Silage Advice

We have put together many of the frequently asked questions, but please contact us if there's something we haven't covered.

You can also access a range of expert advice and practical tips through our new intiative Cut to Clamp.

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Cut to Clamp aims to raise the profile of good silage as a vital part of modern farming, showing how it can really make a difference to overall farm efficiency and profitability. Our step by step guide covers all 6 key stages of silage production; Cutting, Wilting, Harvesting, TreatingClamping and Feeding.

  • What colour of wrap should I use?

    When black plastic is warmed by the sun it absorbs much of the heat and becomes more permeable to oxygen; the outer layer of forage also warms up. White plastic reflects more of the sunlight which reduces the activities of spoilage microorganisms in the bale.  Green plastic is intermediate between black and white.

  • How many layers of wrap should I apply?

    More layers reduces oxygen permeability and the risk of damage to the wrap. A minimum of 4 layers (50% overlap) is essential but increasing to 6 is beneficial as it provides a better oxygen barrier. Some work at CEDAR with black wrap found that 10% DM losses with 4 layers were reduced to just 1% by using 6 layers; the silage also had a better feed value. You should also use more layers with more mature or stalky forages, and very high DM grass, eg haylage, as well as with square bales.

    Min. layers
    Round <40% DM 4
    Round 40-50% DM 6
    Square <50% DM 6
    >50% DM or stalky 8


  • How important is sealing the clamp?

    The silo must be sealed as quickly as possible to minimise the time exposed to air initially and allow the fermentation to get started quickly. Line the walls, leaving enough over to allow a 2 metre overlap with the top sheet.  Place a second sheet over the top.

    Despite being much thinner, oxygen impermeable cling film sheeting has been shown to be more effective than conventional clamp sheeting. It also sucks down onto the surface of the silage, giving a better seal. It should be used to line the walls and cover the top but as it is UV sensitive a conventional sheet will need to be placed on top.

    Check the sheeting periodically for holes and seal with suitable tape. Air infiltration during storage as a result of poor compaction and sealing can increase DM losses by as much as 8%.

    Touching Tyres

  • Potential reasons for bales being mouldy on opening?

    Moulds can only grow in the presence of air. So, if bales are mouldy when opened it is an indicator that air has been available to them. There are a number of potential reasons for this:

    • Delayed wrapping
    • Air pockets under the wrap, eg due to misshapen bales are badly wrapped bales
    • Poor wrapping
    • Not enough layers of wrap
    • Poor quality wrap allowing air to infiltrate slowly
    • Damage to the wrap
    • Degradation of the wrap, eg old bales exposed to the elements

    No additive will be able to prevent bales going mouldy if they are exposed to air for a long period.

  • What is aerobic spoilage?

    What is aerobic spoilage?

    This is often wrongly referred to as secondary fermentation but it is not part of the fermentation process as it only occurs in the presence of air and fermentation, by definition, occurs only in the absence of air. It is initiated mainly by yeasts which can grow using a variety of different substances particularly residual sugars and lactic acid. They convert these to carbon dioxide and water and, in the process producing a significant amount of heat and high DM losses. After the initial yeast activity, moulds join in. They are able to grow on a wider range of substances so spoilage accelerates.  Such silages will also have reduced palatability.
    good silage diagram

    What are the major influences on aerobic spoilage?

    • Exposure to air (most important)
      – crop DM
      – speed of filling
      – compaction
      – effective sealing
      – feedout rate and technique
    • Fermentation
    • Ambient temperature

    What types of silage are most susceptible to aerobic spoilage?

    • High dry matter silages
    • Silages with high residual sugars
    • Silages fed in warm weather
    • Aerated silages, eg mixed in TMR
    • Silages with high numbers of lactate assimilating yeasts at opening

    How can I minimise aerobic spoilage?

    • Narrow clamp face
    • Shorter chopping for high DM silages to ensure good compaction
    • Good compaction to minimise trapped air
    • Good sealing and weighting of clamps
    • Ensure no damage to sheeting or bale wrap
    • Fast feedout (15 cm/d winter; 30 cm/d summer)
    • Twice daily feeding in hot weather
    • Keep clamp face tidy and tight (block cutter/shear grab)
    • Keep the sheet off the open clamp face
    • Use an effective additive
  • How important is it to roll the clamp?

    Good consolidation is probably the single most important step in silage making. It minimises the amount of air trapped in the silo initially which will mean wasteful aerobic processes will cease sooner, allowing fermentation to begin. Yeasts numbers will also be lower when the clamp is opened for feeding, reducing the risk of aerobic spoilage. Rolling will also bruise the forage a little, helping to release more sugars for fermentation.

