Silage production

The goal of silage production is to minimize biological degradation and conserve digestible nutrients. To do this, oxygen must be eliminated and silage acidity must increase rapidly so that lactic acid bacteria grow and stabilize the silage. The four phases of ensiling are the aerobic phase, lag phase, fermentation phase, and stable phase. By the end of the fermentation phase, silage temperature should reach a final low pH and bacterial growth will stop. Fast silo filling, good packing, and tight sealing are necessary for oxygen elimination. Particle size and moisture content will affect packing. Silage with good aerobic stability (bunk life) has minimal heating after feed out.

From the minute forage is cut in the field until it is consumed, biological degradation processes are occurring which decrease the nutrients available to the cow. The goal should be to minimize this degradation and conserve the greatest amount of digestible protein and energy for the cow. The main benefit of silage production over hay production is lower field losses. However, without proper silage management, silage storage losses can be significant.

Silage Preservation:

Goal –To reduce oxygen and increase acidity rapidly, so that lactic acid bacteria grow to stabilize and preserve or “pickle” the forage.

Four Stages of Ensiling:

1. Aerobic Phase – As much oxygen as possible should be eliminated from the silage by good packing and sealing. The amount of oxygen left in the silage will also depend on the silage’s moisture content, silo filling time, manual silo packing, and the fineness of chop of the silage. Despite the best management, some oxygen will remain in the silage. This oxygen is used up during the aerobic phase. Cells in the forage take in oxygen and turn it into carbon dioxide. This is called plant respiration. Aerobic (oxygen-loving) bacteria also use up oxygen as they degrade the silage at this time.

Sugar + Oxygen -----> Carbon Dioxide + Water + Heat

The aerobic phase usually lasts about one day but it can be more or less. Well-sealed and packed silos will have a shorter aerobic phase (as little as a few hours). Poorly sealed and packed silos will have longer aerobic phases. Long aerobic phases are a problem. They result in higher silage fiber (ADF and NDF) and less energy (NEl). Digestible sugars are used and wasted by the bacteria. Long aerobic phases also result in high silage temperatures (above 100oF (38oC)), high dry matter losses, mold growth, mycotoxin production, and browning reactions (Maillard reactions). Browning reactions convert digestible protein into bound protein (ADF-CP or ADIN) which the cow cannot digest.

2. Lag Phase – After the oxygen is used up in the aerobic phase, plant cells are broken down and used as a food source by bacteria. Plant enzymes break down complex carbohydrates (starch and fiber) into simpler sugars that are easy for bacteria to use.

Enzymes also break down plant proteins at this time, making the protein more soluble. The bacteria will use the cell juices produced during the lag phase to grow during the fermentation phase. Silage pH drops (acidity increases) from a pH of 6.5-6.7 at ensiling to 5.5 to 5.7.

3. Fermentation Phase – By 2-3 days after ensiling, cell juices are available as a food source, oxygen has been eliminated, and silage pH has declined to a level at which the lactic acid bacteria can grow (5.5-5.7). The lactic acid bacteria begin to grow, multiply, make lactic acid (C3H6O3) and some acetic acid (C2H4O2), and increase silage acidity more.

Lactic acid is a stronger acid than acetic acid, meaning it reduces the silage pH more than acetic acid. There are two types of lactic acid bacteria: homofermentative and heterofermentative. The homofermentative bacteria produce almost exclusively lactic acid. The heterofermentative bacteria produce lactic acid, acetic acid, ethanol, and carbon dioxide. The homofermentative are more desirable because they work faster, saving more nutrients for the cow to use and preserving the silage better. Of the total acid in well-preserved silage, at least 70% will be lactic acid or 3-6% lactic acid (%DM).

The initial number of lactic acid bacteria in the silage will be higher with longer wilting times, warmer temperatures, and higher silage moisture. The lactic acid bacteria need moisture, sugar, and oxygen-free conditions to grow. It is more difficult to ferment alfalfa silage because of its naturally lower sugar content and high acid-buffering capacity. Acid-buffering capacity is the ability of forage to resist a pH drop. Differences in acid-buffering capacity are due to the organic acids and minerals contained in various types of forage. Corn silage is naturally easier to ensile because it contains more sugars and has a lower acid-buffering capacity.

