Silage is grass that has been ‘pickled’. This is used to preserve the grass for later events, like in winter season when there is no grass growing.
The grass is cut and then fermented. This process must be carried out under acidic conditions (around pH 4-5) in order to preserve nutrients and provide feed that cows will like to eat. Fermentation at higher pH results in silage that has a reduced pallatability, and lower nutritional quality.
Preparing the grass
First, the lay must be cut when the grasses contain their highest nutritional values. This is usually just before the grass is fully mature. This is important because all forms of preserved grass, such as hay and silage, will have lower amounts of nutrients than fresh pasture, everything must be done to make the end product as nutritious as possible.
Grass is allowed to wilt in the field for a few hours to reduce the moisture content to around 60-75%. This moisture level will allow for optimum fermentation. If the grass is left out longer, it may get too dry, or it may get rained on - and both these will reduce proper fermentation.
After the grass is cut, it is chopped into smaller pieces and then compacted to get out as much oxygen as possible. This is important because the microorganisms, called lactic acid bacteria, that naturally habitats the grass are needed to carry out the fermentation, like living in oxygen-free environments. If the silage is to be stored, piled in a large pit, tractors and other machinery are usually driven over the grass pile until it is compacted.
The next step is to seal the compacted grass with plastic to keep oxygen out. Mounds of silage are covered with huge polythene (plastic) sheets and weighted down to ensure maximum compacting. Silage can also be stored in bales which are covered with a plastic wrapping.
The oxygen challenge
Removing and keeping out oxygen is a key part of making good quality silage. This is because fermentation occurs under anaerobic (oxygen-free) conditions, otherwise the correct type of microorganisms won’t grow.
While oxygen remains, plant enzymes, bacteria and unwanted microorganisms react with the plant sugars and proteins, reducing the amounts of these nutrients in the silage.