22 November 1996


Consolidation is crucial for high dry matter silage. Silage expert and nutritionist

Prof Mike Wilkinson offers management guidelines

SHORT chopping and careful consolidation are crucial to the successful preservation of wilted silage.

With the trend towards spreading grass to achieve speedier wilting, it is even more important to remove air from the silo when filling with drier crops.

Water in the crop can be removed rapidly after mowing, even in showery weather conditions, by spreading it out over the whole ground area.

Research at Hilsborough in Northern Ireland and at Wye College, University of London, has shown that dry matter contents of over 30% can be achieved in a 24-hour wilting period by spreading.

The crop need not be tedded after the initial spreading, and should be rowed up just before harvest, taking care that the rake is set to brush the stubble without contaminating the wilted crop with soil.

Wilted crops require fewer trailer loads than direct-cut crops, because less water has to be carted a hectare. Density of dry matter is also usually higher in the silo for wilted silage than for wetter crops, so wilting effectively increases the capacity of the silo and at the same time reduces effluent loss.

But drier silages are more prone to aerobic deterioration than wetter silages. Aerobic deterioration is the process of spoilage after air is introduced into the silo following opening for feed-out. Residual sugars and acids are oxidised by micro-organisms in the silage to carbon dioxide and water. There is loss of protein and amino acids, and an increase in ammonia. The silage becomes hot and moulds develop as the decomposition process continues.

Rate of aerobic deterioration depends on the original fermentation in the silo, and on the initial development of yeasts and moulds in the period immediately after filling. If a lot of air is left in the crop at filling, as the result of poor chopping and poor consolidation, then it is likely that the silage will be more prone to rapid heating and moulding once the silo is opened. Fermentations which inhibit growth of yeasts and mould, or which involve the secondary fermentation of lactic acid to acetic or butyric acids are most likely to leave silages which are stable in air during feed-out. In other words, a well-preserved silage is more likely to heat up and go mouldy than a poorly-preserved one.

Some well-preserved wet silages can be unstable in air during feed-out, especially if the fermentation has been restricted by the addition of an acid-type additive. The problem is sometimes not as apparent with wetter as with drier silages because their higher water contents act as heat sinks, and there is less rise in temperature as a result. Also, moulds prefer drier to wetter conditions, so their growth can be slower in wetter silages than in wilted material.

Yeast growth can continue under anaerobic conditions – lack of oxygen – throughout the storage period, and their rapid development on subsequent exposure to air is assisted by having a relatively high population at the outset. Rapid and effective consolidation during filling does not therefore guarantee a stable silage at feed-out if the population of yeasts is high in the crop at harvest. Prolonged field-wilting, for over 48 hours, may result in elevated yeast counts, but more research is needed to establish what factors influence the population of yeasts in grass crops.

Residual sugars, and starch in the case of maize and whole-crop cereal silages, are probably most important in affecting the degree of instability in air. In wilted silages, sugars are most likely to remain after the storage period because the fermentation is restricted by lack of available water for the bacteria. Silage which is sticky when squeezed in the hand is evidence of a high content of residual sugars.

Grass silages with high residual sugars have relatively higher proportions of their total ME as FME than is the case with extensively fermented silages. The main difference between a cluster 4 silage – the best for intake – and a cluster 3 – well-preserved but not as high intake – in the SAC classification of silages is that the content of residual sugars is usually much higher in cluster 4 silages than in cluster 3 silages. Thus it is highly desirable to achieve a cluster 4 silage if high intake is the objective, but unless the feed-out process is rapid and air prevented from getting into the silo, loss of nutrients due to spoilage after the silo is opened may result in a significant reduction in feeding value.

Silo management is therefore crucial at filling, when short chopping and careful consolidation of thin layers can help to reduce the growth of undesirable aerobic micro-organisms; it is also vital at the feed-out stage when a smooth face can assist in restricting the penetration of air into the mass of silage in the silo.

To assess whether or not the shear grab or loader is doing a proper job, dig into the exposed silo face for about 0.5m (1.6ft). If the silage gets progressively hotter as you move into the silage mass, then you know spoilage is occurring with significant nutrient loss.

Spoilage cannot be cured at this stage – it can only be reduced by speedier feed-out and more attention to keeping the exposed face of the silo as undisturbed and as smooth as possible.

Additives can be used at harvest to restrict heating and moulding during feed-out. Salts such as sodium sulphite or calcium formate, and acids such as propionic or octanoic can be used to delay the deterioration process. There is now a re-emergence of additives which have been in use for many years, but which are now specially marketed to be used with silages such as maize, whole-crop wheat and high dry matter grass silage – with the objective of controlling aerobic spoilage during feed-out.

Inoculants are also available which are specifically recommended for higher dry matter crops such as those commonly ensiled in big bales, and which are claimed to restrict the development of moulds and undesirable aerobic bacteria, such as listeria, in the ensiled crop.

But additives are no substitute for poor silo management, and should ideally be used only to solve a difficulty which cannot otherwise be addressed by paying attention to good silo management. An example would be when feeding out unstable silage in hot weather when the deterioration process is accelerated by high ambient temperatures.

Above all, tight consolidation is a must with all high dry matter crops, and should be the major objective when chopping, loading and removing the silage from the silo.n

Tight consolidation and thin layers at silo filling are crucial to successful preservation of

wilted silage, says Professor Mike Wilkinson.


Good management involves:

&#8226 Short chopping.

&#8226 Thin layers at silo filling.

&#8226 Tight consolidation.

&#8226 Smooth face at feed-out.

&#8226 Additives can control spoilage.