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.

  • How long should I leave between cuts?

    In the UK five to six weeks is usually required for regrowth if the traditional three or four, high-yielding cuts are to be made. If quality is more important to you than quantity you might consider cutting more often for a lower yield of higher feed value forage. This material is harder to ensile wet without an additive and will benefit from wilting. Remember that more cuts require more labour and the overall yield will be lower but cutting a younger crop means less delay in regrowth.

  • What is silage?

    Silage is a forage crop that has been preserved by pickling in acid.  In a natural fermentation the acid is produced by bacteria present on the crop that, in the absence of air, convert plant sugars mainly into lactic acid.

    There are 4 distinct phases associated with ensiling.

    1. Initial aerobic (oxygen present) - begins immediately after cutting and continues during the initial period after ensiling.  It is not helpful to the subsequent fermentation because sugars are wasted as a result of plant and microbial respiration that breaks sugars down to carbon dioxide, in the process producing heat.

    2. Fermentation – When all the oxygen has been used up and conditions become anaerobic (no oxygen present) the main fermentation occurs. This is brought about by lactic acid bacteria already on the crop or added by inoculation.

    3. Storage – When fermentation is complete and this storage phase can last a few weeks or several years.  During this phase there will be minimal microbial activity and the silage should remain more or less stable provided that the clamp remains sealed.

    4. Feedout – When the silo is opened for feeding and the silage is exposed to air again. This allows aerobic microorganisms, such as yeasts and moulds, to become very active again and can lead to high DM losses.
  • How should I store bales?

    Store bales in a dry, sheltered area preferably on a prepared base. They should not be overhung by trees and should be at least 10 metres from a water course, ditch or field drain.

    Stack height depends on the dry matter of the silage. Bales made of lush, leafy grass should not be stacked at all. If the DM is 25-35% they can be stacked two high and if more than 35% they can be stacked three or four high. Stacking them on their ends is another option as the ends have more layers over them so provide extra protection, but they tend to be less stable so you can’t stack them so high.

    Stack within 24 hours of wrapping and do not disturb them again for at least three weeks if possible.

    If you are concerned about vermin damage bait the area around the stack – not within the stack.  If birds are a problem it would be wise to net the stack. Some people say that mounting a plastic bird-of-prey (obtainable from gun shops) is also very effective. Also, make sure you protect the stack from the attentions of livestock.

    Inspect the bales for damage regularly and seal any holes. Use those bales first.

  • Why are bales more likely to go mouldy?

    The grass in bales is not so highly compacted as in a clamp and is not chopped as short.  These factors, together with the fact that the grass for bales normally has a high dry matter content, mean that more air is trapped in the bale initially, allowing the yeasts that initiate aerobic spoilage a longer period to grow and multiply before the air gets used up.  This means that there are higher numbers of yeasts present later if air should get in so spoilage will begin faster. Bales also have a high surface area to volume ratio which means that a greater area is potentially exposed to air. About one third of the volume of a bale is contained in the outer 3 inches.

  • Advantages of haylage over hay
    • Dust free so healthier and no need to soak
    • Bales are denser so less storage required
    • Bales are wrapped so can be stored outside
    • More palatable so there is less wastage
    • Higher nutritional value so less purchased feed
    • A more natural feed and more easily digested
    • More cost effective
  • How soon can I feed silage?

    Ideally you should leave a clamp closed for at least 4 weeks but if you are desperate, you can feed it within a few days even though the fermentation will not have stabilised yet.  By opening the silo before the silage has settled properly you also run the risk of increased aerobic losses as the face will not be tight and will allow air to penetrate deep into the clamp. This is especially important with higher DM forages that are not so well compacted initially.

    There is often a temptation to open maize clamps immediately. Apart from the risk of increased aerobic spoilage, with maize you also need to be aware that starch digestibility will increase gradually for several months after ensiling. The ME could increase by 0.5 MJ/kgDM over the first couple of months. This is because the prolamin protein matrix that the starch granules sit in is gradually degraded, making the starch more accessible over time. Thus, it is essential to have maize silage analysed frequently so the starch level in the ration formulation can be adjusted as required.

    If you feel you are going to have to open the clamp immediately, rather than risk the main clamp you would be better to make a small separate clamp or some bales.


  • Is there a risk of botulism when feeding silage to horses?

    Botulism is caused by the bacterium Clostridium botulinum and is rarely associated with well-made silage. In cases where this disease has been linked to silage, it has always been because an animal carcase has been ensiled along with the forage. The rotting carcass provides a niche environment in which this disease organism can potentially thrive.  

    Clostridia will not thrive in higher DM silage (> 30% DM) and as haylage is normally baled at > 50% DM there would not normally be an issue – just check there are no animal carcasses in the bale as you are feeding it. If there are, discard it.

  • Can I simply switch from hay to haylage?

    The large intestine contains bacteria to help digest the fibre in feeds. They are very sensitive to changes in the diet so make the switch gradually, say over a week, to allow the bacteria time to adapt.  This should prevent stomach upsets.