    With grass in excess of 27% DM fill the clamp quickly and in thin layers (maximum 6 inches / 15 cm). You need to be using plenty of weight on the silo; it is advised that packing tractor weight (tonnes) should equal the forage delivery rate (tonnes fresh weight/hour) x 0.25. You also need to use single wheels, not duals, which simply spread the weight over a bigger area, reducing the point pressure. Do not over-roll as the spongy nature of the grass will pump air in and out, providing more oxygen for respiration and heat production. Direct cut some wetter grass to seal the top before sheeting.

    With grass below 20% DM, load with a steep ramp and take care not to over-roll. Use dual-wheel tractors or a crawler to load the clamp as excess contact pressure and rolling will increase effluent. Do not load the clamp too high or over-compress near the walls as wet grass exerts a greater pressure on the walls.

    Do not roll the clamp for more than about 30 minutes in the evening. It is more important to get the sheet on as most of the trapped oxygen will get used up quickly and be replaced by carbon dioxide.  

    If filling over more than one day, do not roll immediately after uncovering the next morning as this will simply push out the carbon dioxide and replace it by oxygen. Only begin rolling after a new layer of grass has been placed in the clamp.

  • How important is clamp management at feedout?

    Having produced a well fermented, high quality silage, it is vital that the feeding out procedure is managed effectively because high losses and reduced animal performance may result from aerobic spoilage.  Unfortunately, the best preserved silages are the most susceptible to aerobic spoilage because several of the products of a poor fermentation (acetic acid, butyric acid, ammonia) are inhibitory to the yeasts that initiate spoilage.

    Tips for Clamp Management at Feedout:

    • Have as narrow a face as possible
    • Avoid overhangs
    • Keep the sheet off the open face
    • Maintain a tidy face (block cutter/shear grab)
    • Keep the face straight
    • Clear up fallen silage regularly
    • Discard mouldy silage
    • With high DM silage and/or hot weather move across the face faster by taking shallower bites with the shear grab
  • Do I need to use an additive with clamped grass silage?

    With all silages it is important that the pH comes down quickly as well as to a level low enough to stabilise the silage. Grass can often be low in sugars and/or have low numbers of lactic acid bacteria on it. Use of a proven additive will help to ensure a successful fermentation. With low DM (< 20%DM) an additive is usually considered essential, especially if harvesting conditions are not ideal and/or you are using grass with a high clover content. Some inoculants have also been shown to bring about improved animal performance, even when the untreated silage would have fermented well anyway.

    What about high DM grass silage?

    With high DM grass the fermentation is slower and the silage will stabilise at a higher pH. The slow fermentation allows more time for undesirable microorganisms to be active as well as increasing the breakdown of true protein.  Inoculation speeds up the process as well as making it more efficient with lower DM losses.

    High DM silages are also at higher risk of aerobic spoilage at feedout with potentially high losses and the risk of mycotoxins so good management is essential but additives are also available to help reduce aerobic spoilage.

    Remember – an additive is not a substitute for good management.

  • How do I calculate how much silage is in store?


    Use our Clamp Stock Levels Tool

    1. Calculate the average DM of silage in the clamp as follows:

    • Take several cores representing the whole clamp
    • Mix the cores together very thoroughly on a clean, dry poly sheet then sub-sample.
    • Weigh a suitable dish and measure exactly 100g of forage into it. Note the total weight. Repeat for 3 separate silage samples.
    • Dry the sample down using a microwave oven or put it in a 60oC oven for 24 -48 hours (eg the warming oven of an Aga). 
    • Weigh the dish + dry sample
    • Calculate the DM for each sample then average them.

    %DM = (dish + dry sample) – empty dish

    2. Use the silage DM and silage DM density from the tables below to tell you how much fresh silage or silage DM you would have in 1 square metre of clamp space.

    3. Calculate how many metres cubes (m3) of silage you have in your clamp based on the width, length and height, eg 20m x 50m x 3m = 3,000 m3, eg
    From the table, a 30% DM grass clamp with a 3m high face would contain 205 kg (0.205 tonnes) of silage DM per m3 of clamp space.
    4. Calculate how much you have in the whole clamp (3,000 m3) = 900 x 0.205 = 615 tonnes of silage DM.

    Depth of clamp (m)

    Grass (kg/m³)
    % DM
          Depth of clamp (m)
    2 3 4
    20 156 778 176 881 191 954
    25 172 690 193 773 208 831
    30 186 620 207 689 221 738
    35 198 565 218 624 233 666


    Maize (kg/m³)
    % DM Depth of clamp (m)
    1.5 2 2.5
    25 185 730 195 770 200 800
    30 205 680 215 720 225 750
    35 215 620 230 660 245 700



    For bales, weigh a few or estimate their weight then calculate your total fresh silage stocks. Do an oven DM on samples from several bales then calculate DM stocks as follow:
    Tonnes DM = Tonnes FW x % DM ÷ 100

    *DM – dry matter; FW – fresh weight

    Fermented wholecrop cereal silage of 50% DM will have a freshweight density of about 600kg/m3 (200kgDM/m3).