Silage pre-testing

Published 11 April 14

Silage grass pre testing and bacterial warfare

With first cut imminent in many areas it is important to consider a number of factors during the silage making process to ensure quality is maximised. 

Grass quality has a very important role to play not only in producing silage of high nutritional value, through maximising D Value but also in achieving the correct initial lactic fermentation, says Tom Goatman, DairyCo extension officer.

A good silage fermentation depends on the acidification of the crop in anaerobic conditions. This is achieved by the rapid conversion of water soluble carbohydrates (WSC) to lactic acid resulting in a rapid fall in pH and preservation of the ensiled crop. 

A number of factors can influence this process and pre testing grass before harvesting can help identify and manage some of the risks associated with achieving a good fermentation. A pre cutting grass analysis will include information in relation to 2 key parameters, WSC Level and Nitrate nitrogen.  

A minimum water soluble carbohydrate (WSC) content of 3% in the fresh crop is required to achieve a good fermentation.  Without a good level of WSC the lactic acid producing bacteria will not have a sufficient energy source to rapidly multiply and quickly drop the pH.  If the pH of the crop does not fall fast enough other undesirable micro organisms will not be inhibited and will start to multiply.

These micro organisms will convert lactic acid to weaker acids such as butyric acid and degrade protein producing substances such as ammonia.  This causes the pH to rise and produce a secondary fermentation with a significant loss of DM and quality.  A typical DM loss of approximately 5% can be expected if the crop has undergone a good fermentation contrasted to losses of around 10-15% in a poor fermentation.

Grass Nitrate levels are important to consider as high levels increase the buffering capacity of the crop making it more difficult to achieve the required pH drop.  Nitrate in the grass plant leads to the production of ammonia when ensiled, this in turn reacts with water to form ammonium hydroxide which is alkaline, buffering the pH.

Ideally Nitrate-nitrogen levels should be below 0.1%, high levels indicate that nitrogen taken up by the sward has not been fully metabolised to protein.  This risk increases in cool growing conditions and with high N applications from fertilisers and slurry.  A sustained period of dry weather followed by rain can also increase the risk of high Grass N levels as this promotes a sudden uptake of Nitrogen by the grass plant.

When it has been identified that the crop is suitable for cutting it is important that harvesting equipment is set up correctly.  Ensure that the cutting height is not too low to avoid contamination by soil borne bacteria; a cutting height of more than 5cm should achieve this.  It is also important to check any raking equipment is set up correctly to avoid similar soil contamination.

Once the grass crop is cut the next important consideration is the target dry matter to ensile the crop at. Increasing the dry matter of the crop helps to inhibit undesirable bacteria growth during fermentation and also helps to raise the ph that the silage will remain stable at.  A typical target of between 25-30% has been demonstrated to minimise field, clamp and effluent DM losses. 

The maximum time the crop should be wilted for is 24 hours, above this there will be increased DM losses due plant respiration and leaf shatter.  Low DM silage needs very acidic conditions to achieve a stable pH and will reduce DM intakes

Careful clamp management will also help to ensure that a good fermentation takes place.  It is important that there is a clean area to tip the grass on and that soil/slurry is not being tracked into the silo as this will introduce undesirable bacteria into the clamp.

Make sure the clamp is filled quickly and evenly avoiding large deposition of grass in one go, as it is difficult to consolidate. Chop length can be adjusted shorter for drier crops (25-40% DM) and longer for wetter crops to aid clamp consolidation.

It is important that the clamp is not over rolled and that is not rolled the following morning as this will expel CO2 and draw in O2.  A lactic fermentation will begin when all O2 present in the clamp is exhausted.