    You also need to get used to handling haylage so as not to over or under feed your horse. A typical 500 kg 16hh riding horse requires a minimum daily roughage intake of about 1% bodyweight, ie 5kg of hay, or, due to its higher moisture content, 7 to 8.5kg of haylage. But although you need more haylage by weight you need much less by volume as, again due to its higher moisture, haylage is much denser than hay. At the beginning it is best to weigh out the required amount each day until you get used to judging it.

    Haylage dry matter has about 10% higher nutritional value than hay so if you are feeding the equivalent in terms of dry matter, you can also cut down on supplementary feed as less will be needed to meet the horse’s energy requirements.

  • Grass silage


    The 1970’s and 80’s saw a dramatic shift from conserving grass as highly weather-dependent hay to the more flexible system of producing grass silage. The introduction of big bale silage brought the additional benefits of transportation, flexible storage and feeding.

    Grass silage is basically pickled grass. The aim is to retain as much feed value as possible by encouraging lactic acid bacteria to ferment grass sugar to produce lactic acid.

    The lactic acid lowers the pH and prevents the growth of spoilage micro-organisms, allowing a stable preservation of grass as silage. To achieve this, there must be sufficient sugar available and fermentation must occur as quickly as possible in and air must be excluded throughout (anaerobic conditions).

    This can be done in a silage clamp or in big bales. Both have the same objectives:

    • Rapid removal of air (compaction)
    • Rapid fermentation of grass sugars to lactic acid
    • Maintenance of anaerobic conditions in the clamp/bale during storage
    • Growing and harvesting factors influencing forage quality.

    Getting the right microbial population is essential to make good silage. High numbers of the right kind of lactic acid bacteria (LAB) are what you want for a fast, efficient fermentation. They will outcompete less desirable bacteria that waste sugars by fermenting them to weaker acids, eg acetic acid, or non-acid alcohols. A fast fermentation to a low, stable pH will also prevent a clostridial fermentation.


    • Follow good crop management practices to encourage yields without reducing sugars.
    • Avoid late application of fertiliser. Apply nitrogen at least eight weeks before mowing.
    • Try not to surface apply slurry within ten weeks of mowing (and preferably not at all) due to the presence of undesirable micro-organisms.
    • Injecting can be done up to 2 weeks from harvest.
    • When reseeding, select the variety which meets your requirements for the harvest date, yield, quality, soil type and climate.


    • Rapid wilt (24 hours maximum) to about 30% DM to reduce effluent and concentrate sugars if conditions permit.
    • As a guideline, 1% of moisture is lost per hour of sunlight in bright conditions. It can be greater with mower-conditioners and tedding.
    • When wilting, remember that high DM grass (35%+) has a greater risk of mould.
    • Wilting for 12 hours or more has become standard practice to reach the higher dry matter silage required by the modern dairy cow. However, longer wilting results in the proliferation of spoilage organisms, leading to the loss of valuable nutrients.

    Advantages of Grass Silage

    • Easy to grow  
    • Less weather dependent than hay
    • High protein
    • Grazing balancer
    • Quantity vs quality flexibility
    • Multiple cuts possible
    • Allows big bale system flexibility

    Disadvantages of Grass Silage

    • Variable fermentation and quality
    • Can be low pH (pH <4)
    • Can be labour intensive
    • Weather dependent
    • Later cuts more costly
  • What are the most important factors influencing silage quality?
    • The grass type and variety
    • Grass sugar content
    • Fertiliser
    • Stage of growth at cutting
    • DM at ensiling
    • Chop length
    • Contamination
    • Additive
    • Speed of ensiling
    • Exposure to air
    • Feeding technique
  • What is clostridial secondary fermentation?

    This occurs before the silo is opened in the absence of air and when a stable pH value is not achieved initially, often because of insufficient sugars or contamination from soil or slurry.  A particular type of undesirable bacteria (clostridia) convert the lactic acid already produced into butyric acid, a weaker acid, so the pH increases.  Some clostridia also break down amino acids, resulting in high ammonia levels and some extremely unpleasant products, eg putrescine and cadaverine.  Dry matter losses will be high and the final silage is unpalatable and has a low feed value.

    Good silage bad silage diagram  

    Fermentation of ensiled grass

    The major influences on clostridial secondary fermentation

    • The dry matter/sugar contents of the crop ensiled
    • Initial clamp management
    • Contamination (soil/slurry)
    • Use of an effective additive
  • What are the brown fumes sometimes seen around a silage clamp?

    This gas is formed when nitrates are broken down immediately after ensiling and is a mixture of nitrogen oxide gases, some of which are highly toxic. It has a yellow-brown colour and a bleach-like smell. It is heavier than air so tends to hang around near the base of the clamp.  Levels as high as 500 times the safe dose can be found.

    With wet grass the gas dissolves in the moisture and within 2 to 3 days will have disappeared but with high dry matter grass there may not be enough moisture to trap it and it is free to be released. When dissolved in moisture it forms nitric acid, very high concentrations of which can turn silage bright yellow-orange with a pH of below 2. It can also dissolve in the moisture in lungs if breathed in.

    This gas is a particular problem with enclosed clamps since open clamps allow the gas to disperse more quickly. With enclosed clamps make sure there is adequate ventilation and in all cases keep people and animals well away. It will usually disperse in a few days but can hang around for several weeks.