Silage Density in Bunker and Bag Silos


Brian J. Holmes                                                                    Richard E. Muck

University of Wisconsin-Madison                                        US Dairy Forage Research Center

460 Henry Henry Mall                                                                   1925 Linden Drive

Madison, WI 53706                                                                        Madison, WI 53706                                                 


Bunker Silo Filling Procedure

          The recommended filling procedure for bunker silos begins by filling the back end of the storage by pushing forage up a sloped filling face in a progressive wedge technique. With this preferred method of filling, the forage is added in thin (<6-inch) layers to this filling face until the storage is full.  The progressive wedge method allows a plastic cover to be applied to the top surface soon after that area is full.


Packing Density

          Attaining a high density in a silo is important for two primary reasons.  Firstly and most importantly, density and dry matter content determine the porosity of the silage.  Porosity, in turn, sets the rate at which air moves into the silo and subsequently the amount of spoilage which occurs during storage and feedout.  Ruppel (1992) measured dry matter loss for alfalfa silage and developed an equation to relate the loss to density.  Table 1 summarizes those results.  Secondly, the higher the density, the greater the capacity of the silo.  Thus, higher densities generally reduce the annual cost of storage per ton of crop by both increasing the amount of crop entering the silo and reducing losses during storage.  The factors affecting density in bunker silos are not well understood.  General recommendations have been to spread the crop in 6-inch layers and pack continuously with heavy, single-wheeled tractors.  In a survey of alfalfa silage in 25 bunker silos, Ruppel et al. (1995) found tractor weight and packing time (min/T AF or min/ft2) were the most important factors affecting density.  However, both factors only explained a small fraction of the variation observed, and layer thickness was not measured.


TABLE 1.  Dry Matter Loss as Influenced by Silage Density (Ruppel, 1992)

Density (lbs DM/ft3)

Dry Matter Loss, 180 days (%)














The objectives in a study conducted by Holmes and Muck (1999c) were to measure density in a wider range of bunker silos and to correlate those densities with filling practices. The range of densities and dry matter contents are shown in Table 2.  Ranges of dry matter densities were similar for both haycrop and corn silages.  Densities on the low end suggested little packing, whereas the highest densities were in the range observed in tower silos.  Average dry matter densities were slightly higher than a recommended minimum density of 14 lbs DM/ft3.  Forage becomes denser in response to the weight of forage piled above it. Densities were positively correlated with the height of silage above the core.  To put densities on a common basis, all densities were adjusted to the median depth below the surface (7.1 ft) using Equation 15 of Pitt (1983) and assuming a compressibility of 2.2 ´ 10-9/psi.  Adjusted dry matter density was positively correlated with average packing tractor weight, packing time, and dry matter content and inversely correlated with the initial depth of the crop layer when spread in the silo.


TABLE 2.  Summary of Core Samples Collected from 168 Bunker Silos (Holmes and

Muck , 1999c)


Haycrop Silage (87 silos)

Corn Silage (81 silos)







Dry Matter (%)







Wet Density (lbs/ft3)







Dry Density (lbs/ft3)







Avg. Particle Size (in)







SD = standard deviation.


          The linear regression which explains 18% of the variation of estimated dry matter density (DMD) is expressed as:

          Est. DMD (lbs DM/ft3)  =  (8.5 + PF × 0.0155) × (0.818 + 0.0136 × D)

where average depth (D) and packing factor (PF) are calculated as:

D      =   avg. silage depth (ft)  =  (height at wall + height at center) ÷ 2.



W      =    Proportioned average tractor weight (lbs) for all tractors packing silage.

                          Example:  Two tractors pack 100% of the filling time; tractor #1 weighs

                          25,000 lbs and tractor #2 weighs 15,000 lbs.  Then the proportioned average

                          tractor weight is 20,000 lbs  =  (25,000 + 15,000) ¸ 2.  If tractor #1 packs

                          90% of filling time and tractor #2 is used 50% of the time, the proportioned

                          average tractor weight becomes:

                                      19,286 lbs  =  (25,000 ´ 0.9 + 15,000 ´ 0.5) ´ [90 ¸ (90 + 50)].


