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.

  • Is a high lactic acid content in the silage a problem?

    Lactic acid per se is not a problem but silages with particularly high lactic acid levels may have undergone an extended fermentation leading to high levels of total acids in the silage and a high buffering capacity.  It is the buffering capacity and the total acid content that should be considered, not the lactic acid content alone.  Silages with a higher proportion of lactic acid in the total acids tend to have a lower buffering capacity.

    Recognising the type of fermentation that has occurred and balancing the diet properly will prevent any problems with these silages.

  • What is the best way to fill the clamp?

    The Dorest Wedge

    The Dorset Wedge has been the traditional method used. Use as steep a ramp as possible as this maximises the effectiveness of rolling. This system minimises exposure to air during filling and avoids exposing earlier cuts to air again when the clamp is opened to put in the next cut. Dorest Wedge

    More recently horizontal layering has been introduced as an alternative.

    Potential benefits of layering:

    • Better compaction as the crop is spread thinner over a larger surface area
    • More room for two machines on the clamp – important if the forage is coming in fast
    • Easier to roll a flatter surface
    • No risk of clamp slippage with wet crops or between cuts
    • More consistent forage quality as you move through the clamp

    Potential disadvantage of layering:

    • Bigger surface area exposed to air during filling which could increase respiration DM losses
    • To put in another cut you need to unsheet the whole clamp – could increase DM losses
    • It may take longer to fill the clamp if only one machine on it as bigger surface area to roll

    Whichever method is used, as the clamp is filled keep the sides higher than the middle. This ensures the silage next to the walls can be rolled properly.

    If a clamp is opened to put in another cut, put a layer of fresh forage on top before rolling commences. Running tractor wheels over silage that has already stabilised has the effect of aerating this material, causing aerobic spoilage.  This will result in a line of poor silage across the clamp.  

  • What silage parameters influence intake?

    Intake is influenced by many factors including dry matter, chop length, digestibility and fermentation quality. High DM, high digestibility and short chop length increase intake.

    Many intake predictions are based on fermentation parameters such as pH value, fermentation acids and ammonia but it is well known that such predictions often do not tie in well with animal performance.

    Some work at the Agricultural Research Institute of Northern Ireland (now AFBI), Hillsborough showed that many of the measurements traditionally thought to indicate poor intakes in fact have little or no effect on intake, eg pH value, individual volatile fatty acids, lactic acid. They found that foul-smelling silages had little impact on intake and sweet-smelling silages were not necessarily palatable.  Similarly, the buffering capacity of the silage had no effect and feeding sodium bicarbonate to neutralise acid silages made no difference to intakes.

    Instead, they found that it was the individual nitrogen components, eg soluble protein and ammonia,that had the most influence on intake, followed by oil, ME, DM and digestibility.

    They concluded that it is probably the breakdown products of digestion in the rumen that are having a direct effect on intake rather than the silage composition itself, although this will obviously influence the outcome of digestion.  

  • Silage effluent

    How much effluent can I expect?

    The amount of effluent produced is dependent mainly on the dry matter of the crop and ensiling.  If grass is ensiled at above 25% DM effluent production will be negligible.

    Effluent production is very uneven, about 50% of the total being produced in the first week. The maximum daily effluent production is about 10% of the total produced.

    Pattern of effluent flow

    What factors affect effluent production?

    • Dry matter or stage of growth of the crop
    • Rain/heavy dew on the crop
    • Chop length – more cut ends release more effluent
    • Acid additives cause increased effluent
    • Enzyme additives have been associated with increased effluent
    • High clamps increase hydraulic pressure
    • Heavy rolling increases effluent with wet crops
    • Aerobic activity in the clamp produces water

    Why is reducing silage effluent so important?

    • The wetter the silage, the more effluent you will need to store.
    • Silage effluent represents a loss of readily available nutrients.
    • Typically it has a DM of 6%, an ME of 12.5 MJ/kgDM and a CP of about 19% DM.
    • Silage effluent is the most corrosive effluent produced on farm.  
    • It is also the most polluting being 200 times worse than raw sewage.

