Much thought, planning, labor, and effort goes into planting, harvesting, and storing fermented forages. High quality, well-fermented, and properly stored forages set the stage to maximize forage dry matter and nutrients. Decisions revolving around the harvest and storage of forages often impact the production of milk or meat, cow health and farm profitability for the entire year.
As upright silos are phased out of use, many producers are turning to the use of plastics to preserve their silage pile or bunker. A decade ago, most American farmers using plastic were relying on 5mil Black on White film made from polyethylene. Now, the market has expanded to include thin underlays, one-layer covers, two-in-one rolls, reusable top covers, and other options. Along with the influx of plastic options came a load of new terms for producers to familiarize themselves with. Two terms that we see often are “oxygen barrier” and “vapor barrier.” Confusion between the differences in vapor barriers and oxygen barriers is common within the agriculture industry. After all, the outward physical appearance of these plastics is similar. However, there are vast differences in the design, polymers, and results achieved between oxygen barriers and vapor barriers.
The term “vapor barrier” comes from the construction industry. The function of a vapor barrier is to retard the migration of water vapor, and they are most often used between the external and internal walls of a building. Polyethylene, the type of plastic found in everything from grocery bags to garbage cans, makes a fine vapor barrier. Vapor barriers are not intended to retard the migration of air, however. While plastic oxygen barriers may be used within the construction industry, they are typically manufactured from more durable material – think brick, steel or concrete. In fact, twelve inches of cast-in-place concrete or brick makes a good quality air or oxygen barrier.
Viewed under a microscope, polyethylene contains microscopic holes or as some nutritionists say “it breathes”. While a properly designed polyethylene plastic or vapor barrier can repel most moisture forms, it does not stop oxygen infiltration. Oxygen molecules are much smaller than water molecules and penetrate traditional black and white polyethylene covers and thin vapor barrier underlays. Over time, this diffusion of oxygen causes significant spoilage in the top two to three feet of the forage surface.
Barriers to water, whether in liquid (rain), solid (snow and ice) or vapor (atmospheric moisture-humidity), are important to silage. However, water infiltration restriction is only a part of the process. By definition, proper silage fermentation is anerobic or occurs without oxygen. It is the limiting of this oxygen that is the single most important factor in maintaining forage quality and reducing spoilage. This limitation of oxygen becomes even more crucial in long-term forage storage. Limiting oxygen is one of the main reasons we harvest at specific crop moistures, fill quickly, pack well and seal immediately and effectively.
Upright silos, with their poured-in-place concrete walls, concrete staves, or glass-lined steel walls, are excellent oxygen and moisture barrier materials when properly maintained. As the industry shifted storage away from upright silos and toward bunker silos and piles, the use of plastic made from polyethylene became commonplace. With this shift, larger surface areas of forage were placed at risk and spoilage became commonplace and accepted. The introduction of true oxygen barrier plastics for agriculture use changed those dynamics.
Agriculture oxygen barrier plastics are complex multi-layer barriers. Since polyethylene is not a good oxygen barrier, other polymers are added. Some oxygen barrier plastics use a nylon (polyamide) core layer surrounded by polyethylene. While decent oxygen barriers, these plastics can suffer structural issues, can become rubbery or brittle, and have limited ability to be recycled. The best oxygen barrier plastics currently on the market employ ethylene vinyl alcohol (EVOH) for the oxygen barrier core component. This core sits in the center of the plastic layers like meat in a sandwich. This core is surrounded by polyethylene, similar to the bread of a sandwich surrounding the meat. Unfortunately, EVOH and nylon do not bind well to polyethylene on their own, thus other “glue” polymers are required. Thus, all true oxygen barriers currently in agricultural use are a minimum of five, or more commonly seven to nine layers. This increased number of microscopic layers accounts for the increased price of manufacturing or purchasing an oxygen barrier versus a polyethylene sheet. Too much EVOH in the mix will result in the film becoming brittle with age, so the best films have the correct combination between high resistance to the passage of oxygen and long-life flexibility.
So, how can a producer know for sure if they are buying a vapor barrier or an oxygen barrier? The most important tool available to the customer is the Oxygen Transmission Rate (OTR)* of a plastic. This is a plastic industry standard test, and any manufacturer of a film should happily share that information on their website. An oxygen transmission rate of under 15 (cc/m2/24 hr) generally means that the plastic performs well in blocking oxygen ingress. The purchaser should be looking for a lower number- this means less oxygen passes through over time.
While there are vapor/moisture barrier tests, the truth is that for agricultural purposes, all polyethylene plastics will block vapor if they are of decent quality and without tears or holes. Polyethylene/ PE underlayer thin plastics can sometimes give a good visual result on surface spoilage if the packing and covering is done well and quickly, by eliminating air pockets between the forage and the film. However, because of the low OTR of these films, up to 900 cc/m2/24hr, the shrink and aerobic instability of these forages will be high.
In summary, the primary and critical component of an oxygen barrier plastic is to stop outside air from entering the silage pile or bunker. This barrier also prevents the outward flow of protective gases produced during fermentation within the forage mass. Finally, oxygen barrier plastics also result in a vapor barrier. High quality oxygen barrier plastics properly placed, sealed, and in the absence of breaks should result in top-layer forage quality corresponding to the interior of the silage mass, if forage was similar at harvest. If forage taken from these areas look, smell, and test vastly different it is advised to review the covering and sealing process and examine the covering material.
*DIN 53380-3. OTR tests are made at 0.21 bar or 21% O2 under 23 degrees C / 50% Relative Humidity.