Silage Clamp Covers

Black plastic sheets, that’s what you cover a clamp with isn’t it?

Well it’s certainly a very popular choice and has been for many years but like just about every aspect of agriculture, the scientists have been working out what’s best.

How we all ended up using black plastic

The early silage clamps of the 1940’s were literally silage pits, as they were dug into the ground. In fact silage was produced as early as the 1880’s when a series of wet summers forced desperate farms to try and find some way of producing a winter forage other than hay. They buried grass into trenches and produced some of the first recorded silage. In these cases the covering of the clamp was generally soil, usually the soil that was dug out to produce the pt. This was placed over a layer of hessian sacking or large leafs used to separate the forage from the soil and make opening the clamp a little easier.

Experiments were carried out in New Zealand in the early 1960’s with uncovered clamps and covered clamps using different material to cover the silage and measuring the losses of dry matter. The uncovered clamps recorded over a third of the silage wasted or lost in run off. The clamps covered and protected from the rain had (unsurprisingly) lower effluent losses but still suffered similar overall losses. Clamps covered with plastic did considerably better with a total of just under 12% lost. These experiments identified two of the key roles of the cover, to keep out the rain and the air. In the UK the use of plastic gained popularity as plastic films became more affordable and available and did a better job of preserving the silage.

Today every farmer understand the importance of keeping rainfall and air – or more importantly oxygen – out of the clamp and a black plastic sheet does a good job of this doesn’t it?

Maybe not a good as you might think, for one, no polythene sheet is entirely impermeable to oxygen.

Health and safety warning – it might not be completely impermeable, but it will still suffocate you!

This oxygen permeability is also dependant on the temperature of the material itself. Ironically, a black polyurethane sheet is quite possibly the worst colour is could be for this job. Not only does this increase in temperature raise the oxygen permeability, it also encourages increased microbial growth in the top layer of the silage. The yeasts, moulds and aerobic bacterial activity in this layer are all consuming the valuable crop. 

Alternatives to black plastic sheets

So lighter colour sheets (or white on black layered sheets) are a more and more common sight across the country. It’s also common for baled silage and Ag bagged silage to be wrapped in white plastic films. Obviously the effectiveness of the wrapping or sheeting is much more important in these higher surface area silage storage systems. In a clamp the covering may account for around 1kg of plastic per tonne of dry matter stored, in baled silage the plastic will be over 5kg per tonne. Bagged silage will be closer to the baled figures.

To overcome the permeability issues of polyurethane, alternative materials have been developed. Today the market place is full of oxygen barrier films or cling films that are used in combination to give a better seal of the clamp. And these products have inherently better sealing properties and they have the added advantage of reducing the amount of trapped pockets of air soiling the crop. We are all well aware of the increased silage wastage noted on shoulders or edges of the clamp. Areas where compacting is difficult and ensuring a good cover to silage contact during sheeting is difficult to achieve. These films literally cling to the silage and reduce this wastage. 

So an oxygen barrier film, covered with a light colour waterproof sheet will give you a good seal, but remains vulnerable to vermin damage and those with a heavy boot! Nets and weights are the subject of another in this series, but we can easily be heading to a 4 or 5 layer system!

Combination silage sheets

Applying more than one cover to the clamp will obviously increase the labour costs and time to carry out the work. Its can also make disposing of the material more difficult as the clamp is emptied. Although many of the materials “can be recycled” it is usually that the film will need to be kept separate from the sheeting to enable the recycling to be processed.

As an alternative, combination sheets offer the benefits of the oxygen barrier, the rain protection of the polyurethane but with durability and robust properties of the netting.

On the down side, the lap joints of different sheets is a vulnerable area and combination sheets can be a real handful to haul around on the silage.

How can I avoid plastic sheets?

And finally, what options are there for those wanting to avoid the use of plastics? Well organic materials have been tried, waste materials and using the top of the clamp as a seedbed. These are more suited to the AD industry as the “cover” can be included in the feedstock ration but losses are considerably higher than plastic covers.

If you want to discuss the options for covering your silage in more detail or any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com

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