    How can I reduce effluent?

    Wilting is the most effective method – above 30% DM there should be almost no effluent.

    EffluentWhat are my options for disposing of silage effluent?

    Silage effluent has a significant value as a fertiliser. It can be spread on land but should be diluted 1:1 with water, especially in dry weather, otherwise there is a risk of leaf scorch. Make sure it is only applied to land with no risk of run off entering open drains or watercourses.

    Fresh silage effluent can be fed and can be stored for up to one year if formalin is added as a preservative at a rate of 3 litres per 1,000 litres effluent.

    Are there any health risks associated with silage effluent?

    It can be mixed with slurry in open air slurry stores but this combination results in the production of toxic hydrogen sulphide gas which is lethal in small doses, so never mix them in an enclosed tank or within a building.

  • Why is soil contamination an issue?

    If the silage has an ash content above 10% DM it is a good indicator of soil contamination. Soil is a good source of enterobacteria, clostridia and listeria which increases the risk of a poor fermentation, especially with wetter forages, and can lead to animal health issues.

    The risk is highest in wet weather and with lodged crops. If there is a risk of soil contamination, wilt to at least 30% DM and/or use an additive to increase the rate of fermentation.

    Minimising soil contamination:

    • Fix muddy gateways
    • Trap moles and roll the fields before mowing
    • Avoid handling the grass in wet weather
    • Increase the cutting height
    • Avoid running machinery over the cut grass
    • Ensure tedders and rakes are set high enough
    • Clean clamp machinery, especially wheels, before you start
    • Keep the clamp apron clean – don’t allow field machinery and clamp machinery to run over the same area
  • What happens to the fertiliser I apply?

    Fertiliser N is usually supplied as nitrate-N or ammonium-N.  The latter is converted rapidly to nitrate-N in the soil and this is the form of N that the plant takes up and uses to transport N around the plant.  In order to use it to make proteins it must first be converted back to ammonia.  This combines with organic acid-based molecules to form the amino acids, the building blocks of proteins.

    At any one time, therefore, N is present in plants in a number of forms.  Shortly after fertiliser application it will mainly be as nitrate-N but over time this will change to protein-N.  As the plant grows the total N concentration will also reduce.

  • When can I apply slurry, FYM, silage effluent, dirty water?

    Slurry and FYM are heavily laden with undesirable  bacteria which can survive on the grass for many months.


    • compete for sugars required for fermentation
    • can produce toxins


    • can produce a secondary butyric fermentation
    • spores in milk can result in some cheeses blowing

    Ideally slurry should not be spread on silage ground at all and certainly not after grass growth has begun, ie within about 10 weeks of 1st cut. Application after the end of January will increase the chances of contamination and if possible it should not be applied between cuts.  FYM should not be applied after Christmas. If you do need to apply slurry late you will need to wilt rapidly to at least 30% DM to minimise the risk of a poor fermentation.

    The heavier and less dilute the dressing of slurry/FYM, the more damage to the grass sward and the greater risk of contamination of any subsequent cut.

    Silage effluent and dairy washings are potent sources of contaminating bacteria and should not be applied to silage ground.

    If you are injecting, rather than spreading, slurry you can apply up to two weeks before harvest.

  • How much fertiliser should I apply to grass and when?

    Grass requires a good supply of all three of the major nutrients, nitrogen (N), phosphorus (P) and potassium (K) and in some areas sulphur (S) is now recommended too.

    Nitrogen is the key to yield and the average rate for first cut for dairy farms is around 100-125 kgN/ha (80-100 units/acre) and for second cut 75-100 kgN/ha (60-80 units/acre).  Higher rates will increase yield but care has to be taken to ensure there is no environmental risk and high rates mean there will be more chance of high nitrate levels at harvest.

