Penn State Corn Silage Bunker Density Study Summary Report

Two-year study results on the density of corn silage bunker silos, provides some management recommendations for improving silage density and silage quality in bunker silos.
Penn State Corn Silage Bunker Density Study Summary Report - Articles



The science and art of producing high quality corn silage has changed dramatically in the past few years. Today corn silage hybrids are selected for high grain yields with highly digestible nutrients and fiber levels. Crop managers are more closely monitoring crop maturities to ensure optimum dry matter levels at harvest. Harvesters adjust chop length, height and processing to deliver a potentially high quality feed to the storage structure. And nutritionists monitor forage analyses to maintain feed consistency at the feed bunk. A great challenge to successful silage production occurs in the silo, an upright or a bag or a pile or bunker. In a silo, microbial processes can be modified to enhance fermentation, through management practices such as harvest moisture, length of harvest period, oxygen exposure, and silage inoculation. However the most important factor influencing silage quality is the density of the silage mass.

Optimum silage fermentation occurs in an oxygen free environment. More dense silage provides ideal conditions for rapid oxygen depletion at the start of fermentation and minimizes the introduction of oxygen back into the silage pack during storage and feedout. Both of these benefits result in a higher quality forage ingredient.

Research at The University of Wisconsin and Cornell identified a significant advantage for bunker silos with a density pack greater than 14 pounds of dry matter per cubic foot of silage. In a 1992 study, Curt Ruppel, Cornell determined that bunker silo dry matter losses approach 17 to 20% at silage densities less than 14 lbs dm/ft3. Investigations by Agricultural Engineer Dr. Brian Holmes at The University of Wisconsin measured bunker silo densities and recorded management practices to determine that silage delivery rates, silage dry matter content, depth of silage pile, average packing tractor weight, packing layer thickness and packing time had considerable influences on silage density.

This study was initiated to determine bunker and silage pile densities in south central Pennsylvania. It was anticipated that by informing producers of existing silage densities and recommended packing procedures that forage quality of silages would result. In addition knowledge gained by the investigators would enable more confident recommendations on bunker silo management.

Tremendous support and encouragement for this educational program was received from many individuals. Dr. Greg Roth, Crops and Soil Sciences Department, Penn State assisted in design, implementation, statistical analysis and encouragement. Dr. Dennis Buckmaster, Department of Ag and Biological Engineering Department, Penn State provided funding and direction. Sky View Labs, Jennerstown, PA provided reduced cost forage analysis in 2004. Dr. Brian Holmes, Agriculture Engineering Department, The University of Wisconsin provided experienced recommendations for sampling procedures and data collection and evaluation and strong motivation. Feed industry interest in this project has been very positive and helpful. Finally the cooperators, dairy managers and livestock feeders, who were interested in investigating forage storage management and offered their facilities, knowledge, experience and time in order to collect and interpret this information.

Pennsylvania Bunker Density Study

Investigations into the existing bunker silo densities in South Central PA began in late winter 2004. Paul Craig, Extension Educator, Dauphin County Extension and Dr. Greg Roth, Professor, Department of Crop and Soil Sciences, Penn State initiated a bunker silo density survey. A Stihl gas-operated drill and a 2 inch diameter core sampler were used to drill into the face of each corn silage bunker at 12 locations. The density of the silage, at these 12 locations, was calculated based on volume, forage mass and dry matter content. The 12 location points were combined to determine an average bunker silo density value.

Samples were collected at three levels of each bunker or pile. The bottom level was approximately 4 feet above the base, the top level approximately 3 feet below the surface and the middle level located half way between. Four points were sampled on each level. Positions were numbered 1 to 4, left to right. Points 1 and 4 were taken within 8-10 feet of the outside wall or edge of the pile and points 2 and 3 taken at approximately 1/3 and 2/3 of the width.

In 2004, twenty-two bunkers were sampled; in 2005 twenty-one bunkers were tested. There were fourteen bunkers that were sampled in 2004 and 2005. More than 525 density calculations were measured. Densities ranged from 6.0 to 21.7 lbs dm/ft³. The average value for each bunker or pile, 12 points per bunker, varied from 8.3 to 16.8 lbs dm/ft³.

