Grass Silage


6 min read

What is pasture silage? Losses when making silage How much silage do I have? Why analyse pasture silage? How to sample silage Assessing silage quality Designing a storage area

Pasture silage is pickled pasture, preserved through the conversion of its sugars into lactic acid by bacteria. This process extends the shelf life of the pasture beyond what it would have if exposed to open air. To produce high-quality silage, both the quality of the original pasture and the fermentation process are crucial. Ensuring optimal dry matter content, rapid wilting, proper compaction, and effective sealing can help retain the nutrients and feeding value of the silage. It's essential to feed out silage quickly once exposed to air to prevent spoilage. Furthermore, consider the design and location of your storage area to minimise losses and environmental impact.

It is impossible to produce high quality silage from low quality pasture, no matter how good the fermentation is. Both the quality of the ensiled pasture and the quality of the fermentation must be considered.

What is pasture silage?

Silage is pickled pasture. When pasture is ensiled, its sugars are converted into lactic acid by bacteria. It is the lactic acid which pickles the pasture, allowing it to be preserved for a lot longer than it would have been if left in the open air.

With well-preserved silage, losses in feeding value during fermentation will be small, and the final silage will be only slightly lower in feeding value than the original pasture.

Minimum values for high quality silage is 10MJME/kgDM and 16% crude protein.

Good quality pasture silage is a good source of energy and protein for a milking cow and can be used as a fibre source when feeding high sugar or starch feeds. However, a poor quality pasture silage (made from low quality pasture, or ensiled with low quality fermentation, or both) will not support high milk yield and will only be suitable for dry cows, or as a fibre source to reduce risk of acidosis.

Losses when making pasture silage

Losses occur as sugars and protein in the grass is broken down by enzymes, and bacteria. This process starts as soon as the grass is cut. Losses decrease quality as well as quantity, because it is the highly digestible components which are most rapidly broken down.

Losses during harvesting

Losses depend on the dry matter (DM) of the pasture. The optimum DM for silage is 25-30% because total DM loss is minimised.

  • Cut in the morning of a sunny day, for rapid wilting. Cutting after 1-2 days’ sunny weather will result in good sugar levels in the pasture, even when cut in the morning.
  • Avoid wilting for any more than 24 hours.
  • Compact the silage well. In a stack or pit, use the heaviest wheeled vehicle available.
  • Tractor wheels should not sink into the pile of pasture any further than the depth of rubber.
  • For baled silage make sure that a high density baler is used.
  • Seal the stack completely with a weighted, airtight cover. Wash old polythene before use to avoid contamination with the wrong bacteria.

Losses while grass is in the stack, pit or bale

Once the silage is sealed, nothing can be done to change the fermentation process. Poor fermentation (e.g. air in the stack) leads to major losses of protein quality. In poorly preserved silage protein is broken down into ammonia, which decreases the feeding value of the silage.  Getting things right while the grass is being harvested will maximise the chance of having a good fermentation:

  • A fast wilt to 25-30% DM will leave good sugar concentrations in the pasture.
  • Quick compaction and effective sealing will keep out oxygen, making conditions more suitable for bacteria to convert sugars into lactic acid.
  • Fast production of lactic acid will quickly reduce the pH, to prevent protein losses.

Feeding out losses

Silage begins to break down once exposed to air,and will begin to heat up as micro-organisms turn the remaining sugars and protein into heat and energy. When feeding out, aim for as little time as possible between exposing the silage to air, and the cow eating it.

There are several ways to limit losses from silage while feeding out:

  • Remove at least 20 cm off the whole stack face each day, so silage at the face is not exposed to air for a more than one day.
  • Cut silage off the face, rather than pulling it off. This keeps a smooth surface at the stack face, which reduces air penetration into the stack.
  • Leave the stack face open on dry days to avoid heat build-up under the polythene.
  • Do not feed out more than 1 day in advance, especially in summer.
  • Cows will be able to eat more of the silage they are offered if it is fed out on dry paddocks or feed out areas, along fence lines, or in feed bins or troughs.

Do not allow cows access to spoiled silage.

How much silage do I have?

Well preserved silage, made from high quality grass, is a valuable feed. To make the best use of it, we need to know how much we have, and how much we are feeding each day.

