Are Back Walls Necessary?

Perhaps your mental image of a silage clamp if different from mine. Your image is probably one of the clamps on your own farm, or clamps you have previously worked with, or perhaps a neighbour’s clamp that you secretly covert! To me, a clamp is the typical three sided structure, that is longer than it is wide. But why are they this shape and do they have to be this layout?

 The height, width and length of the clamp are subjects dealt with elsewhere in the series, but the layout is what we are going to explore in this piece. Assuming you have decided to have a clamp with walls of some sort, you have also decided that you are going to limit your access to the space. That is (yet) another blindingly obvious statement, but it’s one that is fundamental to what you are trying to do in designing a new silage clamp. For the avoidance of any doubt – the sole purpose of this whole process is:

 To safely store the harvested silage crop for later use with the minimum of losses.

And it’s that final bit that is key – with the minimum of losses. Most of what we are trying to do in the design process is to limit the losses.  The size and shape of the open face is the key driver in the design process and the losses at feeding out stage can be the most significant – and costly.

So why would we be considering have a clamp that open at both ends?

There are a couple of situations where this can be an attractive solution. Perhaps the easiest to consider is an AD plant running predominantly on virgin crop for its feedstock. To maximise gas yield, it is critical to maintain a stable menu for those bacteria in the tank. Changes in the feedstock ration require a re-profile of the bacterial soup in the tank. Incredibly the bacteria will re-populate to accommodate this – all by themselves, but it takes time and gas yield will drop during the process. So it’s important to minimise these changes to the ration.

Whatever the feedstock, maize or whole crop, this material is available to harvest over a very limited time period. Once ensiled it needs to be left for a couple of weeks for the ensiling process to protect the crop. So how do you feed the plant whilst waiting for the pickling to complete? If the scale of the operation is sufficient, you may have an entire clamp of old crop that’s still available to feed from but in most cases this is unlikely. So by leaving the end of one clamp open at both ends, it is possible to maintain access to the old crop whilst this year’s crop is ensiling. Clamp management needs to be practiced to make this cycle work year on year and if you’re feeding two feedstocks then you need to consider access to both materials.

For the livestock farmer, the same principals can be applied to the large scale 365 day housed herds, but what about the smaller, traditional herds that graze during the summer, do they need to consider this?

Well maybe, maybe not, it all depends on calving patterns and feed requirements. Before climate change took hold, first cut grass silage was traditionally considered the highest feed value in a three cut system. Today in multi cut “rocket fuel” systems this might not still be the case, but it’s still likely that late season silage will not be the same value as that cut in May and June. So for a seasonal calving herd, you need to have access to the right silage at the right time. Practically this might mean access to both ends of a clamp would be desirable.

So what are the implications of having access to both ends of the clamp?

We don’t have to build a wall at the end so it must be cheaper, right? Not necessarily – unfortunately there are some other costs to consider. By not having an end wall, the sides will need to be a little longer to make up for the loss in capacity. This can generally be calculated by adding the equivalent in height to the length of the walls. So if the clamp is 3m high, make the side walls 3m longer if you’re leaving out the back wall.

Without a back wall, the SSAFO regulations require an apron to contain any seepage or spillage. The cost of this and the additional side walls will, in most cases, wipe out all of the savings from the back wall. There are also considerations regarding the increased area collecting dirty water and vehicle access to “back” of the clamps. If site layout means the back of the clamps is tying up with existing aprons or roadways then these “costs” will not be realised.

In the final analysis, it takes careful consideration to weigh up the benefits against costs. For further information on the benefits of back walls or any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com