Anyone who has ever fitted side sheets to an empty silage clamp as the chopper is heading out to the fields will have wondered – is this really worth the effort?

Sheeting silage has been the most common solution to forming a gas tight seal on the fresh crop since the first crops were ensiled in the 1940’s. Creating an airtight seal at the base is not difficult and before it was common to build clamps with walls, simple plastic or polythene sheets sufficed to seal the top of the heap. An airtight seal is fundamental to ensuring a rapid fermentation and preventing in-clamp losses. But sheeting the side walls of the clamp has become “optional” so should you keep up the struggle or what are you missing out on?

Well the fundamental principle remains, the clamp needs to be as airtight – or sealed as possible – on all sides. An AgBag or sausage of silage has plastic on all sides. A silage bale, be it round or square, is wrapped in plastic so shouldn’t the clamp follow the same rules?

As already discussed, creating a seal at the base is part and parcel of the floor slab design (more on that subject elsewhere) but the walls are more problematic. Clamp walls can come in many different forms, earth banks, plywood panels, wooden or concrete railway sleepers, mass concrete, precast concrete cantilever panels or horizontal panel and post systems to name a few. Each system has its own characteristics with regard to impermeability.  Obviously railway sleeper are extremely permeable and the only feasible solution here would be to line the walls with plastic sheets. The term plastic is used to cover a multitude of material used to form silage sheets the details of which will be covered elsewhere.

So do you only have to sheet sleeper walls?

Earth walls don’t have joints but the substrate is unfortunately gas porous so a side or wall sheet is vital for airtightness in addition to effluent containment. So it’s critical to sheet timber, sleeper or earth walls but do you need to sheet concrete walls?

Precast sections are generally impermeable within the body of the unit due to the nature of the substrate but the joints between units are potential leakage points. Good unit design with interlocking joints help, but ultimately the seal relies on some sort of mastic sealant in the joint. Whilst this will (or at least should) be effective when the wall is new, the mastic needs to be maintained as it’s subject to physical and chemical attack. - so do you need to sheet them?

As ever, the decision needs to be taken after weighing up the pros and cons of the action. So the advantages for side sheeting need to clarified. Firstly the sheets provide a good effective gas barrier. In addition the sheet provides a physical barrier between the highly acidic effluent and the walls. The inclusion of wall sheets also makes sealing the edge of the top sheet much simpler. Oversized side sheets covered with the top sheet provide a good gas seal and minimise the chances of rainwater running off the top sheet and into the silage. Side sheets are a relatively cheap solution as they can often but cut from last years top sheets.

So what are the down sides?

Fitting the sheets is a labour intensive and potentially dangerous operation, the sheets are heavy and are difficult to handle in windy conditions. Avoiding snags and damage to the sheet on the top edge of the wall, support columns or handrail supports is also difficult. Due to the likelihood of damage – particularly from wind, sheeting is best left to the “last minute”. This makes the chances of an accident much higher as the pressure is definitely on at this point. Side sheets getting caught up in the feeding out process and contaminating the ration are also a risk.

The actions of the physical barrier can also be double edged. If you inspect a “new” concrete wall after the first season when the walls have been sheeted, small ‘rivers’ of effluent attack can often be seen. So how can this be, surely the sheet has protected the wall?

Silage effluent is only really aggressive when in comes into contact with oxygen. If the effluent seeps between the sheet and the wall, it collects in the folds and creases in the sheet where oxygen rich air is present – leading to effluent attack on the wall. If the sheet was eliminated and the compaction of the silage was good, then the erosion would probably be avoided.

Therefore choosing to sheet the clamp walls can be either vitally important, a safe option or an unnecessary waste of time with undesirable consequence - so is there a compromise?

Going short?

Full side sheets have all sorts of issues as we have seen above, but they do help project the edges of the silage - the shoulders - and these are the most vulnerable area to wastage. A short side sheet that extends only 300mm - 600mm (1 or 2 feet) down the walls can be a really good solution to this area. This sheet can be a remnant or offcut from top covers and only needs to be fitted to the wall as the clamp is close to full. This is not only easier but a whole lot safer than installing sheets on an empty clamp from a ladder or telehandler!

This solution avoids the issues of trapping effluent behind the side sheet as it is free to gravitate down to the floor and drain away. Being short, these sheets tend to end above the level where effluent starts to collect anyway.

If you need further information or help on decided whether to sheet or not, or for any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com

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