Factors Increasing the Sugar Content of Hay Crop Silage

Sunny Days with Long Daylength

Mowing Late in the Day When Sugars are Highest

Short Wilting Period

Fast Silo Filling

Good Silo Sealing

Good Silage Compaction

Adapted from Pitt (1990)

4. Stable Phase – By the end of the fermentation phase (about 2 weeks), silage temperature should reach a final low pH (4.3-4.5 in legume silage and 3.8-4.0 in corn silage). This is the stable phase in which the silage is preserved and the bacteria have stopped growing.Unstable silage is characterized as having low levels of lactic acid (less than 65-70% of the total acid).

If the available sugars run out before the silage reaches low pH, the fermentation will stop before the silage reaches the stable phase. In higher moisture silage, the bacteria stop growing at a lower pH. So, higher moisture silage needs more sugars for the lactic acid bacteria to grow on and make acid to lower pH. In drier silage (35%DM or more), pH isn’t as important as it is in wet silage. Dry, higher pH silage may be O.K., nutritionally. However, dry silage is more susceptible to heating and mold problems because it doesn’t pack as well and more oxygen remains in it. Dry matter losses can be high with dry silage.

Wet forages have lower sugar concentrations. Forages ensiled at less than 30% DM are at a greater risk of being unstable and having clostridia. Clostridia bacteria are one of the most common undesirable bacteria that may persist in unstable silage that has no oxygen. They produce butyric acid and break down protein. They usually are associated with silage that has a pH above 5.0-5.5. Unwilted silage may need a pH as low as 4.0 to totally inhibit clostridia. Silage with clostridia usually smells rancid. With clostridia, there will be higher silage dry matter losses, poor silage palatability, and a higher level of ammonia nitrogen that is poorly used by the cow. The only good thing about a silage that has undergone a clostridial fermentation is that it usually is fairly aerobically stable (it has good bunk life).

Goals for Stable Silage:

  % Dry Matter (DM)
Lactic Acid 3-6%
Acetic Acid <2%
Butyric Acid <0.1%
Propionic Acid 0-1%

Adapted from Mahanna (1997)

Goals when Evaluating of Silage Temperature:

Not More than 15-20oF (8-11oC)above Air Temperature at Ensiling Time

Not More than 1-2oF (0.5-1oC) above Air Temp. During the Stable Phase

Protein Goals for Silage:

Ammonia Nitrogen (NH3-N) Less than 10-15% of N in grass/legume silage, less than 5% in corn/cereal silage
Bound Protein (ADIN, ADF-CP) <12% of the crude protein (CP). If greater, use Available CP to balance ration.

Adapted from Mahanna (1997)

Getting Oxygen Out of Silage:

Based on the above discussion, it is obvious that some attention should be paid to getting oxygen out of the silage when it is put up. This primarily involves three things: fast filling, good packing, and tight sealing. Fast filling means no delays other than overnight. Good packing is fairly easily accomplished in the tower silo by using a distributor and by topping the silo with a wet load of forage. It is more difficult to pack dry silage, mature silage, and long-cut silage.

Good packing is harder to get in a bunker silo. Wheel tractors rather than crawler tractors should be used. Single-wheels rather than dual-wheeled should be used. One should also pack at more than 800-1000 hour-pounds per ton of silage. This means that you should take your total tractor weight and divide it by 800 to determine your maximum filling rate in tons per hour. So, if you have a total tractor packing weight of 38,000 pounds (17,273 kg), 47.5 tons (43,182 kg) can be delivered per hour. Alternatively, if you have a desired filling rate (tons/hour) multiply it by 800 to determine how many tons of packing tractor weight you need. Packing method is important too. Spread out the forage as soon as possible when it arrives. Spread and pack the forage in a progressive wedge at a 30o angle. Don’t try to pack more than 6 inches (15 cm) at one time. The difference between good packing and poor packing could mean differences in silage temperature of 10-20oF (6-11oC) and significant differences in dry matter losses. Finally, don’t forget tight sealing. Tower silos should be checked and treated for cracks and pores. Bunker silos should be sealed with 4-6 mil plastic that is held down with tires (or split tires) that are touching and sealed at the edge with silage or sand bags. Leaving a bunker silo uncovered is equal to using 30% of the top three feet of silage as silo cover. The silo will have a cover. It will either be the forage itself or plastic. That silage which is used as a “cover” and which aerobic organisms eat and spoil, is the part of the forage which is most digestible and of most benefit to the cow. Sealing has been estimated to have a 4:1 return on investment with hay crop silage and a 2:1 return on investment with corn silage.

