What’s the difference between Farm and AD silage clamps

Silage is silage and a silage clamp is a silage clamp – isn’t it?

Yes and no and as always this is only part of the story, so what are the differences between the design of a silage clamp for an AD plant and one for a livestock farmer?

Well let’s start off with the similarities. Both anaerobic digestion and livestock silage clamps are designed to produce conditions to make the friendly bacteria happy (lactobacillus planetarium). They are also both designed to make a feedstuff that will – ultimately – feed the same digestive bacteria be they inside an AD tank or inside a cow.  These clamps are also designed to be filled, compacted and emptied with the same sorts of machinery. Both types of clamp are also (currently) covered with the same legislation – the SSAFO regulations, so there are many similarities. Any good farm silage clamp will produce silage suitable for an AD plant but what should you consider to design a bespoke AD clamp.

How to design an AD silage clamp

The basic rules of designing a clamp still apply to AD or farm clamps. The SSAFO regulations must be adhered to whatever type of clamp you are building, but there are lots of ways of meeting the requirements. The efficiency rules that target minimising losses are also the same. The golden law of 2m per week usage to control wastage of material on the exposed face still applies. Obviously on an AD plant you will be out feeding for 52 weeks of the year rather than just for the winter. That means you need to design the clamp layout that is at least 110m long for each feedstock you are producing.

Why 110m long when 52 weeks times 2m per week is 104m?

For good reason and let’s do the maths. We will assume you are growing a wholecrop feedstock that’s ready for harvest on mid June. Once you have clamped the crop, you need to give it at least 2 weeks for the silage to be “produced” by the fermentation process. That means you can’t open the clamp until early July and once you do, you really should be blending the old crop with the new crop for a week or more to ensure you don’t get a feedstock shock to the bacteria population. Next year the season might delay the harvest until late June and then we are in all sorts of bother. So a 55 week supply is good target feed period.

Livestock farmers have some more complex feed requirements than an AD manager. Feed ration requirements for livestock will vary depending on the production phase of the herd. This is particularly true for a dairy herd where you may want to assign more of the (hopefully) higher feed value, first and second cut grass silages to the recently calved or high yielding cattle. The young stock and dry cows making the most of the less energy dense later cuts. In an AD plant, it would be ideal – but probably unachievable – to feed exactly the same ration make up every day, year round. This difference will influence the design layout of the silage clamps and is a significant difference between farm and AD plant.

Understanding the length of clamp required is one thing, but how do you cope with blending multiple AD feedstocks without building clamps the length of Heathrow’s number 1 runway? The usual solution is overlaying crops within the same clamp.

Let’s assume you have 1000t of ryegrass silage and 2000t of wholecrop. The ryegrass might be ensiled in early May but the wholecrop not harvested until mid June. A 3m high double clamp (with a dividing wall) that’s 55m long and 24m wide would give the required storage capacity but how to fill it? The usually solution would be to put all the grass into clamp one and then top it up and fill clamp 2 with the whole crop some weeks later. By filling the entire clamp this way you will not be able to achieve the 2m per week rule with a balanced ration.

As an alternative, put 1m of grass silage into both clamps and then overlay with the wholecrop in June. This way you get access to the same feed ration for the full 52 weeks with no variation in the blend. However there will be a small amount of wastage when you uncover the grass to place the wholecrop on top so is this a good idea?

If you are not moving through the silage face at 2m per week, the in-clamp losses can soon jump into double digit percentages and this will dwarf the losses from opening and overlaying the grass. The alternative is to vary the feedstock from mostly ryegrass to entirely wholecrop – this will have a massive impact on the bacterial population in the tanks and a huge reduction in the gas yield.

 AD clamp size and scale

The other significant difference between an AD silage clamp design and a livestock farm clamp is the size and scale. Average livestock herd size in Europe are getting ever larger but their silage requirements are still someway below AD appetites. Where farm clamps are sized in hundreds and thousands of tonnes, AD clamps are thousands and tens of thousands of tonnes. Does this make any difference?

Not really, larger clamps reduce the costs per tonne stored and larger clamps are also easier to safely overfill. The main difference comes in the design of the walls for very large clamps.

 Designing walls for AD clamps

As we have previously covered, designing large scale clamps with an eye on the cost per tonne stored will led you to increased wall heights. This is a good solution, as long as the designer understands the implications. It’s easy to underestimate the influence of increasing height and whilst it’s the topic of another in the series, in essence the load on the wall is a factor of the height cubed! The means for a 3m high wall the height factor is 3x3 = 9 but increasing by just 1m you get 4x4 = 16 more than half as much again.

The massive volume of silage required for an AD plant will, inevitably, lead to a harvest team using the largest of machinery to clamp the crop in a timely manner. Large machinery filling the clamp can make producing good quality silage a little easier so we can anticipate big loading shovels, heavy compacting tractors with compactors or train wheels. All of this will put more load on the walls unless you control the vehicle movements. You can – and should - get the walls designed for these loads but as time goes by, inevitably bigger machines will find their way onto the clamp. We have covered the safe overloading of clamps earlier in this series and the AD industry is most at risk from not following this advice.

The floor design between an AD clamp and a farm store ultimately have to do the same job and meet the same regulations. However the different scale might have an implication on the relative cost between tarmac and concrete floors. Basically a tarmac floor cost includes a significant chunk of money to get the specialist machine to site and set it up. Once it’s there it can lay a large area of floor slab, so if you have a small area then tarmac will not be cost effective but if it’s a 10,000t clamp floor then it will be a different story.

In conclusion, farm and AD silage clamps are the same – only different. If you want to help in designing the ideal low loss clamp for either a farm or AD project or to discuss any other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com

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