Careful thoughts and practices are involved in all phases of producing the highest-quality forage for dairy and livestock.
Today, corn silage hybrids are developed by the seed industry for high grain yields with highly digestible nutrients and fiber levels.
Crop managers closely monitor seeding rates, fertility, pest levels and crop maturities in order to set the stage for harvest.
Harvesters then monitor and adjust chopping length, height and processing to deliver highest qualities to the storage structure. And then nutritionists monitor forage analysis to maintain feed consistency. An enormous challenge to maintaining high-quality silage occurs during storage.
All of the management decisions and production inputs and costs involved before storage can be significantly lost if proper storage conditions are not maintained.
The keys to producing and maintaining the highest quality, in any silage structure, is to limit silage exposure to air and water. Air allows yeasts and other undesirable microbes to develop.
These microbes consume fiber and nutrients, reducing feed value and may also affect forage dry matter (DM) intake. Moisture additions dilute acid levels in the silage, which allows the rate of microbial growth to increase dramatically.
There are also seasonal considerations in silage management. All microbial activity depends on temperature. The rate of microbial- caused shrink of silages is more noticeable in warmer seasons than during cold conditions.
To minimize air exposure in a silage pile or bunker, a dense pack of the forage mass must be created at fill and maintained during feedout.
As harvest equipment has advanced, the potential for delivery rates of fresh chop to exceed the packing rate potential often results in poorly packed silages; and improperly covering exposed forage allows air and moisture to enter the forage mass.
Work on silage density and porosity at the University of Wisconsin by Dr. Brian Holmes has determined that silage producers can change filling and packing practices to affect silage densities. Holmes recommends a density of silage bunker and piles of greater than 15 pounds of dry matter per cubic foot of silage to minimize shrink.
Bunker density study
In 2004, we began an on-farm investigation to measure the existing levels of bunker and silage pile densities on dairy farms across Pennsylvania.
Since then we have sampled 192 bunkers on 75 different farms. Unfortunately, less than 30 percent of the sampled bunks/piles measured attained or exceeded the goal of 15 pounds of DM per cubic foot. Values ranged from 8.6 to 17.2 pounds of DM per cubic foot.
We measured silage density by using a gas-operated drill and a 2-inch diameter core sampler to drill into the face of each corn silage bunker/pile at 12 locations, three levels (bottom, middle and top) and four positions across each level.
Two positions were taken along the outside wall or edge of the pile and center at approximately 1/3 and 2/3 of the width. We collected borings and weighed the forage mass.
We recorded the depth of boring and determined DM content at each location. The 12 points were then combined to determine an average bunker silo density value. In total, we sampled more than 2,100 points.
Across a silage face there is significant density variability. Greatest densities are found on the lowest levels of all piles. In our sampling the lowest level averaged 15.5 pounds of DM per cubic foot, with a range from 7.1 to 21.7 pounds of DM per cubic foot.
The top level is the least well- packed with an average value across the 192 bunkers of 11.2 and a range in densities of 5.9 to 16.1 pounds of DM per cubic foot.
The interior of a pile contains the highest densities at all levels. In our study, from more than 180 bunks, the left and right outside areas each averaged 12.9 and ranged from 5.9 to 18.9 pounds of DM per cubic foot.
The interior areas averaged 13.9 and 14.1 and had ranges from 7.3 to 21.7 pounds of DM per cubic foot.
Management factors
Today silage harvesting equipment has the capacity to deliver 150 to 200 tons per hour of silage to a bunker or pile.
A general guideline for the amount of packing equipment needed to match up with high harvesting rates is to have 800 pounds of packing weight per ton of silage delivered.
For 100 tons per hour harvest rate, a producer would need 100 x 800 = 80,000 pounds or 40 tons of packing equipment to achieve high density levels. It becomes very easy for harvesting equipment to overwhelm packing.
Holmes has studied what factors affect silage density levels. He notes that delivery rate; packing layer thickness; packing equipment weight; packing time; and dry matter content are key factors that influence silage pile density.
On his website is a prediction equation that producers can use to input their harvest and packing procedures to estimate their silage densities.
Producers can then change their values to see what effect adding additional tractor weight or numbers can have on final packing results.
Producers that we have worked with during this project have been able to improve their final packing densities by adding extra weight to packing equipment, adding additional packing equipment, packing longer and increasing tire air pressures while packing.
Soil and silage compaction principles
There are two principles of compaction, axle load and contact area. In soils, equipment is modified to reduce compaction, but in a bunk or pile these principles are reversed.
The heavier the axle load, the deeper into the soil or silage the compaction effect will be. A light tractor has less compaction potential than a bigger, heavier tractor.
A heavier tractor compacts to a greater depth with each pass. More trips over the pile with a heavier tractor produces a denser pack at lower levels in the soil or the pile. In silage production, the heaviest packing tractor weight is recommended.
The second aspect of compaction is contact pressure. Contact pressure is related to the surface area or “footprint” of the equipment and primarily influences the uppermost levels of the soil or silage.
A flotation tire, a track tractor, or dual wheels produce a larger footprint, resulting in less compaction than a narrower tire at the upper levels of the pile.
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 or overinflating the tires would be more advantageous to increase surface density.
Covering and sidewall plastic
Because the top and the outside edges are the least-packed area of a silage pile or bunk, these areas have the greatest risk for the introduction of air and moisture, which leads to microbial shrink.
Ensuring a tight, well-maintained, oxygen-limiting barrier is required. On many farms we sampled, the use of sidewall plastic has been added to bunker management practices.
Producers drape plastic over the walls before filling, lay this plastic toward the center of the pile and then cover the entire pile with a second layer of plastic to reduce air and moisture entry on the sides and the top of the bunk.
In 2007-08 we evaluated this practice in cooperation with Dr. Limin Kung at the University of Delaware. Silage samples and densities were collected from 10 bunkers with sidewall plastic and 10 bunkers without.
Samples were collected along the edges of these 20 bunks and compared to samples collected at the same time at the center of each bunk.
The greatest effect found was related to NDF digestibility of the silage. Bunks without sidewall plastic had approximately 4 percentage units lower digestibility levels in the silage located along the wall area compared to silage from the center of the same pile.
Bunks with sidewall plastic had no difference in digestibility along the wall compared to the center.
Sidewall plastic bunkers had, on average, higher dry matter levels and greater levels of acetic acid. Acetic acid is an important inhibitor of yeast and mold growth.
On smaller, narrower bunks, losses on sides can be significant. We found that there were no dry matter density differences in these 20 bunks.
Frequently, producers comment that sidewall plastic limits them from packing close to the wall. As a packing tractor moves across a pile, all four wheels can pack. However, along a wall only the outer wheels travel over this area, reducing by half the packing effectiveness.
A wall helps to limit silage movement outward, but the benefit of axle load and contact pressure is greatly reduced along the top, outside areas.
In our study we found similar but lower densities in the outer areas in both sidewall and non-sidewall bunks, indicating that using sidewall plastic does not limit packing in wall areas by traveling close but not too close to the walls.
Conclusions
Growing the most digestible, highest-yielding corn silage crop is of limited value when storage practices significantly reduce the amount of milk yield or meat production potential of the silage crop. Results from our study indicate that managers can affect their silage packing densities and forage shrinkage. FG
- Paul H. Craig
- Extension Educator Penn State University
