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 naturally-occurring 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 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).
 

 

Managing Packing Density

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.

 

 

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:

 

• 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).

• Chop length and processing – Smaller particle size facilitates compaction, but needs to be balanced against feeding objectives.

• Storage 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!

• 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).

• 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.

• Adding 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.

• Packing layer depth – Thinner is better and the old rule-of-thumb of six inches as a maximum should really be applied.

• 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 good-quality silage.  While the old mantra “pack, pack and pack some more” is a good rule-of-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.

  

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:

 

1)      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)

 

2)      Dry weight density:

               Dry matter (%) ¸ 100                                        C

               Dry weight density = fresh weight density x C

 

Dry Matter Packing Density Determination During Filling:

It is also possible to monitor packing density during filling, as part of the overall management program.  This is done by digging down into the silage using an auger or corer and carefully collecting forage for weighing.  Once weighed, a sample can then be taken for dry matter measurement.  A suitable plastic bag is then placed open into the hole and filled with water to the level of the silage surface.  The bag plus water is then weighed.  The packing densities can then be calculated:

 

1)      Fresh Weight Density:

               Weight of sample (g) ¸ 454 (conversion to lb)     D

               Weight of water (g) ¸ 454 (conversion to lb)       E

               Fresh weight density in lb/ft³ = D x (E ¸ 62.3)

 

2)      Dry Weight Density:

               Dry matter (%) ¸ 100                                        F

               Dry weight density = fresh weight density x F