How to make grass silage

This quick reference guide explains how to make grass silage while assuming that soils are free from compaction and have optimum nutrient status. It is important to seek further advice if needed.

What is grass silage?

Farmers produce grass silage by cutting grass and preserving its nutritional value by using bacteria to ferment the sugars. This produces lactic acid to reduce pH and stop microorganisms from spoiling grass silage.

What to consider when making grass silage

Important considerations when selecting the best grass for silage making should include:

  • Planned duration of the ley
  • Number and timing of cuts in a season
  • Silage only or dual-purpose
  • Quantity and quality
  • Clover or no clover
  • Is drought tolerance required

Addressing these points will help ensure that the type of silage grass seed mixture is fit for purpose. The best grass seed for silage production will contain a number of ryegrass varieties (perennial, hybrid or Italian), with the possible addition of an alternative grass species, such as timothy, and white or red clover.

  • Ensure all varieties are high-ranking on the latest Recommended List
  • Consider the relative merits of diploid and tetraploid varieties

Diploids are more persistent and create a denser sward, better suited to wetter conditions and where long-term grazing is also required Tetraploids have a more upright growth habit and can be faster establishing.

  • Ensure the heading date range within the mixture is as tight as possible (one week optimum) and coincides with target cutting dates in order to maximise quality

How to make grass silage

How to make grass silage

When making grass silage, it is usually the case that as quantity increases, quality decreases. This is because the more mature (and higher-yielding) crop will have lower nutritional value, for reasons explained earlier in this guide.

There is, therefore, an inevitable compromise, with decisions on when to cut grass for silage in the UK. This is best determined by the class of livestock to be fed and stock performance targets.

For maximum yield without significant compromise of quality, most crops are best cut approximately one week before heading.

How long to wilt grass for silage?

Wilting to achieve an optimum silage dry matter of 30-35% (clamp) and 35-40% (bale) should ideally be quick and short, so a maximum of 24-36 hours.

  • Using a mower-conditioner will increase the speed of wilting and reduce losses of sugar, protein and dry matter of grass silage
  • Leaf pores only remain open for two hours after cutting, when the speed of moisture loss is five times greater than after pores close – so spread the crop quickly after cutting grass for silage
  • Spread the crop over 100% of the field area, again to increase the speed of wilting grass silage

Clamps and bales

High-quality grass silage can be made in both clamps and bales, and both systems have their place on modern livestock farms. The choice depends on individual farm circumstances and a range of variables.

Using a silage additive for clamped grass will not salvage poor-quality forage. However, when the right product is selected for the right purpose, they may help make good silage even better. Here are some tips:

  • Ensure the forage harvester is blowing all chopped material into the trailer
  • Set forage harvester chop length according to grass dry matter content
  • Sheet the clamp sidewalls
  • Consider using a ‘clingfilm’ as a second top sheet to reduce oxygen ingress
  • Ensure all machinery entering the silage pit have clean wheels
  • Fill the clamp quickly in thin layers whilst ensuring sufficient compaction
  • Sheet down quickly after finishing
  • Weigh down the top sheet sufficiently

How to make grass silage

Using bales

  • Create a box-shaped swath and use a baler (round or square) to form uniform bales with ‘square’ edges
  • Use a baler with a chopping function to produce denser bales with less oxygen trapped inside; chopping also releases sugars to assist a rapid fermentation
  • Wrap bales as soon after baling as possible
  • Use good quality wrap and an effective wrapper to apply at least six layers
  • Move bales for stacking as soon after wrapping as possible
  • Stack bales on a level surface free from sharp objects and at least 10m from a watercourse; bales are best stacked on their sides and no more than three high (or four if over 35% DM)
  • Protect bales from bird and rodent damage.

How to make grass silage

Setting up a multi-cut silage system

Cutting grass earlier in the season and at shorter intervals will mean it is closer to the optimum D-value at the point of ensiling and should therefore result in a higher feed value forage.