Length of Cut of Silage:

The shorter forage is cut, the better it will pack and the more sugars will be available for the silage bacteria. But, remember that we are feeding cows that need long forage to re-chew as a cud to produce saliva and avoid acidosis. In the total ration, at least 15% of the particles should be longer than 1.5 inches. Therefore, some fine-cut silage is tolerable so long as it is compensated for with some longer cut forage in the ration. Generally, it is recommended to cut hay crop silage at a theoretical length of cut (TLC) of 3/8 inch (1 cm) and corn silage at ¼ inch (0.64 cm) TLC. Knives should be kept sharp to prevent shredding and minimize effluent.

Particle Size Separators, such as the Penn State Particle Size Separator, are now being used on many farms. These separators have two stacked screens that are 0.75 inch (1.9 cm) and 0.31 inch (0.79 cm). Forage is placed on the top of the stacked screens. The sieves are then shaken sideways for two minutes. The three weights of the material remaining on each screen and underneath the bottom (0.31 inch) screen are taken and divided by the total weight.

General Recommendations for Particle Size:

  Coarse, 0.75 inch Medium Fine, <0.31 inch
Processed Corn Silage 20-25% 30-40% 35-50%
Unprocessed Corn Silage 10-15% 35-45% 35-45%
Hay Crop Silage 20-25% 30-40% 35-50%
TMR 10-15% 30-50% 40-60%

Note: Some researchers suggest that only 6-10% of the TMR needs to remain on the top screen. The exact recommendation depends in part on the make up of the NFC’s in the diet and feeding management. The author has had the most success across diets with the above recommendations

It is not only important to have enough long fiber but also not have too much long fiber. Too much long fiber will not pack as well. Cows are also more likely to sort through their ration and pick and choose what they eat if there is too much long fiber.

Aerobic Stability (Bunk Life) of Silage:

When silage is fed out of the silo to cows, we want it to be stable and not heat when it gets out in the air. Heating is caused by yeast, molds, and aerobic (oxygen-loving) bacteria that are breaking down the silage, reducing its nutritional value, and making the silage less palatable for the cow. Mycotoxins may be produced which when consumed, negatively impact the cow.

Feeds can have a bunk life of less than an hour to as long as several days before they begin to heat. Silage that has some oxygen infiltration during storage due to poor sealing or poor packing will have a shorter bunk life. Oxygen increases the amount of aerobic bacteria in the silage and their ability to grow quickly once they are exposed to more oxygen at feed out time. Silage that has undergone a good fermentation with a low pH will have longer bunk life. Also, legumes have a natural buffering ability and they will be more stable in the feed bunk. It has been found that acetic acid inhibits the growth of molds and yeast more than lactic acid. So, in the case of bunk life, a bit higher ratio of acetic to lactic acid may be helpful. This should be weighed, however, against all of the advantages of higher lactic acid in silage. Silage that was rained on before chopping often has a shorter bunk life because it is more likely that aerobic soil bacteria and fungi have found their way onto the forage. Forage that was stressed by drought or insects also often has a shorter bunk life.

The feeding surface silage should be kept as fresh as possible. At least 4-6 inches (13 cm) should be removed from the surface each day in the summer and 2-4 inches in the winter. This can save 5-10% of silage dry matter and reduce the loss of the most available nutrients in the silage. Actually measure feedout rate by marking a line on the feedbunk wall by the bunk face and measuring how far the face is away from it in 7 days.

What Size Should Your Bunker Silo Be?

  1. How many Pounds of Dry Matter will be Fed Each Day?
    500 cows * 15 pounds DM/day = 7500 pounds DM/day
  2. How many Cubic Feet will be Needed Each Day?
    (Degree of Packing: Poor = 13 lbs/cubic ft, Average = 15, Good = 17 lbs/cubic ft) Assume Good Packing: 7500 / 15 = 500 cubic feet/day
  3. How much Silage Needs to be Removed from the Surface Every Day? Assume 6 inches (0.5 foot) per day
  4. How many Square Feet need to be Removed Each Day?
    Cubic Feet divided by Feedout Rate 500/0.5 = 1000 Square Foot Removed Each Day
  5. How Wide Should the Bunker Be if the Wall is 20 Feet High?
    1000/20 = 50 Feet Wide
  6. How Long Should the Bunker Be?
    Assume 360 Day Feeding Period 360 Days * 0.5 foot per day = 180 Feet Long

It is important to limit the amount of silage surface area that is exposed to air. In a bunker silo, attention should be paid to maintain a smooth, tight bunk face. It is recommended that when unloading silage from a bunker, shave down from the top. Never drive into the pile and lift the silage up. This exposes the silage to too much air. The important thing is that no more silage is disrupted than what is needed at that feeding time.

