Silage Clamp Walls - How High?
This is a complex question to answer, it depends on lots of factors, not least how much space you have. That might sound like a blindingly obvious statement and no space is actually “free” space as there is always an alternative use or cost involved. So what do we need to consider?
By the simple laws of physics, the higher we make the walls, the more expensive they will be. That’s not just as simple as a 5.0m high wall being a bit more than a 2.5m wall, it’s much greater than that. It’s all to do with the level effect, a 5ft long spanner puts soooo much more force on a bolt that a 2.5ft bar and so it is with walls. You need to do the maths to calculate the overall costs of per tonne (or cubic metre) of storage space.
For a 2,500 tonne clamp, 50m long we would need this the to be the following widths/height combinations:
| Wall Height | Clamp Width Required |
|---|---|
| 2.4m | 28.8m |
| 3.0m | 23.0m |
| 3.6m | 19.1m |
| 4.0m | 17.3m |
| 4.5m | 15.4m |
| 5.0m | 13.8m |
Wall Cost
The cost of the walls will fall into the following compared to the base cost of 2.4m high walls.
| Wall Height | Relative Cost |
|---|---|
| 2.4m | 100% |
| 3.0m | 106% |
| 3.6m | 104% |
| 4.0m | 132% |
| 4.5m | 138% |
| 5.0m | 152% |
This is the total cost relationship for the walls to the clamp, 5.0m high walls cost 52% more for this 2,500 tonne clamp but the actual wall units cost over 71% more per metre run of wall – it’s just we need less metres of wall in the higher clamp.
Floor Cost
The floor areas are much smaller for the higher clamps so we need to include these in our calculations as well
| Wall Height | Wall Cost | Floor Cost | Total Cost |
|---|---|---|---|
| 2.4m | 100% | 100% | 100% |
| 3.0m | 106% | 80% | 89% |
| 3.6m | 104% | 66% | 80% |
| 4.0m | 132% | 60% | 86% |
| 4.5m | 138% | 53% | 83% |
| 5.0m | 152% | 48% | 85% |
The mathematics dons amongst you will have noted that the 3.6m high clamp is the lowest cost per tonne stored in this example – or is it?
The Influence of Dirty Water & Sheeting Costs
Written off over a 5 year period, the difference between the 5m high and the 3.6m high clamp is less than £1,700 per year. The greater area of silage in the lower clamp will result in increased sheeting costs and greater potential surface wastage that erodes some of this difference in capital costs. Perhaps a bigger cost is that of dealing with the rainfall that drops on this larger surface area. This water is classified as “dirty” as long as the clamp has silage in it so it’s reasonable to assume you will be looking at no less than 250 days worth of rainfall. For large parts of the UK that’s over 700mm of rainfall.
You need to quantify these costs to assess the potential savings. The lower clamp has a footprint 265m2 greater than a 5m high clamp. Sheeting costs are in the region of £0.20 per square metre (at the time of writing), so over a 20 year period that’s a total of £1,060.
The larger area clamp will collect an extra 185 cubic metres of dirty water based on the usage above. With spreading costs generally around £2.00 per cubic metre, that’s £7,400 over the same period. These figures have (quite by accident) wiped each other out so that in the long term, there is almost no difference between the lifetime costs of the 3.6m clamp versus the 5.0m high clamp.
So the other factors come into play, how much space do you have, what are the opportunity costs of using the space for something else, etc etc?
I mentioned potential wastage of the crop earlier in these calculations, but didn’t quantify the figures. These losses are a much bigger influencer on the shape of a clamp than the height of the walls as we will explore in other parts of this series.
If you need further information or help to decide what height your clamp should be, or for any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com
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