         L       =     Layer thickness (inches) of the spread but unpacked crop in the silo prior to driving over it during the first packing pass.

         N      =     Number of tractor-packing equivalents, where N = 1 when one tractor is packing continuously during the filling process.  This value can be fractional, reflecting one or more tractors packing intermittently.  For example, if one tractor packs continuously during the filling process and another packs 50% of the filling time, N = 1 + 0.5 = 1.5.  If there is only one packing tractor and it packs for 11 hr/day and the silo is filled 10 hr/day, then N = 11/10 = 1.1.

         DM   =     Dry matter content (decimal).

                  For example, 35% dry matter forage is used as 0.35 in the equation.

         C      =     Crop delivery rate (T AF/hr) to the silo.


          Use of rear duals or all duals on packing tractors had little effect on density.  Other factors such as tire pressure, crop, and average particle size were not significantly correlated with density.  Thus the low r2 of the regression of dry matter density vs. the 5-parameter packing factor probably reflects variability in accurately estimating parameters such as initial depth of the crop and packing time per ton rather than missing factors important to determining density.  Holmes and Muck (1999d) have developed a spreadsheet to simplify the process of solving those equations. The spreadsheet can be downloaded from the Team Forage web site with URL:


          One practical issue raised in the study was packing time relative to crop delivery rate to the silo.  Packing time per ton was highest (1 to 4 min/T AF) under low delivery rates (<30 T AF/hr) and generally declined with increasing delivery rate.  Packing times were consistently less than 1 min/T AF at delivery rates above 60 T AF/hr in the survey.  These results suggest that farmers using contractors to harvest their silage crops probably will need to pay particular attention to spreading the crop in a thin layer and would benefit from using several packing tractors simultaneously.


          Rapid forage harvest and delivery to storage requires a corresponding rapid rate of storage filling.  A producer selecting a self-propelled forage harvester may see a doubling of forage delivery rate (Table 3) compared to a large pull-behind harvester, provided transportation is also increased.  This requires larger and perhaps more push-up/packing tractors at the bunker silo.

                         TABLE 3.  Forage Harvester Average Capacity (Shinners, 2001)



Capacity (T AF/hr)

Forage Harvester Type




Pull, 250 HP




Self-propelled, 450 HP





         The desire to improve feed quality drives the need to have rapid harvest and silo filling.  As herd size increases, the quantity of forage harvested also increases.  However, the window of harvest opportunity remains the same. Any bottlenecks in the harvest/delivery system cause cost increases due to equipment downtime and/or forage quality losses.  Some custom operators offer a complete service of harvesting through silo filling.  With an adequate complement of equipment and labor force, bottlenecks found on many farms can be eliminated, and rapid harvest/delivery can be accomplished.  This service requires a higher out-of-pocket cost to the producer but will be covered by the preservation of feed quality. 


         The spreadsheet of Holmes and Muck (1999d) was used to estimate density for a hypothetical case where a producer increased harvest rate from 50 T AF/hr to 100 T AF/hr.  The storage averaged 9 ft tall, the forage dry matter was 35%, forage layer thickness was 6 inches, and one tractor (30,000 lbs) was being used to distribute/pack the forage.  The results are summarized in Table 4.


          The packing tractor should be as heavy as possible to achieve high forage density.  Tractor weight can be augmented by adding weight to the tractor within the limits set by the manufacturer.  Weight can be increased by adding iron wheel weights, adding liquid to tires, or adding front end and 3-point hitch weight. As the harvest rate increases, the need for more than one pushing/packing tractor increases.  One 40,000-lb tractor will handle a harvest rate of about 90 T AF/hr while two or more 33,000-lb tractors may be needed between harvest rates of 90 to 120 T AF/hr. Dual wheels all around will improve traction and tractor maneuverability on a slippery surface. Tractor rollover protection (ROPS), the use of a seat belt and selecting an experienced operator helps to improve safety in an inherently unsafe process.  A shuttle shift transmission is very convenient for the operator making frequent changes of direction while packing.