You can estimate the yield of your own silage crop:

  • Weigh a silage wagon when it is empty, and when it is fully loaded, across a weigh bridge. The difference in the two weights is the pasture in the wagon.
  • Count the total number of wagon loads which have gone into the stack.
  • Multiply the total number of loads by the estimated weight of grass in a wagon load to get an estimated fresh weight of the silage you have made.
  • Estimate the DM% of the silage, and use this to estimate the total kgDM which have been harvested.
  • Total DM, divided by the hectares harvested, gives an estimated yield of kgDM/ha.

How much is in my pit?

The amount of silage/m³ in a stack or bunker will depend on the moisture content of the silage, how well compacted the silage was, and the height of the stack (deep bunkers have more compaction at the bottom of the heap). A good average for silage in the stack is 180 kgDM per cubic metre (kgDM/m³).

The weight of silage in storage is then calculated using the following equation: Silage weight (kg DM) = volume of stack or pit (m3 ) x silage density (kg wet weight/m3 ) x DM %. The volume of the storage pit can be estimated by multiplying the dimensions: width x the length x the approximate height of the silage.

Calculations would be simple if the silage was stored as a rectangle or square block. However, given that most silage storages are irregular shapes, the dimensions can be estimated, taking into account the taper on the top of the storage and the slope of the sides and ends.

How can I estimate the DM% of my silage?

Use the squeeze test. You can use to estimate DM% in fresh grass, and silage.

  • Break up the grass or silage into lengths of 2-3cm (1”).
  • Roll into a ball about twice the size of a golf ball, and squeeze in your fist for 30 second
Juice runs out easily 18-20% DM
A little juice runs out, with difficulty 20-25% DM
Your hand is damp, and sample stays in a ball when you stop squeezing 25-30% DM
Sample does not stay in a tight ball when you stop squeezing Over 30% DM

Why analyse pasture silage?

To assist your management. An analysis will tell you the quality of the silage so you can decide whether to feed milking or dry cows and allows you to value silage based on it Metabolisable Energy (ME) content.

How to sample silage

Take at least five handfuls of silage from different places and mix them together. Send a subsample of one litre (half a bread bag) for analysis. The sub-sample should be representative of all the silage in the stack or bale.

You can take silage sampled from unopened stack or bales to tell the quality of the feed before it is fed out. (It will also not have been spoiled through exposure to air) or sample silage as it is being fed out. 

Assessing silage quality by sight and smell

If you don’t have a feed analysis for your silage, you can estimate quality from its appearance.

Feed quality: High quality silage has lots of leaf, and very little stem in it. The more stem in silage, the poorer its quality.

Colour: Well-preserved silages are green, yellow, or pale brown. Dark brown silage is generally poorly preserved.

Smell: Well preserved silage has a sweet, tobacco smell. Foul, rancid smells indicate the presence of butyric acid from poor preservation.

Moulds: Moulds grow where silage has been exposed to air. Some fungi can produce toxins.

Designing a storage area

Well-placed supplementary feed storage areas can save money through reduced spoilage, fuel use and travelling time. Ideally, you want to store feed as close to where you will be using it as possible. A good design will help to reduce spoilage from rodents, surface or rainwater, and prevent water pollution by capturing toxic leachate and run-off. Placement should also take the food safety minimum distances into account.

Creating good storage areas:

  • Storage should be easily accessible
  • At least 45 metres from any farm dairy
  • At least 50 metres from waterways, open drains or dams, and the catchment areas of bores; further away if the land is steep, soils are coarse textured, or ground water may enter sensitive waterways.
  • Away from gullies or other places where run-off catchment water can flow into the storage area not prone to flooding or that has a high water table away from any subsurface drains.
  • Silage leachate should never enter waterways as it is highly toxic to aquatic life.
  • The walls and floor of a silage bunker must be capable of withstanding the hard knocks of tractors and other machinery as well as resisting corrosion from leachate.
  • The silage bunker floor should be watertight and extend out beyond the bunker walls
  • Properly covering stacks to keep the oxygen out and prevent rainwater entering is crucial to reduce the volume of leachate.
Last updated: Sep 2023

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