Summary Bunker Silo Density Calculations
# Bunks/Piles Sampled2221
Total Sample Points272252
Range of Bunk Averages8.3-16.4 dm/ft³11.1-16.8 lbs dm/ft³

Density Score Card

Earlier investigations have recommended that a bunker silo dry matter density goal should be a minimum of 14 lbs dm/ft³. Of the 22 bunkers sampled in 2004 only seven (7), 32%, achieved a silage density greater than 14 lbs dm/ft3 and eight (8), 36% averaged less than 12 lbs dm/ft³. In 2005, ten (10), 48%, exceeded 14 lbs dm/ft³ and only three (3), 14%, were below 12 lbs dm/ft³.

Bunker Silo Densities20042005
Exceeding 14 lbs dm/ft³7 out of 22 - 32%10 out of 21 - 48%
12-14 lbs dm/ft³7 out of 22 - 32%8 out of 21 - 38%
less than 12 lbs dm/ft³8 out of 22 - 36%3 out of 21 - 14%

Year to Year Comparison

There were fourteen (14) bunkers sampled in both 2004 and 2005. These bunkers had a change in density values from -8% to +74%. Overall, the improvement average for all 14 bunkers was 1.3 lbs dm/ft³ or 12.8%. Impact was greatest with producers who had the lower densities in 2004.

Density Trends by Bunker Silage Level

It is evident that the bottom level of a silage mass has the greatest density, followed by the middle level and then the top. This effect is caused by self packing of the silage as the increasing depth of silage results in additional weight to increase silage density. The trend also shows that too frequently the bottom and the middle level densities were sufficient for storage but the top level density was not packed adequately for optimum fermentation and storage. Unfortunately the top level is the area of the silage that is most likely to be exposed to air and moisture. Lower densities can result in rapid dry matter losses and potential spoilage in this zone.

Bunker Level Densities
Bunker Level2004 Average Density2005 Average Density
Top11.2 lbs dm/ft³11.9 lbs dm/ft³
Middle12.9 lbs dm/ft³13.9 lbs dm/ft³
Bottom14.0 lbs dm/ft³15.1 lbs dm/ft³

These findings indicate that additional packing attention on the upper portion of a pile or bunk is recommended. Perhaps, but highly unlikely, the slowing down of the delivery rate or adding additional packing equipment close to the completion of the filling process could be a management strategy. Other considerations would be to put slightly wetter silage on top of the pile, perhaps incorporate inoculants and especially ensure an airtight cover or seal as quickly as possible. Safety risks increase with a second or third packing tractor and as wall heights are challenged.

Overfilling Wall Heights

Placement of silage above wall heights is not advantageous. Consistently, density samples from over-filled bunkers had a significant decrease in densities in areas above wall heights. One farm with 8 foot side walls, filled to nearly 14 feet at the highest point, had densities of: Bottom - 16.1; Middle - 14.8; and at the Top - 11.3 lbs dm/ft3. Covering with securely anchored plastic sheets is vitally important to prevent additional spoilage but it cannot substitute for reduced silage densities.

Bunker Densities by Position

A significant difference in silage densities across the face of the pile was noted. Samples taken from the outside edges, within 8-10 feet of the walls, consistently had lower values than interior samples on each level of silage sampled. This trend was evident in both 2004 and 2005.

Density Trends by Bunker Position
Position2004 Average Density2005 Average Density
Left Outside (#1)12.413.4
Left Center (#2)12.914.0
Right Center (#3)13.214.1
Right Outside (#4)12.313.1


The lower densities measured along walls or the outside edges of piles results from the challenge of operating equipment in these areas. Traveling along a wall with large equipment requires experienced operators to achieve desired affects. Silage in the center of a pile will be traveled over at least 2 times as equipment works across a pile. The area within the width of equipment (8-10 feet) will only be traveled on one time in this scenario. This results in lower pack in this area. Unfortunately concrete walls are not air tight with many joints and cracks that can allow air to interface with a large area of silage that does not have a very dense pack. Extra attention to this situation is needed.

Side Wall Plastic

Many bunker operators are now incorporating an extra sheet of plastic on the sidewalls to minimize oxygen and moisture exposure on wall faces.

Effect of Level and Position on Silage Density

The following graph illustrates the relationship between silage densities for 21 bunkers at each level (top, middle and bottom) and position across each level. No matter what level of silage the densest silage was in the center of the pile and the lowest densities were found on the outside edges. The bottom layer was denser than the middle layer which is denser than the top level. Results from 2004 are very similar.

Factors Affecting the Density of Silage

Dr. Brian Holmes, University of Wisconsin, Agricultural Engineer is a leading investigator of optimum bunker silo management. Dr. Holmes has uncovered that the following factors have the greatest affect on bunker and pile silage densities. These include: delivery rate into the bunk or pile, crop dry matter content at harvest, depth of silage pile, average packing tractor weight, number of packing tractors used, packing layer thickness, and packing time. Many of these factors can be managed by silage producers; however, many are not likely to be affected, such as delivery rate.