TABLE 4.   Scenarios for Trying to Improve Silage Density When Forage Delivery Rate is Increased  from 50 T AF to 100 T AF/hr

Variables Changed from the Base Case

Est. Dry Matter

Density (lbs DM/ft3)

No change in packing procedure


Add 20,000-lb tractor for 50% time


Add 20,000-lb tractor for 100% time


Add 5,000 lbs weight to 30,000-lb tractor and do not use 20,000-lb tractor


Add 5,000 lbs weight to both tractors and use both tractors 100% of time


Reduce layer thickness from 6 inches to 4 inches


Use both tractors 100% of time and reduce layer thickness to 4 inches.


Add 5,000 lbs to 30,000-lb tractor and reduce layer thickness to 4 inches


Add 5,000 lbs to both tractors, use each 100% of time, and reduce layer thickness to 4 inches




          In narrow bunker silos, the most logical packing direction is from back-to-front.  In wider bunkers, consider packing in both directions to achieve a more uniform packing of the forage.  In either case, the packing pattern should allow the wheel patterns in the forage to overlap about half a tire to improve uniformity of packing.  When dual wheels are used, try to have a wheel pack the forage left unpacked between the wheels of the previous pass. If packing in both directions between bunker silo walls, remove the drawbar to avoid damage to the bunker walls and/or to the tractor.



                                                                   Feed Out

         Feedout losses can represent up to 30% of the total dry matter loss in the ensiling process (Roth and Undersander, 1995). Losses occur from the exposed face and top as well as from loose silage lying on the floor between feedings. Therefore, only the amount of silage that will be fed in a short period should be uncovered at one time. Plastic can be pulled back from the silage top or cut off each day. At no time should more than three days worth of silage be exposed.


         A common attitude is that “if the cows eat it, it will be fed”. Bolsen and workers at Kansas State University tested the effect of feeding four different levels of spoilage to steers (Bolsen, 2000). Inclusion rates were 0%, 5.4%, 10.7% or 16% of ration DM. Dry matter intake, and the digestion of dry matter, organic matter and starch declined with increasing level of spoilage in the ration, including dramatic declines in fiber digestibilities. Even at the lowest level of inclusion, spoilage totally destroyed the rumen forage mat. This study, along with others, underscores the necessity of minimizing spoilage and of excluding spoiled feed from the ration. 


         The rate and method of silage removal from the face critically affects feedout loss and animal performance. Removal rates should never be lower than 4 inches per day in the summer and 3 inches in the winter. Minimal removal rates are most critical with hay crop silages, high moisture corn and drier silages (Bodman and Holmes, 1997). One method of determining if enough silage is removed at each feeding is to take a 12-inch deep boring into the removal face at the beginning of a feeding. If adequate silage is being removed and the appropriate removal management is employed so a tightly packed, smooth face results, the sample should be cool. If any part of the sample is warm, adjustments are in order.


Pitt and Muck (1993) determined the dry matter loss during feed out of bunker silos as a function of silage removal rate.  They determined the dry matter loss was 3% at the recommended removal rate of 6 inches per day for 35% dry matter silage with a density of 14 lbs DM/ft3.  They also concluded dry matter loss was reduced as silage density increased.  Muck and Pitt (1994) state that dry matter loss is proportional to silage porosity.  Porosity is inversely related to dry matter density and dry matter content.  Based on this information, Figures 1-3 were developed to establish the dry matter loss as a function of dry matter density, silage removal rate, and dry matter content.  In Figure 1 (9 inches per day removal rate), the dry matter loss during removal is less than 3% when the dry matter density is greater than 14 lbs DM/ft3 and forage is ensiled at less than 40% dry matter.  For the forage ensiled at 50% dry matter, the dry matter density must be greater than 17 lbs DM/ft3 before the removal dry matter loss is 3% or less.  Note that porosity must be less than 55% for the removal dry matter loss to be less than 3% when the removal rate is 9 inches per day. In the situation where the silage face removal rate is 6 inches per day (Figure 2), the porosity must be 43% or less for the removal dry matter loss to be 3% or less.  As the forage dry matter content increases, higher and higher dry matter densities are needed to keep the removal dry matter loss under 3%.  If the dry matter density is less than 13 lbs DM/ft3, it will be difficult to keep dry matter loss under 3% for any dry matter content graphed with a 6-inch silage face removal rate.   Figure 3 presents the removal dry matter loss as a function of dry matter density when a 3-inch silage face removal rate is used.  Under these conditions, the porosity must be less than 23% to keep the removal dry matter loss under 3%.  The dry matter density must be greater than 16 lbs DM/ft3 for this to occur.  Face removal losses will be higher than 4.5% when the dry matter density is less than 14 lbs DM/ft3.  In fact, dry matter loss can be as high as 10% when forage is 50% dry matter and density is 10 lbs DM/ft3. As the silage face removal rate decreases from 9 inches per day to 3 inches per day, forage must have a lower dry matter content (more moist) and/or a higher dry matter density to assure dry matter loss is kept under 3%.


