Ammonia and amino acids areneeded for microbial synthesis in the rumen.

Another nitrogen transformation that mayoccur during hay or silage-making results fromthe browning reactions that take place whenexcess heat is produced (Table 1). Excess heatcause chemical reactions that combine aminoacids with plant sugars (usually derived fromhemicellulose) to form a compound resemblinglignin.10 This reaction results in increased levelsof acid detergent fiber (ADF) and acid detergentinsoluble protein (ADIP; Table 2). Excess heat

Table 3: Effect of dry matter content andgreen forage type on pH required to avoidclostridial growth (Libensperger and Pill, 1987).

Dry Matter Stable pH(%) Grass Legumes20 4.16 4.2625 4.26 4.4530 4.43 4.6035 4.63 5.0440 4.90 5.5645 5.14 --50 -- --


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is an indication of aerobic degradation due tothe presence of air (oxygen) in a silo. Again,these undesirable reactions can be avoided bycompacting silage as much as possibleimmediately after unloading fresh forage into asilo.• Set the harvester to obtain a theoretical

length of cut (TLC) of 1.0 cm for greenforages and 0.6 cm for corn silage. Shortparticle length enhances sugar availabilityfor fermentation and facilitates silocompaction. However, at least 20% of theparticles should exceed 2.5 cm in length toinsure enough effective fiber in the silage.

• Filling a silo should be a continuous processwith delays no longer than overnight. Thelast load of the day should always be packedparticularly well to reduce oxygenpenetration overnight.

• Avoid contamination of the silage mass withsoil (from mud on tires), which tend tocontain high levels of undesirable species ofbacteria (clostridia).

• Pack the silage as much as possible to expeloxygen and favor the growth of lactic acidbacteria.

• Judiciously use additives to potentiallyimprove fermentation patterns (optional).

• Seal the open surface of the silo with aplastic sheet to prevent oxygen from

reentering the silo; poor covering (or nocovering at all) causes large losses of drymatter, undesirable fermentation andunpalatable silage. Plastic sheet should havea low level of permeation by air and becarefully placed and anchored to seal thesilo completely. Holmes (1997) tried toquantify the value of feed saved (i.e., notspoiled) by using proper covering technique.He calculated that the time spent coveringand uncovering a silo is worth $60-100/hour.

• Leave the silo close for at least two weeksfor fermentation to reach its stabilizationphase.

• Plan to build silos such that, uponunloading, 5 to 10 cm of silage can beremoved daily (so that the rate of dry matterremoval is greater than the rate of oxygenpenetration in the silo’s front).

• Use equipment that leaves a smooth surfaceto minimize exposure to oxygen and risk ofsecondary fermentation.

• When unloading a silo, remove only theamount needed for one meal (one day) andavoid spoilage of silage in feed bunks(mangers) by cleaning them every daybefore offering freshly unloaded silage tocows.

In summary, what are the best management practices formaximizing the quality of silage preservation?

In summary, one should remember that the quality of silage depends primarily on the quality of theforage entering the silo. Dry matter losses and quality changes in silage cannot be entirelyeliminated, but they can be minimized by using good management practices:

u Cut the green forage in the early morning if the anticipated period of wilting is several days, butcut the green forage late in the day if drying conditions are good and chopping is anticipated forthe next day. Research has shown that afternoon mowing consistently produces the lowest pHlevels in alfalfa that is wilted one day and ensiled at 35% dry matter (Muck, 1998).

u Enhance the rate of drying in the field (to reduce respiration losses) by using a conditioningmower or increasing exposure to sunlight (optional).

u Ensile the forage when it has the proper moisture content (depending on silo structure and foragecrops, between 30 and 50% DM) to minimize effluent losses and maximize level of silagecompaction.


10 Dairy Updates

u Set the harvester to obtain a theoretical length of cut (TLC) of 1.0 cm for green forages and 0.6cm for corn silage. Short particle length enhances sugar availability for fermentation andfacilitates silo compaction. However, at least 20% of the particles should exceed 2.5 cm inlength to insure enough effective fiber in the silage.

u Filling a silo should be a continuous process with delays no longer than overnight. The last loadof the day should always be packed particularly well to reduce oxygen penetration overnight.

u Avoid contamination of the silage mass with soil (from mud on tires), which tend to contain highlevels of undesirable species of bacteria (clostridia).

u Pack the silage as much as possible to expel oxygen and favor the growth of lactic acid bacteria.

u Judiciously use additives to potentially improve fermentation patterns (optional).

u Seal the open surface of the silo with a plastic sheet to prevent oxygen from reentering the silo;poor covering (or no covering at all) causes large losses of dry matter, undesirable fermentationand unpalatable silage. Plastic sheet should have a low level of permeation by air and becarefully placed and anchored to seal the silo completely. Holmes (1997) tried to quantify thevalue of feed saved (i.e., not spoiled) by using proper covering technique. He calculated that thetime spent covering and uncovering a silo is worth $60-100/hour.

u Leave the silo close for at least two weeks for fermentation to reach its stabilization phase.

u Plan to build silos such that, upon unloading, 5 to 10 cm of silage can be removed daily (so thatthe rate of dry matter removal is greater than the rate of oxygen penetration in the silo’s front).

u Use equipment that leaves a smooth surface to minimize exposure to oxygen and risk ofsecondary fermentation.

u When unloading a silo, remove only the amount needed for one meal (one day) and avoidspoilage of silage in feed bunks (mangers) by cleaning them every day before offering freshlyunloaded silage to cows.


