Silage Packing Density
A critical management control point for
producing high-quality silages
by R. Charley, Ph. D., Forage Products Manager-Animal Nutrition, Lallemand
The process of producing silage
involves harvesting a fresh forage crop
at a near neutral pH from the field,
putting that crop into an enclosed
storage system of some sort (e.g. bales,
bags, bunkers, pits, piles, towers, etc.)
and acidifying the material to reduce
its pH which prevents the growth
of spoilage organisms. Acidification
is achieved, either in part or in total,
by the production of organic acids
from microbial fermentation within
the ensilage (the ensiled material).
Microbes, or bacteria, either naturallyoccurring in the crop or applied as
an inoculant, convert fermentable
substrates, predominantly sugars, into
organic acids that reduce the pH and
effectively pickle the crop, producing
acidified silage.
The most important acid produced,
in terms of rapidly decreasing the
pH, is lactic acid, though it is also
advantageous to have some acetic
and propionic acids produced, to
achieve good stability of the material
at feedout and to prevent heating
and mold growth. The bacteria that
produce lactic acid in ensiled materials
are facultative anaerobes, i.e. they can
grow aerobically (with oxygen) and
anaerobically (without oxygen), but
produce lactic acid most efficiently
in anaerobic conditions. Thus to
reduce the pH as quickly as possible,
to minimize nutrient losses and the
potential for the fermentation to be
overtaken by clostridia, it is important
that the ensilage become anaerobic
as quickly as possible. Oxygen is the
enemy, both in terms of achieving
the desired fast initial fermentation to
preserve the forage and for preventing
spoilage after opening.
©The Saskatchewan Stockgrower - March 2008
The Importance of Packing Density
The process of harvesting fresh forage and
placing into storage structures tends to aerate
the forage (enabling oxygen to permeate the
ensilage). Rapid removal or displacement of
this entrained oxygen is critical to achieve fast
fermentation required to maintain quality
in the resultant silage. Left just in a loose,
covered pile, the oxygen would ultimately be
utilized by the mixed microbial population
in the forage at harvest, producing a rather
smelly pile of composted material that would
most likely turn clostridial once all the oxygen
had been eliminated. To ensure that good
preserving fermentation is achieved and to
minimize nutrient and dry matter losses, it
is important to displace as much oxygen out
of the ensilage mass as possible, by packing
the material thoroughly and effectively. The
effect of packing density on dry matter losses
in silage is shown in Table 1.
In addition, Lynch and Kung (2000) showed
that decreasing packing density resulted in
a slower ensiling fermentation (Figure 1),
which in turn influenced the level of yeasts
in the silage at opening (>100 CFU yeasts/g
in the more tightly packed silage compared
to 100,000 CFU yeasts/g in the more loosely
packed silage).
Table 1
The Effect of Packing Density
on Dry Matter Losses in Corn Silage
after 180 Days Ensiling
Silage Density
(lb DM/ft3)
10
14
15
16
18
20
Dry Matter Loss
(%)
20.2
16.8
15.9
15.1
13.4
10.0
Source: Ruppel, 1992
Figure 1
31
Table 2
Continued...
Results of Silage Density Tests for Haylages and Corn Silages
Managing Packing Density
Storage Unit
The packing density achieved is a result of
a number of factors, many of which can be
controlled by the producer. In a survey of
168 bunker silos in Wisconsin, Holmes and
Muck (1999) found packing densities ranging
from below 7 to just over 27 lb DM/ft3 and
Visser (2007) reported a wide range of
packing densities for corn silages and haylages
in various structures from a survey that
covered 177 storage units across Minnesota
and Wisconsin (Table 2). Thus, it is clearly
important to understand and manage the
factors that can improve packing density.
Bunker
Pile
Bunker/pile
12 ft bag
10 ft bag
9 ft bag
8 ft bag
The interrelationship between a number of
the controllable management factors and the
expected packing density can be explored
using an excellent planning tool available on
the University of Wisconsin Web site (www.
uwex.edu/ces/crops/uwforage/storage.
htm). This link provides a “live” spreadsheet
where the planned or actual values can be
input by the producer or advisor and the
resultant expected packing density will be
automatically calculated. By changing values
in the spreadsheet, one can also see how
adding weight to packing tractors, changing
the delivery rate of the forage to the pit,
etc., influences the final density achieved.