Dr. Holmes has created an Excel-based spread sheet that can be used to estimate bunker silo densities based on individual producers and their existing practices. Factors such as adding weight or slowing delivery rates or adding a second tractor can be changed and the program will estimate what the resulting density level should be. The spreadsheet can be downloaded from the internet.

Recommendations for Increasing Density

A significant portion of silage harvest is done by custom harvesters. The potential exists for harvest rates to significantly exceed the ability of on-farm packing equipment to have the size and weight necessary to provide adequate packing at high harvest rates. Since slowing down delivery rate is not a realistic management practice, increasing the weight of the packing equipment is probably the most likely alternative to increase silage pack. Increasing the weight of the packing tractor(s) is possible with wheel weights and liquids, front and/or rear hanging weights, and a distribution blade. Dual wheels can provide additional tractor weight and stability. Adding one or more heavy tractors, especially as the pile is nearing completing is advantageous. Many custom harvesters are able to provide a large packing tractor as part of their service.

Most bunks are filled using a wedge process rather than a drive over method, more common in piles, for better results. Extra attention to wall areas in bunkers by a second tractor can be beneficial. These areas require an alert and deliberate operator. This may not be possible on narrow bunks.

Reducing the layer thickness of silage that is compressed with each pass will increase densities. A 6 inch layer compacts more effectively than a 12 inch layer. Poor distribution of fresh silage on the pile creates uneven layers from 6 to 24 inches thick throughout the pile. Harvest rate, operator experience and packing equipment used will affect management of layer thickness.

The top level of a pile is always the least well packed. In addition this area is more likely to be exposed to runoff and oxygen. Extra time spent packing the surface will improve densities in this area. This sealing off of the bunk would be the time to slow down harvest rates, add an extra tractor or pack for a longer time prior to rapid covering.

Soil and Silage Compaction Principles

Soil and silage compaction affects are created very similarly. There are 2 relationships to any type of compaction. These are axle weight for deep compaction and contact pressure or foot print for the surface area.

The heavier the axle load, the deeper into the soil or silage the compaction effect will be. A light tractor would have less compaction potential than a bigger, heavier tractor. In silage production the heaviest packing tractor weight is recommended. A heavier tractor compacts silage to a greater depth with each pass, (see diagram, on left). More trips over the pile with a heavier tractor produce a denser pack that decreases as the pile is increased in height.

The second aspect of compaction is contact pressure, (see diagram, on right). Contact pressure is related to the surface area or "footprint" of the equipment and primarily influences the upper most levels of the silage. A floatation tire, a track tractor, or dual wheels produce a larger footprint resulting in less compaction than a narrower tire. In a bunker the surface level always has the lowest level of compaction. More compaction, on the top level, is necessary. On top, a narrower tire or single wheeled heavy tractor would be more advantageous to increase surface density.

Principles of Soil Compaction

Many times dual wheels are selected for compaction and stability against rollover risks and on a properly weighted tractor can provide excellent silage compaction levels. However, because the top layer of a bunk or pile is not affected as greatly by the first principle of compaction, axle weight, the surface compaction affect must be considered. A second tractor with extra weight and single wheels could be incorporated into a packing procedure where the narrow contact pressure tractor is operated primarily on the upper levels and the wall areas while the largest tractor distributes silage in six inch layers and compacts utilizing the axle weight principal to greater depths.


Growing the most digestible, highest yielding corn silage crop is of limited value when storage and feeding practices significantly reduce the amount of milk yield or meat production potential of the silage crop. Results from the 2004 sampling season indicated that there was significant room for improvement in bunker and pile silage packing practices. Results from 2005 indicated that many producers were able to modify their management practices to improve silage packing densities, (range -8 to +74%). Over 14 bunkers the average improvement was 12.8%.

Producer interest in silage density is impressive. Requests for density determination were received from across the region. A greater appreciation of management affects has been recognized by many silage producers. One cooperator commented that this type of investigation was "invaluable" as far as he was concerned. Another mentioned renting equipment at harvest for the sole purpose to increase silage density packing potential in his bunkers. A third saw density improvement on his 750 ton capacity bunker improve by more than 74%. It is conservatively estimated that he reduced his storage dry matter losses by 15%, probably more. At a silage value of $35.00 per ton, he saved nearly $4000 in potential silage losses and probably produced higher quality silage.