The determination of silage density is important for three reasons. To determine if adequate packing has occurred, to determine the face removal rate and to manage feed inventories. Two methods of density determination are available. Some consultants are using a silage probe similar to that used in the Wisconsin Bunker Density study that provides a known volume and weight from which density is calculated (Holmes and Muck, 1999c). A very practical approach that can be used on farms involves marking on the silo walls where silage removal starts and ends, weigh out the silage in the feeder wagon and determine density. Silage volume is calculated:                         


Silage Volume (ft3) = Ave. Silo Width (ft) x Distance Between Marks (ft) x Ave. Silage Height (ft)

Figure 1. Dry Matter Loss vs Silage Density During Silage Removal From a Bunker Silo

Face at the Rate of 9 Inches/Day.


          Figure 2. Dry Matter Loss vs Silage Density During Silage Removal From a Bunker

Silo Face at the Rate of 6 Inches/Day.

          Figure 3. Dry Matter Loss vs Silage Density During Silage Removal From a Bunker Silo

Face at the Rate of 3 Inches/Day.


                         where Average Silage Height  = (Height of Silage at the Wall [ft] + Maximum Silage Height [ft])/2. 

         Silage density is calculated by dividing silage weight by silage volume. An example where 72,000 lbs. AF silage is removed from a silo 40 feet wide, with an average height of 11 feet and a 4-foot slice of silage removed follows:

         Silage Density = 72,000 lb. AF/ ((40 feet) (4feet) (11feet)) = 40.91 lb. AF/ft3.

To determine the density on a dry matter basis, multiply the As Fed density by the percent dry matter:

         DM Density = 40.91 lb. AF/ft3 x 0.35  = 14.32 lb. DM/ft3.


                                                         Silo Bags

Relatively little research has been published on the performance of bag silos. Losses are reputedly low with bag silos. Limited research results generally agree with that reputation. Rony et al. (1984) reported a 9.0% dry matter (DM) loss in a alfalfa:grass silage and 6.1% loss in corn silage. Storage time and feed out rate were not reported. Wallentine (1993) reported a 2.5% loss in corn silage also under unspecified conditions. In contrast, Kennedy (1987) found that losses in two bag silos were double those found in bunker silos.


Densities in bag silos are also difficult to obtain. Esau et al. (1990) indicated that wet densities were on the order of 43.6 lbs/ft3. Assuming 35% DM, that would result in a dry matter density of 15.3 lbs/ft3. Harrison et al. (1998) reported considerably lower DM densities for corn silage in 9 ft diameter bags of only 2.68 to 3.18 lbs/ft3. Holmes (1998) calculated DM densities based on filling weight records from several farms and reported a range of 9.1 to 15.6 lbs/ft3. Most of the bags were either alfalfa or corn silage, and there were no obvious trends with crop or bag diameter.


Overall, there are limited data on losses from bag silos, and the densities reported are highly variable. This makes accurate economic assessment of bag silos relative to other types difficult. Information on densities and losses is also important to farmers with bag silos relative to feed inventory and management.


Three farms (Arlington, Prairie du Sac and West Madison) that are part of the University of Wisconsin Agricultural Research Stations have been making bag silage for several years, and all appear to be managing this type of silage well. At Arlington (Arl) and Prairie du Sac (PDS), bag silage is often made for research studies involving small numbers of cattle, and consequently feed out rates may be lower than recommended. At West Madison (WM), bag silage is made to be re-ensiled later in small tower silos. These bag silos are emptied rapidly, typically one third of a bag in a day, and are resealed between emptying events.


