Frank Mickan is the Pasture and Fodder Conservation Specialist with the Department Environment and Primary Industries (DEPI) at Elinbank in Victoria. 

“There are no short cuts when sealing stacks of forage harvested silage,” says Frank Mickan, Pasture and Fodder Conservation Specialist, NRE, Ellinbank. Sticking a few tyres on top to hold the plastic down and throwing a bit of dirt around the edges is not a suitable airtight seal. If mould is present when the stack is opened, then air is present, or has been. The more air, the more mould! Where has this air come from?

Many farmers can open a stack up at feeding time to reveal perfectly stored silage with no visible signs of waste from decomposition or mould. How do they achieve this?

  • Silage made at a suitable dry matter(DM) content.
  • Stacks are tightly compacted by rolling during harvest.
  • Stacks are sealed straight after harvest.
  • Plastic sheet on top of the stack is well weighted.
  • Stack is sealed airtight at the edges of the plastic.
  • Holes which occur are fixed immediately.
  • Stock damage is prevented by well constructed fences and gateway.
  • Crops harvested with loader wagons, double choppers, etc. which harvest relatively long chopped material (~ 5 – 15 cm) should be harvested at about 28 – 35% DM. If the material is much drier than this compaction by rolling is much less effective unless an extremely heavy machine is used. Not enough air will have been excluded. 

Precision harvested material (~ 1 – 5 cm) can be ensiled at about 40 – 50% DM as its length allows easier and better compaction, but the stack must be well sealed or the benefit will be lost. This shorter and drier material will require about one third less storage volume.

If material of either length is becoming too dry as harvest progresses due to machinery delays, dramatically rising temperatures, etc. spread some loads of freshly cut or lightly wilted material. This will ensure a much more satisfactory compaction and air exclusion.

Many operators roll the stack too quickly! Roll slowly to allow the weight of the tractor to compress the material. Spread the harvested material thinly in layers of about 150  – 200 mm. Large clumps or thick layers will not allow complete expulsion of the air despite rolling. If the stack is above ground and has no support walls be very aware of rolling the edges. Use the front wheel to compress the edges by driving to and from the edges on a slight angle turning the front wheel along the stack, but stopping when the rear wheel approaches the edge.

For cement bunkers, if the top of the stack is likely to finish below the tops of the walls, thus making sealing very difficult and possibly allowing water/air entry into the stack via the edges of the walls, consider building the stack edges higher than the middle. The resultant depression in the middle of the stack will allow water to run off, and allow the full extent of the tractor’s weight to compact the edges.

It is preferable to pull a plastic sheet over the stack at night. Place tyres around the stack edges and a few in the middle to weight down the plastic. This will reduce the amount of air gaining access to the stack at night as the hot gases move out of the stack at a greatly reduced rate.

Ideally, the stack should be completed within 1 – 3 days. If at any stage you see steam rising from the stack when the silage face is lifted, or high temperatures are felt upon inserting a hand into the stack face, losses of energy and protein are becoming high. This is often noticeable when filling a stack over a longer period than 3 days, if rolling is inadequate, or the material too dry. 

Seal the plastic sheet at the edges to be airtight. Two excellent seals are shown in Diagram a and b. The folded plastic edge (diagram a) ensures an airtight seal and is easily pulled out when the stack is opened. If the dirt does not cover the edge of the plastic, air entry into the stack is highly probable.

Overlap plastic by at least 0.5 m and preferably seal with a tape specifically designed for silage films. The film must be dry, clean, cool and the tape allowed to shrink before applying it to the plastic sheet. Weight the plastic all over with tyres, dirt, thin layer of soil with grass seed sprinkled on top.

Fence off so that stock cannot stray onto the stack and cause holing in the plastic sheet.    Seal any holes as soon as they are noticed.