DAERA Notes: Dairying

When should grass be cut for silage?

By Christopher Breen
Tuesday, 14th May 2019, 11:00 am
An ideal chop length promotes good consolidation in the clamp and provides enough fibre for the cow to ruminate
An ideal chop length promotes good consolidation in the clamp and provides enough fibre for the cow to ruminate

As a guide you should cut before 50% ear emergence in the sward. For swards based on early perennials cut around 10 May. Swards based on mid-season varieties will be ready to cut around 20 May. Cut late varieties in the first days of June. Walk your crops before these dates and check for ear emergence so that you can plan a cutting date. Each weeks delay after 50% ear emergence results in an extra 2 kg of concentrates to achieve the same milking performance.

Cut grass 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 that grass is mown dry, cutting wet grass means a longer wilt time and a reduction in nutrients. Mowing grass later in the day is more preferable than starting at 9am when dew may still be a problem.

Wilting grass

To speed up the wilting process spread the cut crop 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 will be 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 longer chop length of 50 mm.

Clamp management

The purpose of ensiling grass is to preserve and minimise the loss of nutrients thus improving silage feeding value. Silage fermentation can be divided into two phases:

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 air and make the clamp as air tight 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 an effective cover and weight the cover, paying particular attention at the shoulders of the pit.

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 them are a substitute for good silage making techniques and management, but they should help make a good situation better.

May jobs checklist

Graze swards down to 1,600 kg DM per hectare to maintain sward quality.

Check silos and carry out any maintenance in advance of silage making.

Check there is adequate storage in tanks to collect effluent produced.

Calibrate parlour and out of parlour feeders to ensure accurate feeding.

Spray docks/weeds if conditions are suitable and they are at the right stage for control. If spraying silage ground for docks, generally allow an interval of at least 21 days between spraying and harvest. As this depends on the product used always read the label.