The bagging machine used at Prairie du Sac was a 8 foot Ag Bag model G6000. The West Madison and Arlington stations shared a 9 foot Kelly Ryan model DLX. This provided the opportunity to compare densities from different operators using the same machine. Occasionally, the Arlington station rented a 9 foot Ag Bag machine.


During the 2000 harvest season, all bag silos made at the three farms were monitored. This consisted largely of alfalfa and corn silages. All loads of forage entering the bags were weighed. The distances for each load were marked on the bag and measured after the bag was completely filled. The load samples were analyzed for moisture content by freeze drying.


At emptying, the weight of all silage removed from a bag was recorded. Any spoiled silage not fed was weighed and identified as such on the emptying log. A grab sample from the face of each silo was taken periodically, one per filling load. Spoiled silage was sampled separately.

Average densities for the bags were calculated based on weight ensiled, overall length and nominal diameter. Similarly, density variation by load along the bag length was based on individual load weights and load length along the bag.


Over the course of the 2000 harvest season, a total of 25 bag silos were made at the three farms. The DM contents of the hay crop silages were generally drier than recommended (30 to 40% DM) whereas the corn silages were largely within that range. The bags used at the Arlington and West Madison stations were 200 ft long and generally filled to capacity. Most of the bags at Prairie du Sac were 100 ft long and often not completely utilized.


Within hay crop silages, the highest densities were found with the Kelly-Ryan bagger when used at Arlington (14.5 lbs/ft3) whereas the Kelly-Ryan at West Madison and the Ag Bag at Prairie du Sac averaged about 10% lower. In corn silage, the Kelly-Ryan averaged less than 12.5 lbs/ft3, i.e., lower than that in hay crop silages. The Ag Bag bagger at Prairie du Sac produced higher averages in corn than in hay crop. Kernel processing at that farm reduced density.


One difficulty in comparing the average dry matter density is that DM content varied considerably among bags. A survey of bunker silo densities found that dry matter density increased with DM content (Muck and Holmes, 2000). If a similar trend occurred in bag silos, a comparison of averages may produce a false assessment of differences across crops, baggers, etc. Using linear regressions, we calculated an estimated density for each condition at a constant crop DM content (40%). This allows a truer comparison, particularly across crops in this study. On this basis as seen in Table 5, the Kelly-Ryan at Arlington and the Ag Bag at Prairie du Sac produced similar DM densities in hay crop silages, just over 12.5 lbs/ft3. When the Kelly-Ryan was used for hay crop silages at West Madison, densities were approximately 10% lower than those at Arlington. The operator at Arlington consistently achieved a higher density.


Table 5. Average Density (lbs DM/ft3) at 40 % Moisture


Bagger                Station                          Hay            Corn           Processed/Unprocessed


8’ Ag Bag             Prairie du Sac                 12.6            17.0                 Unprocessed

14.7                                  Processed                                                                                                                                              


9’ Ag Bag             Arlington                         -----            12.1                 Processed

9’ Kelley Ryan                                            13.1            12.1                 Processed



9’ Kelley Ryan      W. Madison                   11.5            11.2                 Unprocessed



With the Kelly-Ryan, densities in corn silage were lower than those in alfalfa silage (3 to 8%). There was less difference between the two farms using the Kelly-Ryan (8%), but Arlington generally used kernel processing whereas West Madison did not. In contrast, the Ag Bag at Prairie du Sac produced considerably higher densities (16 to 35%) in corn silage than in hay crop silage. Unprocessed corn silage at PDS was consistently denser than processed. Four of the five corn silage bags at PDS were produced for a trial comparing processed vs. unprocessed corn silage. The four bags were filled with corn from the same field, and the two bags of each maturity (early vs late) were filled within a day of each other. Consequently, the difference in density due to processing at PDS was not only consistent but also the result of a planned comparison.