Feeding No. 502 11

ReferencesBroderick , G.A. 1995. Performance of lactating

dairy cows fed either alfalfa silage or alfalfa hayas the sole forage. J. Dairy Sci. 78:320-329.

Harrison, J.H. and S. Fransen 1991. Silagemanagement in North America. In Field Guidefor Hay and Silage Management in NorthAmerica, pg. 33. K. K. Bolsen, ed. Natl. FeedIngredients Assoc. West Des Moines, Iowa.

Holmes, B. 1997. pp 17-33. In 13th annualPostgraduate conference, Food Animal ProgramProc., School of Vet. Medicine, University ofWisconsin, Madison, WI.

Leibensperger, R.Y. and R.E. Pitt. 1987. A modelof Clostridial Dominance in Ensilage. Grass andForage Sci. 42:297-317.

Muck, R.E. 1989. Initial bacterial numbers onLucerne prior to ensiling. Grass and Forage Sci.44:19-25.

Muck, R. E. 1998. Preparing high quality alfalfasilage. In Conservación de Forrajes de altacalidad, pp 1-11. E.Jahn, (ed); Serie quilamapu# 100. Seminario Internacional - Conservaciónde Forrajes de alta calidad, Los Angeles, Chile

Pitt, R.E. 1990. Silage and hay preservation. NorthRegional Agricultural Engineering Service,Cooperative Extension (NRAES-5), 152 Riley-Robb Hall, Ithaca, New York 14853.

Pitt, R.E. 1991. Hay preservation and hay additiveproducts. In Field Guide for Hay and SilageManagement in North America, pg 127. K. K.Bolsen, ed. Natl. Feed Ingredients Assoc. WestDes Moines, Iowa.

Oelberg, T.J., A.K. Clark, R.K. McGuffey and D.J.Schingoethe. 1983. Evaluation of covering, drymatter and preservative at ensiling of alfalfa inbunker silos. J. Dairy Sci. 66:1057-1068.

Satter, L.D., R.E. Muck, J.A. Woodford, B.A. Jones,and C.M. Wacek. 1988. Proc. of the WisconsinForage Council’s 12th forage production and usesymposium.

Vanbelle, M. and Bertin, G. 1985. L’ensilage,aspect biologiques nouveaux. Université deLouvain, Faculté des Sciences Agronomiques,Place croix du Sud, 3 Louvain La Neuve, 1348,Belgium.

Endnotes1 Enzymes are proteins that accelerate reactions in

living cells without being consumed in theprocess.

2 Organic acids are acids that are produced by thenormal functions of all living cells; commonorganic acids include formic acid, acetic acid,propionic acid, butyric acid, lactic acid, etc.

3 pH is a measure of acidity or alkalinity. pH valuesrange from 0 (most acid) to 14 (most alkaline).Water has a pH close to neutral (pH 7). A drop inpH indicates an increase in acidity.

4 Buffering capacity is the resistance to pH drop andis often expressed as milliequivalent/kg of drymatter. Plants such as alfalfa, which contain highlevels of salts of organic acids and proteins, have ahigh buffering capacity; which means that a givenamount of acid will lower pH less than for plantsthat have a lower buffering capacity (corn). Inother words, plants with high buffering capacity(e.g., alfalfa) require more acids to reach the samereduced pH than a plant with a low bufferingcapacity (e.g., corn).

5 Respiration basically means to live in presence ofoxygen and use it.

6 Osmotic pressure reflects the concentration ofsubstances in a solution. As dry matter contentincreases, osmotic pressure also increase. At highosmotic pressures, water is drawn out of bacterialand other living cells, which “die of thirst”because of lack of intracellular water.

7 ADIP or acid detergent insoluble protein is ameasure of the unavailable nitrogen in the feed,primarily as a result of heat damage.

8 ADF or acid detergent fiber is correlated with thedigestibility of a feed; the lower the ADF, thehigher the digestibility.

9 NDF or neutral detergent fiber is correlated withthe level of dry matter intake by cows; the lowerthe NDF, the higher the level of intake.

10 Lignin is an indigestible plant compound, which isdeposited in the cell wall and is responsible for adecrease in cell wall carbohydrate (cellulose andhemicellulose) digestibility as a plant matures.


12 Dairy Updates

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