SILAGE
The amount of silage annually made in Ireland has increased steadily over the
last number of decades. In 2005 950,000 hectares of silage was cut compared to
360,000 in 1976. There are many factors for this increase in silage making and
weather would be one of these factors. Rainfall has seemed to increase over the
last few decades. However research has shown that silage has more nutritional
value than hay and this would have influenced farmers to cut silage instead of
hay.
Principles of Conservation
Conservation of grass for winter feed involves restricting the activity of bacteria
and other micro-organisms that would have otherwise caused spoilage or rotting.
In silage making controlled fermentation is used which involves reducing the pH
of the grass to a level at which all microbial activity ceases, pH of 3.8. – 4.
Fertilizer Application
Nitrogen, Phosphorus and Potassium (N.P.K.) fertilizers should be applied on the
grass. P and K fertilizers should be applied the previous autumn according to a
soil requirement. Nitrogen is never applied in the autumn as it will be leached
out of the soil and will go into rivers and streams causing pollution and killing
fish. Also over the winter plants are not growing so nitrogen is not needed.
Nitrogen should only be applied in Spring (mid to late March) at the rate of
100 – 120kg per hectare for first cut silage and at the rate of 80 – 100kg per
hectare for second or third cut silage. Examples of fertilizers that can be applied
would be 18:6:12 or 10:10:20 or in the autumn 0:7:30.
Clover Sward
The sward being cut for silage should contain some clover as clover has the
ability to fix nitrogen naturally and clover could contribute the equivalent of 5 –
9 bags of nitrogen fertilizer per hectare each year for use by the grasses. On the
roots of clover plants there are Rhizobium bacteria which form nodules on the
roots. The nodules contain large numbers of bacteria which obtain energy in the
form of starch from the root tissue. Some of the nitrogen fixed diffuses through
the walls of the bacterial cells as simple compounds which are absorbed by the
clover plant. This relationship between the clover plant and the Rhizobium
bacteria is known as a symbiotic relationship, as both the clover and Rhizobium
bacteria benefit from each other.
When to cut Silage
Silage should be cut around May 20th or before the grass heads out. Grass
should be cut at the leafy digestible stage ensuring a high quality feed. Grass is
grown for 6 to 8 weeks on average. At the 6 week stage grass will have 70 to
75% dry matter digestibility (DMD) but this will drop to about 65% after 8
weeks of grass growth. DMD drops by 0.5% per day after ear emergence. Leafy
silage contains over 14% protein and stemmy silage less than 10%.
Biochemical Process Involved
When fresh green grass is cut it continues to respire until all the oxygen is used
up. Respiration involves the oxidation of carbohydrates to Carbon dioxide which
means a loss in food value in the vegetation. This is done by rolling the silage
pit with a tractor to ensure anaerobic conditions.
When the oxygen is used up and the environment anaerobic ( no oxygen
present) bacteria present on the vegetation begin to cause fermentation, which is
the conversion of carbohydrate to organic acids. This continues until the pH falls
to a level at which all bacterial activity is inhibited and the material is “pickled”
or preserved. When the concentration of the carbohydrates in the vegetation is
high the bacteria which control the fermentation process are Lactobacillus spp.
The acid produced is lactic acid and lactic acid silage is: palatable (tasty) to
animals, highly nutritious and can be kept for years once air is excluded.
High Levels of Carbohydrates in the Grass can be encouraged by:
1. Do not cut silage during or after rain. Water on the grass reduces
carbohydrate concentration.
2. Allow to wilt for 1 - 2 days after cutting. This increases the carbohydrate
concentration.
3. Use – double chop or precision chop equipment. This does not alter the
concentration of carbohydrates but it makes the grass more accessible to
bacterial action.
4. Use a carbohydrate rich additive e.g. molasses on grass cut early in the
year when it is likely to be lush and sappy.
5. Cut on a dry sunny day when the sun has been shining for a few hours.
Practical Procedures involved in Silage Making
Cutting the Grass: The grass is cut in the field using a mower. The field may
be divided into section when cutting the grass especially if it is a very big field
and there is the likelihood of rainfall. Silage is cut in sward and it is cut from
the outside to the inside. It is important that any stones or ponds or wet areas
in the field are highlighted so the person cutting the grass won’t enter them and
destroy the mower. The grass is left for 1 - 2days to wilt.
Picking up the grass: A forage harvester is used to pick up the cut grass and at
the same time it cuts the grass into small pieces and blows it into a silage
trailer.
Removing the grass from the field to the pit: A silage trailer is used to bring the
cut grass to the silage pit. Silage pits are usually located on a slab on concrete.
Storage
The harvested grass is transported to the silage pit and heaped there. An
additive such as molasses or acids are added at this stage if considered necessary.
Matted lumps of grass are shaken loose and the surface levelled to lessen the
risk of air pockets. The grass in the pit is rolled repeatedly with tractors during
the filling process to compact it and get rid of entrapped air. When all the grass
is in the pit and rolling is completed the pit is sealed with at least two layers of
heavy polythene. This should then be further covered with ground limestone,
farmyard manure, soil or old tyres. This ensures that the upper layers of silage
remain compact and reduces the risk of entry of air through the plastic which is
slightly porous. The edges of the polythene sheeting are pulled tight and weighed
down securely to complete the sealing of the pit. This is repeated once or twice
more as the silage subsides over the following two to three weeks.
Additives
Additives are used when the concentration of carbohydrates in the grass
being ensiled is low. Additives are of two types:
(a)Stimulants - are sources of carbohydrates and their presence ensures that
lactic acid silage is formed. Molasses is the most commonly used stimulant
and has 53% sucrose (sugar) and is applied at the rate of 10litres per tonne.
Molasses is applied onto the grass in the pit, an application being made to
each layer of grass before it is rolled.
(b)Inhibitors - are acids which initially encourage lactobacillus activity. They
lower the pH to a level at which the grass is preserved and all bacterial
activity inhibited. Acids used are Sulphuric acid and Propionic acid . These
are applied at the rates of 3 to 5 litres per tonne. The acids are very
corrosive to machinery and acid should not be added to the last load of
grass cut each day in order to clean out the harvesting machinery.