Typical recommendations for feed out rates from bunker silos in the northern Midwest are 4 to 6 in/d from the whole face. Based on average densities from our study, minimum feed out rates of 6 to 8 in/d for bag silos might seem appropriate. However, average densities do not account for variability in density across the face of bag silos and the potential impact on feed out recommendations.


Seven core samples were taken to estimate within-bag density variation on five bags during emptying with average densities shown in Figure 4. The density of approximately the outer 12 in. is of substantially lower density than the center and lower portions of the face. Occasionally, areas that were expected to have a high density had low densities. Such random pockets of low density may explain pockets of mold in the middle of the face that our farm crews have seen in the past. Whether such variations in density occur in other types of baggers is unknown at this time, but will be of interest in future research. Overall, the low densities around the outer portion of the bag and the occasional low density pockets elsewhere suggest that higher feed out rates than indicated by average bag densities may be needed to minimize feed out losses.

Figure 4.  Average dry matter densities measured in five bags at different face locations.



To this point, 15 of the bag silos have been completely emptied. Dry matter losses for these bags are shown in Table 6. The spoilage represents the silage removed from the bag but not fed whereas the gaseous/seepage is a measure of difference between the amount ensiled and the total amount (good and bad) removed from the bag. Seepage losses occurred in only two bags, the two wettest (30 and 32% DM) corn silage bags from Prairie du Sac.


Table 6. Range of Loss ( % DM)


Loss Type                     Range                           Average              Average w/o

                                                                                                         Worst 3 Bags


Gaseous/Seepage           -0.3-15.7                          8.4                              7.9

Spoilage                          0.0- 25.4                           5.8                              1.9


Total                              -0.3- 38.2                        14.2                              9.7


Substantial losses can occur in bag silos. Three bags of 15 had major losses (>25%). In one case, bird damage and an unknown time between the event and repair was most likely the culprit. In the other two, the farm crews noted no unusual damage to the plastic, and so presumably these high losses were related to porous, drier silages as well as a low feed out rate in one case. Thus while it is possible to obtain low losses in bag silos that are similar to those in tower silos (i.e., 10% or less), there is substantially greater risk in bag silos for major losses. Good management (harvesting between 30 and 40% DM, operating the bagger to get a smooth bag of high density, monitoring routinely for and patching holes, and feeding out at a minimum of 24 in/d) is essential to obtain low losses from bag silos. Our results provide evidence that deviation from those practices come with a cost.


·        Dry matter densities based on the nominal diameter of the bags were approximately 12.5 lbs/ft3 for alfalfa silage at 40% DM. Densities decreased with wetter crops at a rate of approximately 0.19 lbs/ft3 per percentage unit decrease in DM content.

·        Dry matter densities in corn silage were 3 to 8% lower with the Kelly-Ryan and appeared to be related to longer particle sizes in the corn silage. Densities with the Ag Bag were 16% higher with processed corn silage and 35% higher with unprocessed corn silage compared to hay crop silage. For both baggers, densities decreased with wetter chopped corn at a rate of approximately 0.31 lbs/ft3 per percentage unit decrease in DM content.

·        Operators affect density. The Kelly-Ryan was used at two farms, and one farm consistently averaged 8 to 10% higher densities than the other did.

·        Core samples taken at the face of bags during emptying found considerable variation in density, largely on the outside edge of the silage mass, suggesting the need for higher feed out rates than might be anticipated for similar average densities in bunker silos.

·        Approximately half the bags had little or no spoilage loss whereas three bags suffered severe losses of 16 to 25% spoilage loss. Removing those three bags from the average had little effect on gaseous losses but reduced spoilage and total losses to 1.9% and 9.7%, respectively. These are losses similar to those in tower silos.

·        Gaseous/seepage losses were higher in low DM silages and at low feed out rates (< 8 in/d).

·        Spoilage losses were primarily associated with drier, porous silages although not all dry and porous silages had visible spoilage. One of the bags suffering major losses had bird damage that was not immediately detected and repaired.

·        Overall, the variation in losses appears to confirm that deviations from good management (harvesting between 30 and 40% DM, operating the bagger to get a smooth bag of high density, monitoring routinely for and patching holes, and feeding out at a minimum of 24 in/d) result in greater losses.



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