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When should grass be cut for silage?

As a guide, you should cut before 50% ear emergence in the sward. For swards based on early perennials, cut around 10th May. Swards based on mid-season varieties will be ready to cut around 20th May. Cut late varieties in the first days of June, writes Christopher Breen, CAFRE Advisor.

Walk your crops before these dates and check for ear emergence so that you can plan a cutting date. Each week of delay after 50% ear emergence results in an extra 2.0 kg of concentrates to achieve the same milking performance.

Grass should be cut at the first spell of good weather. A bright day is ideal to increase the sugar content of the grass giving improved fermentation. Sunshine also promotes a more rapid wilt, reducing the amount of water ensiled.

It is important, where possible, that grass is mowed down dry. Wet grass needs a longer wilting time, resulting in reduced nutrient value. Mowing grass later in the day is more preferable than starting early when dew may still be on the ground.

Wilting grass 

  • To speed up the wilting process, spread the cut crop out over the entire field straight after mowing.
  • Aim for a target grass dry matter (DM) of 30% at harvesting.
  • A rapid wilt prevents excessive sugar and protein losses.
  • In ideal wilting weather, a crop is ready to lift within eight hours if it has been spread out.


  • Ideal chop length promotes good consolidation in the clamp and provides enough fibre for the cow to ruminate.
  • Set chop length at 25 mm for grass with a DM of 30%.
  • When grass is wetter (less than 20% DM), consider a chop length of 50 mm.

Clamp management

The purpose of ensiling grass is to improve silage feeding value by preserving and minimising the loss of nutrients. Silage fermentation can be divided into two phases:

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Phase 1 – elimination of oxygen by chopping grass to the correct length, ensiling at recommended dry matters, filling the silo quickly and distributing grass evenly in the silo.

Phase 2 – production of lactic acid by micro-organisms in the grass. Lactic acid is the primary acid responsible for lowering pH, producing silage and making it stable.

Undesirable micro-organisms can dominate if the pH does not drop rapidly. Where weather allows, wilting grass to 25-30% DM before ensiling can eliminate this problem.

The key is to remove the air and make the clamp as airtight as possible. Ensure the ensiled grass is spread in shallow layers and rolled continuously. Always cover the silo at night.

At the end of harvest, apply a cover and weight the cover effectively paying particular attention at the shoulders of the pit.

If it has been possible to wilt the crop to 30% DM before ensiling, effluent output from the clamp will be minimal. However, if dry matters are lower, keep a close check on effluent output and tank holding capacity for up to six weeks following ensiling.

Use of an additive

Effective silage fermentation produces high levels of lactic acid reducing the crop pH. Silage additives can help this process. A variety of additives are available including bacterial inoculants, enzymes, non-protein nitrogen sources, acids and sugar sources.

It is important to emphasise that none of these products are a substitute for good silage making techniques and management, but they can assist in making a good situation better.

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