“Storing large square bales of silage under sheets of plastic should be cheaper than being individually wrapped”, says Frank Mickan, Fodder and Pasture Specialist, Natural Resources and Environment, Ellinbank. However this is often not the case because many bales are mouldy when unsealed! These mouldy bales

Mouldy silage (see Figure 1) usually occurs due to the presence of air in the stack. Mouldy silage represents a loss in dry matter and silage quality. The amount of loss will depend on the extent of mould and deterioration of the bales. The moulds, fungi, bacteria, yeasts, etc. which set up camp and multiply profusely in these ideal conditions actually “feed” on what you were try to preserve for your animals, the energy and proteins.

Under these conditions the silage also breaks down into carbon dioxide, moisture and heat. The hotter the silage the higher the proportion of dry matter and quality is being lost, and the more these blighters like it!

Following are some possible causes of various degrees of the mould problem.

  1. Small areas of mould on the bale exterior after several months of storage.
    The plastic sheet may have a slow leak due to a small hole or air is entering the stack at the bale-ground juncture, or the folded ends are not airtight.
  2. Large areas of mould on the bale exterior after several months of storage. 
    Large or many holes in the plastic sheet is allowing a lot of air to enter,  or the seal at ground level is not totally airtight, or the folded end flaps are not very airtight.
  3. Large areas of mould on bale exterior, mould “growing” inwards to the bale interior from the outer edges, bales may be warm.
    Very large or many holes in plastic sheet or very poor seal at base of stack or folded ends ineffective. If the bales are tight then the air has been entering for quite some time.
  4. Large areas of mould on bale exterior, mould throughout most bale interiors, bales generally will be hot!
    Very large or many holes in plastic sheet or very poor seal at base of stack or folded ends ineffective or the air has been entering for a long time. Bales may have been baled relatively “loosely”(for large squares) or the crop may have been too mature, ie stemmy.
  5. Any of the above with effluent flowing out of stack bottom.
    This effluent could be from silage made too wet, or as a result of the silage deteriorating from air entering the stack and braking down to gas, heat and water.

For stacks above ground seal enough bales for about 14 – 16 days feed in each compartment. If stacking large square bales into pits in the ground, use plastic sheets to seal stacks into compartments containing 16 – 20 days feed. This is suggested because if the top of the sheet is holed by cattle, rabbits, dogs, kids, etc. then only the holed compartment will “go off”, provided the seal between compartments is effective. When you start feeding out, the air can only go back to the next compartment. Putting dirt around the edges of the stack and or down the pit sides will help to form a good seal.

If the plastic is billowing in windy conditions, too much air is probably entering somewhere! See diagram 1 for recommended sealing in pits.

Use proper plastic tape specifically made for silage film. Grey duct tape is no good. Ensure the plastic is dry, clean and cool before applying plastic to holes. Cut tape to length, let it shrink back, then apply it.

Using dirt to seal at the bale-ground interface can ensure an excellent airtight seal. Dig a trench before or after the stack is built. Place the plastic sheet into the trench, folded so that the edge sticks back up out of the ground trench. Dirt is then placed into this trench against the plastic. The jutting plastic sheet edge is then easily pulled up when unsealing the stack.

An alternative is to lay the sheet edge on the ground and cover it with dirt, ensuring the film edge is covered.