    It is important to take into consideration the contribution from other sources of nitrogen, eg slurry or clover, when calculating the amount to be applied and also the residual N in the soil.  There will be little in a new ley after arable cropping but there will be plenty in a re-seed from permanent pasture. Many badly fermented silages are the result of applying too much N fertiliser so, if in doubt, err on the side of caution.

    Fertiliser should be applied immediately after cutting, a delay of only one week leading to a 6% reduction in yield.

    There is no point in applying nitrogen before grass growth begins as it could result in environmental pollution.  For many years T sum 200 has been used as a guide to when fertiliser application can begin, provided ground conditions permit.  Typically 40 kgN/ha would be applied in February to early March with the rest applied in late March to early April, but at least 6 weeks before cutting.

    Fertiliser should be applied immediately after cutting, a delay of only one week leading to a 6% reduction in yield.

    A general guide to nitrogen uptake is 2.5 kgN/ha (2 units/acre) per day from the date of application to the date of cutting, but this assumes active uptake is possible.  Dry periods will not count. Use this and your intended cutting date to calculate the maximum amount of fertiliser you should apply.

    If spring application of fertiliser is delayed the application rate should be reduced because the harvest date will not be delayed by as much.

    Care should be taken to spread fertiliser evenly and accurately. Uneven application will lead to uneven plant nitrate contents and an uneven fermentation in the clamp.

  • When should I take 1st cut?

    There is no fixed time for this. It will depend on the type of grass, the weather, whether you are looking for quality or quantity, etc.  Past experience is probably your most valuable guide using stage of growth as your main indicator.

  • Tips for making silage in wet, catchy conditions

    In such conditions it might be best to aim to make good, low DM silage than attempt to wilt it and still end up with relatively low DM silage but with a poor fermentation.

    • Don’t attempt wilting
    • Get the grass in quickly
    • Minimise conditioning
    • Lengthen chop
    • Minimise rolling
    • Use a proven inoculant
  • Why are high nitrate levels at cutting a problem?

    High nitrate levels at harvest (>1000ppm or 0.1%) are an indication that the fertiliser nitrogen applied has not all been converted to protein. This can occur if too much N fertiliser is applied or it is applied to near to harvest.  It can also occur when there is rain after a prolonged dry spell which will cause rapid N uptake.

    Such crops will also have low sugar levels and a higher buffering capacity. Low sugar, high non-protein nitrogen and a high buffering capacity all tend to make a poor silage fermentation more likely. There is also the risk of toxic silo gas being produced.

    How can I avoid high nitrate levels at harvest?

    The best way is to get your application rate right in the first place!  However, prolonged periods of dry weather cannot be anticipated and can ruin even the best made plans.  Remember that, after a prolonged dry spell, rain will cause rapid nitrogen uptake by the plants.

    If high nitrates are suspected make sure you have a representative sample of the grass analysed.  If the nitrate level is above 1,000 ppm (0.1%) you should not ensile.  Wait a few days and analyse again.

    Is there anything else I can do to help if nitrates are high

    Ensiling at a higher dry matter will help to reduce the risk of a poor fermentation as will application of a silage inoculant designed to make the initial fermentation faster and more efficient

  • What is the best way to wilt?

    As soon as the crop is cut it starts to deteriorate. Sugars released during mowing will be used by a wide variety of bacteria that can grow in the presence of air, reducing the sugars available for fermentation. Yeasts and moulds can also proliferate in the dying plant material, increasing the risk of aerobic spoilage later. So, whatever dry matter you are aiming for it is important to get there quickly.

    There are a number of things that will speed up the drying process:

    • Don’t cut unless you have a long enough weather window
    • Cut when it is warm and windy
    • Condition the grass to speed up moisture release
    • Maximise exposure to the sun and wind by spreading it over as much of the field surface as possible
    • Ted it once or twice to increase drying – make sure the tedder (and rake) height is set not to scoop up soil

    Even in good conditions it is not advisable to wilt for more than 24 hours because field losses and nutrient losses will probably exceed any benefits resulting from increased dry matter levels.  A mini-wilt of no more than 12 hours probably gives the greatest benefit overall.