Best practice for making silage for AD plants

For crop-fed AD plants,quality of feedstock is the key to efficient biogas production throughout the year. Tim Elsome, General Manager of FM BioEnergy, explains the importance of creating good silage to ensure you get the highest gas output from your feedstock.

Between 25-40% of dry matter is estimated to be lost in silage clamps, with up to a third of these losses due to poor practices when the clamp is being unloaded. It therefore makes sense to adopt best practice at all stages of the ensiling process.

We’ve highlighted the top eight areas where dry matter losses can be prevented andprovided our top tips for best practice at each stage; from harvesting the crop to removing feedstock from the clamp and feeding the digester.

1. Harvest period

As well as choosing a crop and variety that has been shown to produce good biogas yields, the crop should be cut at the ideal stage of growth so that energy content is maximised in order to optimise the dry matter yield.

For a typical maize crop, this will be when the whole plant dry matter is between 32-36%. Harvesting earlier will sacrifice yield potential and greener crops will lose more fluid in the clamp. Later harvested crops with more fibre content may be less digestible subsequently in the digester. If in doubt, it is better to harvest a little early rather than too late.

2. Chop length

Maize for use in AD plants is generally chopped smaller than for forage use; 4-6 mm is optimal. The largest possible surface area is best for biogas production, as it aids rapid bacterial breakdown in the digester. However, shorter chop lengths can lead to difficulties in clamping and compaction, and operators need to achieve a balance.

Chop length will also depend on crop maturity. For maize, the suitability of the chop length can be tested by squeezing a handful of the harvested crop. If a lot of moisture runs out, you should delay harvest or increase the chop length. However, if the material does not stay compressed after squeezing, you should shorten the chop length.

3. Silage additives

Silage additives help to ensure effective fermentation and also act as an insurance policy against spoilage in the clamp, such as yeast and mould formation. If possible, use a biological additive such as Silasil Energy XD, which is easy to apply and specially formulated for use on silage for bioenergy production, rather than for livestock feed.

Silasil Energy XD is supplied as a freeze-dried product which is mixed with water on site and then dosed at very low rates at the point of harvest. Low rate applications mean less time spent carrying water and refilling the tank, allowing more hectares to be harvested each day.

Biological additives boost the population of beneficial bacteria in the clamp, helping to begin fermentation rapidly and achieve the optimum pH as quickly as possible. They also help prevent the growth of spoilage moulds and ensure silage remains in good condition.

4. Timely filling

To prevent dry matter loss due to aerobic bacteria feeding on the plant sap, it is important to create anaerobic conditions as quickly as possible, aiming to be completed within three days. Minimising the time between harvesting the crop and filling the clamp is therefore critical.

Match harvest rate to the speed at which the clamp can be filled and compacted, and space out trailer deliveries to allow timely spreading of material.

To ensure safety when emptying the clamp, avoid making it more than 4m high and never fill above the specified design level.

5. Compaction

Poor compaction slows the fermentation process and increases spoilage. It is important to ensure good compaction across the whole clamp, but particularly at the side walls. A useful target for clamp density is 700-750 kg of fresh material per m³, which typically equates to 230-250 kg of dry matter per m³, depending on the dry matter at harvest.

Silage with higher dry matter requires greater compaction, but conversely can often be more difficult to compact. Pay particular attention when compacting as poor compaction and consolidation is one of the most common reasons for yeast and mycotoxin formation, particularly with high dry matter crops.

Compact the clamp systematically in layers of 15-20 cm, with at least one tractor dedicated to compacting the clamp while another spreads the material. Ensure there are no dents in the surface as this may prevent the sheeting from forming a close seal, resulting in detrimental air pockets. Avoid extra packing at the end of the day as this is unlikely to have an effect on the clamp and may be counterproductive.

To achieve consistent results, consider using dedicated silage compaction equipment. As a rule of thumb, rolling machinery should weigh a quarter of the weight of the maize delivered to the clamp each hour.

6. Sheeting

As soon as the clamp has been filled, it should be covered immediately to create the anaerobic conditions necessary for fermentation and to prevent further aerobic losses.

Dedicated oxygen barrier films are increasingly common and should be protected from damage and the elements with heavier sheeting. Make sure that side walls are also sheeted and ensure adequate overlap (at least 1m) on the floor, where sheets meet, and around the top of the clamp.

Protect plastic sheeting with a woven sheet which is well weighted down, not only to ensure that it stays in place, but also to help avoid losses from the top of the clamp. Weights should be placed in parallel lines to the clamp face and spaced one to two metres apart (depending on clamp size). To ensure continued protection on opening, peel back the sheeting from one weighted tier to the next as required.

7. Opening the clamp

The first consideration when opening the clamp must be the safety of workers and others. Do not dig under the clamp face, as this can lead to the clamp caving in, with potentially fatal consequences for staff and damage to machinery.

Do not be tempted to open the clamp too soon. Normally, at least eight weeks is required from covering to opening, although using a silage additive such as Silasil Energy XD will accelerate the fermentation process, allowing the clamp to be opened in as little as two weeks.

Do not leave the sheet hanging over the front of the clamp as this can encourage mould growth. Roll the sheet back on top of the clamp and weight the front edge well. Discard any poor-quality material from the shoulders and front of the clamp, as this is often the cause of instability in the digester.

8. Removal of material from the clamp

Keep the cut face tight and clean, and try to ‘shave’ silage from the exposed face where possible. Using the right tools will make this much easier. A shear grab will provide a cleaner, more even face than other options, but the knives must be well-maintained as blunt knives can undermine the clamp face allowing in air, particularly with drier silage. For high volumes, consider investing in a dedicated silage defacer or scraper.

Do not make the face any bigger than you need to as exposed silage can lose a significant portion of its dry matter once exposed to the air. The usually quoted figure is to progress into the clamp at no less than 1m per week in cold weather, and at a minimum rate of 1.5 to 2m per week in warmer conditions.

Exposing the clamp to the air will allow any yeast and mould contained within to respire and cause secondary losses, evidenced by increased silage temperature. An increase of 10°C, which is not uncommon, will causes losses of 3% per day. Silasil Energy XD prevents this through the production of acetic acid which inhibits yeast and mould growth.

Finally, only take what you need and feed it into the digester immediately. Don’t be tempted to pull down two days’ worth of material and leave it in a heap where exposure to the air will begin to break it down, resulting in lost biomass. Remove any spoil or spillages from the area to avoid contaminating good feedstock.