Producers are encouraged to go this Web site
and juggle factors to discover the importance
layer thickness plays in the whole ensiling
process.
Factors that can be managed to achieve
higher packing densities, which minimize
nutrient and dry matter losses and enhance
stability at feedout include:
1. Forage maturity/dry matter – Wetter forage
compacts more easily, but can be prone to
seepage and more likely to suffer a butyric
fermentation (especially haylages). Drier
forage is more difficult to pack and keep
compacted and is more likely to have higher
levels of yeasts and molds (increased risk of
instability at feedout).
2. C
hop length and processing – Smaller
particle size facilitates compaction, but
needs to be balanced against feeding
objectives.
3. S
torage structure – In towers, gravity will
take care of compaction of the material
lower down in the tower, while the upper
lays will not be adequately packed. From
previously given data, it is evident that
good management of bagging machines
is essential!
32
Haylage
Number of
Units Tested
31
14
3
1
14
15
1
Range of Packing Densities
(lb DM/ft3)
Corn Silage
Number of
Units Tested
9.9 – 27.2
8.2 – 22.9
14.7 – 36.3
9.5 – 11.8
3.4 – 24.8
4.3 – 27.2
8.3 – 15.9
Source: Visser, 2007
4. Forage delivery rate – This needs to be
matched against optimum packing time/
number of tractors (see below) since it
is often not practical to control delivery
rate (especially if forage is harvested by a
custom chopper).
5. Packing tractor weight – The estimated
amount of packing weight required can
be calculated by multiplying the estimated
tons of crop delivered to the silo in an hour
by 800. Weights can be added directly to
the front of the tractor or the three-point
hitch or by filling tires with water.
6. A dding packing tractors – Optimally
packing time should be one to three
minutes per ton of forage (fresh weight). It
may take more than one packing tractor to
achieve this without impacting the forage
delivery rate.
7. Packing layer depth – Thinner is better
and the old rule-of-thumb of six inches as
a maximum should really be applied.
8. Silo height – Greater height increases
silage density in the lower layers, but for
piles and bunkers this can lead to upper
layers being less well packed and can
cause significant safety issues, both while
filling and at feedout.
Bottom line
Achieving good packing density (minimum
15 lb DM/ft3) is essential for making goodquality silage. While the old mantra “pack,
pack and pack some more” is a good ruleof-thumb, there are tools available to help
producers plan and monitor the packing
operation to achieve the best packing density.
Putting those tools to work may have a
positive effect on your bottom line.
Range of Packing Densities
(lb DM/ft3)
37
21
11
3
10
15
1
6.4 – 23.6
4.9 – 18.7
4.9 – 18.6
3.2 – 12.5
5.7 – 13.5
2.4 – 13.9
5.7 – 10.5
Measuring Silage Density
The surveys mentioned in the companion
article “Silage Packing Density” (Holmes
and Muck, 1999; Visser, 2007) reviewed
measurement of packing density in bunkers,
piles, and bags post-ensiling. Determining Dry Matter
Packing Density:
These measurements are obtained using a
commercially available silage density corer,
essentially an 18-24” piece of 2” stainless
steel pipe with a sharpened cutting edge
at one end. Using a suitable gas or high
power electric drill, the corer can be driven
into the silage at various points across the
face to measure packing density at different
heights, in the center, at the edges, etc. The
depth of the hole left following the coring
is recorded and the sample weighed. To
obtain dry matter (DM) packing density, the
dry matter content of the sample needs to
be measured, using a microwave oven or
a Koster tester. The packing densities can
then be calculated:
Fresh Weight Density:
Weight of sample (g) / 454 (conversion to lb) = A
Depth of core (in.) / 12 (conversion to feet) = B
Fresh weight density in lb/ft3 = A x (0.0191 x B)
Dry weight density:
Dry matter (%) / 100 = C
Dry weight density = fresh weight density x C
©The Saskatchewan Stockgrower - March 2008