Baled silage is expensive enough without pulling the wrapping off to find mouldy and/or unpleasant smelling silage inside when feeding it out 6-12 months later. However, just like a carcase competition, farmers and contractors can learn a lot when the plastic 'skin' comes off.
Whenever the ensiling process has been compromised (e.g. punctured plastic seal, baled too wet or too dry), dry matter (DM) and nutrient losses will occur, as well as animal health issues occasionally. These losses may appear to be small but are usually far greater than first thought. Hence, losses that appear large, are also much larger than imagined. The extra effort to prevent, avoid or repair problems is well worthwhile financially.
Understanding a simple 'act of nature' equation can be useful in determining problems in baled (and forage harvested) silage.
Silage + Oxygen = Carbon Dioxide + Water + Heat produced
Freshly mown grass, silage sealed with damaged plastic film and silage at opening/feeding out all produce the same thing when exposed to air (oxygen). Carbon dioxide, water and heat are produced.
So when determining a problem, if mould is present, air has usually also been present.
If the bales or the plastic film are obviously wetter than expected, or wetter than when wrapped, then the silage is probably breaking down inside and producing this excess water.
Carbon dioxide will have been produced but not seen. Heat is sometimes, but often not, felt because, generally, it has escaped before being detected.
Failed baled silage can be due to: Incorrect dry matter content at baling. Wrapping/sealing problems. Damage to the plastic seal. Quality of the plastic film.
1. Incorrect DM content at baling
If DM content is too high, mould growth can occur in the bale.
With round bales, material that is baled too dry (more than 50-55 per cent DM) cannot be compacted enough to expel excess air. This entrapped air will allow plant respiration and aerobic (requiring oxygen) microbial activity to continue, leading to DM and quality losses.
A certain amount of mould (and yeast) growth may also occur depending on the quantity of trapped air. The problem is dramatically compounded in over-dry bales if the plastic wrap is holed, because the air can enter more quickly and further into the bale (see Figure 1).
If DM is too low, bales will be sunken and misshaped, and will usually have undergone poor fermentation (see Figure 2).
Most bales baled under about 38 per cent DM may undergo a clostridial or enterobacterial fermentation. This is due to the high moisture content and slower rate of fermentation because the material is not chopped as in a forage-harvested crop. The wetter the silage becomes towards the base of the bale, the wetter it was at baling, or the more it has broken down due to incoming air.
Round bales should be 40-50 percent DM. Large square bales should be 40-60 per cent DM.
Use a chopper baler for overdry bales with all knives in place.
Use a fermentation-enhancing silage additive for overwet bales.
2. Wrapping/sealing problems
Despite bales being correctly wrapped with four layers at 50 per cent overlap and 55 per cent stretch, air will still slowly enter the bale over time. These losses are minimal across 12 months, sometimes longer.
However odd-shaped bales have not had the minimum of four layers of plastic over the entire surface of the bale. Underlapping of the film has occurred (see Figure 3), which allows air to enter the bale at a much higher rate where the film is three layers instead of four.
To expose or verify underlapping for the lighter coloured films, cut out a square section (roughly 30 cm square) of the suspect area, and hold it up to the light. Carefully pry apart the film and count the number of layers in the lighter zone.
To prevent this ensure bales are square or slightly convex in shape. Apply extra wrap over underlapped sections.
Pre-stretcher issues can also create problems. The film passes through a pre-stretcher to ensure it forms an airtight seal on the bale. Poorly serviced pre-stretchers may over or under stretch the film. If the film is not stretched enough, is over-stretched, or the film runs through the pre-stretcher the wrong way, then the seal will be inadequate.
With the advent of pre-stretched films onto the market, separate gear sets were required for these new films. In the event of a machine being on-sold, or the correct gears being lost or forgotten to be exchanged, sealing problems are the result.
Dust and rain will settle between the film layers and will prevent a good seal forming. Unused plastic rolls stored in the sun's heat during the day's harvesting will not 'neck down' or stretch as well as they were designed to.
The solution is to service the pre-stretcher regularly. Use the correct gearing for the specific plastic wrap used. Store unused rolls away from direct heat and avoid dust and moisture getting between the film layers at wrapping. D
*Frank Mickan was a pasture and fodder conservation specialist with the Victorian Department of Agriculture for many years. He prepared this and several other definitive guides to silage management.