It is estimated that this could amount to as much as an extra 1 MJ/kg of energy in many cases – so 12 MJ/kg ME silage instead of 11 ME – which sets the platform for increasing milk production from forage.

Germinal’s guide to multi-cut silage making is designed to ensure maximum benefits are gained from this more progressive approach.

Plan ahead

  • Consult your contractor or review your own equipment
  • Consider clamp capacity and/or bale storage area
  • Set targets for timing, tonnage and quality
  • Test soils and slurry over the winter period

Reseed grass leys to ensure quality

Over-winter swards with optimum cover

  • Remove autumn grazing stock by end of December with a sward height of 4-5 cm
  • Walk silage fields to check drainage, mole damage and weed content
  • Apply slurry into the soil, not onto the sward, to a maximum of 25,000 l/ha

Ensure correct crop nutrition

  • Avoid heavy slurry applications within 10 weeks of cutting
  • Apply any slurry into the soil immediately after fields are cleared
  • Then apply bagged fertiliser as recommended by an agronomist but not exceeding two units/acre for each growing day between cuts

Cut early and frequently

  • Take first cuts in late April/early May (depending on season and location) to maximise ME yield
  • You can take subsequent cuts at intervals of four to five weeks to maintain quality
  • Mow no lower than 6.5 cm to ensure rapid regrowth

Wilt quickly for optimum dry matter

  • Cut early in the day with a mower with an effective integral conditioner
  • Ted out the crop within two hours to maximise the speed of drying
  • Aim to pick up the same day for a target of 28-32% DM silage

Apply a proven silage additive to improve fermentation

  • Because protein and nitrates may be higher – which buffer the fermentation
  • Select an additive containing the most efficient fermentation bacteria

Chop long to maintain structural fibre

  • Consider a chop length of 5 cm or longer for good clamp management
  • The typical chop length of a forage wagon can work well with multi-cut

Ensile for the best possible grass silage fermentation

  • Apply best practice approach when clamping or baling
  • Ensile in layers to maximise compaction
  • Roll or compact to squeeze out air
  • Seal effectively to maintain airtight conditions

Feed fibre as needed to balance rations

  • Ensure sufficient ‘scratch factor’ for optimum rumen function
  • Consult your nutritionist to maximise the value of higher energy silage

How to read grass silage analysis

Whether clamped or baled, conserved forage will often make up a large proportion of ruminant diets. Therefore, it's essential to understand the grass silage nutritional value. And you can undertake this through a representative grass silage analysis, with the key metrics outlined below.

AnalysisAbbreviationUnitsRangeTarget value**
Grass silage dry matterDMg/kg150-500280-350
Metabolisable energy (grass silage yield)MEMJ/kg DM8.8-12.0>11
Neutral detergent fibreNDFg/kg DM500-650500-550
Acid detergent fibreADFg/kg DM230-350300
Ash-g/kg DM60-200>80
Crude protein (grass silage protein content)CPg/kg DM100-200150-175
pH--3.5-5.5Depends on DM
Ammonia NNH3Ng/kg N20-300<80
Total fermentable acidsTFAg/kg DM20-300<100 (depends on DM)
Volatile fatty acidsVFAg/kg DM10-90% TFA 25% (as low as possible)
Lactic acid*-g/kg DM20-20080-120
Acetic acid-g/kg DM20-80<25
Butyric acid-g/kg DM0-20<5
Residual sugars-g/kg DM0-150100 (as high as possible)

* For well-fermented silage, lactic acid as the proportion of the total acids should be >75%.

** Different analytical companies use different units for expressing the values. This example shows g/kg DM to convert to % divide the value by 10.

Indicators of in-field silage grass quality

  • D-value
  • ME
  • CP
  • ADF
  • NDF

Indicators of fermentation quality

  • Lactic acid
  • Acetic acid
  • Butyric acid
  • Ammonia N
  • VFAs 

Indicators of silage grass and fermentation quality

  • Dry Matter
  • pH
  • Water-soluble carbohydrate

Talk to the forage production experts

Feel free to ask our grass and forage experts for advice on producing high-quality grass silage.


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