Good bunk face management results in less unavailable nitrogen (ADIN or bound protein) in hay crop silage.

  Good Face Average Face Bad Face
ADIN (%CP) 14.69 9.88 5.07

Ruppell et al.(1995)

If face management was changed from bad to good, 9.6% of the CP could be saved that otherwise would be lost to the cow. Or to put it another way, if the feed representative dug in and got a really good sample of hay crop silage and it tested 20% CP, you could reduce it to 18% Available CP for the cow just by bad bunk management. That’s expensive!


Bolsen, K.K., J.T. Dickerson, B.E. Brent, R.N. Sonon, Jr., B.S. Dalke, C. Lin, and J.E. Boyer, Jr. 1993. Rate and extent of top spoilage losses in horizontal silos. J. Dairy Sci. 76:2940.

Mahanna, B. 1997. Troubleshooting Silage Problems. Pioneer Hi-Bred International, Inc. Website (

Muck, R.E. and Kung, Jr., L. 1997. Effects of silage additives on ensiling. Proceedings of the Silage:Field to Feedbunk North American Conference, Hershey, PA.

Pitt, R.E. 1990. Silage and hay preservation. Northeast Regional Agricultural Engineering Service. Ithaca, New York.

Ruppell, K.A., R.E. Pitt, L.E. Chase, and D.M. Galton. 1995. Bunker silo management and its relationship to forage preservation on dairy farms. J. Dairy Sci. 78:141.

Ruppell, K.A. 1997. Economics of silage management practices: What can I do to improve the bottom line of my ensiling business. Proceedings of the Silage:Field to Feedbunk North American Conference, Hershey, PA

Ruppell, K.A. and D.R. Specker. 1996. Pioneer Field Encounters: Enhancing Value from Seed to Feed. Proceedings from the Seed of Animal Nutrition Conference, October 22, 1996, Rochester, NY.

Seglar, W.J. 1997. Dairy Production Management – Silage Management. . Pioneer Hi-Bred International, Inc. Website (

Shaver, R. Silage preservation – The Role of Additives. University of Wisconsin, Cooperative Extension. A3544.

Van Soest, P.J. 1982. Nutritional ecology of the ruminant. O&B Books, Inc., Corvallis, OR.

Related Links:

Penn State Particle Size Separator

Analyzing Silages For Fermentation End Products
Kung, Jr., Ph.D., University of Delaware, Stokes, Ph.D., University of Maine

Improving the Aerobic Stability of Silages
Limin Kung, Jr., Ph.D., University of Delaware

What the Smells for Silages Can Tell You
Limin Kung, Jr., Ph.D., University of Delaware

Harvesting, Storing, and Feeding Silage to Dairy Cows
B. Harris, Jr., Ph.D., University of Florida

Microwave Drying for Measurement of Forage Moisture
C.R. Staples, Ph.D., University of Florida

Principles of Silage Making
W. Coblentz, University of Arkansas

Bagged Conventional Silage
W. Coblentz, University of Arkansas

Estimating how much silage is in a chopper wagon
Mike Rankin, University of Wisconsin

Feeding Systems, In: Feeding the Dairy Herd North Central Regional Extension Publication
J.G. Linn et al.

Silage Bag Capacity
B. Holmes, University of Wisconsin


Mary Beth de Ondarza

Mary Beth de Ondarza
45 articles

Nutritional consultant for the dairy feed industry at Paradox Nutrition, LLC.

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Dr. de Ondarza received her Ph. D. from Michigan State University and her Masters Degree from Cornell University, both in the field of Dairy Nutrition.

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Paradox Nutrition

Paradox Nutrition

Paradox Nutrition, LLC is a nutritional consultation business for the dairy feed industry. Mary Beth de Ondarza, Ph.D. is the sole proprietor.

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