Grass Silage


Grass silage is becoming more of a “planned” crop rather than a method employed to capture surplus at peak growth. The value of the silage to an animal is dependent on the amount it eats, therefore the need to put the best quality in front of it.

With storage, fermentation develops, producing acids, which help to preserve the crop. This process occurs quickly and most of the changes occur within three to four days at which point the silage could be fed but it is common practice to allow the silage to mature for at least two to three weeks post ensiling. Silage stored in well sealed pits may last several years. Baled silage does not share the same luxury due to the action of sunlight on the plastic.

Dietary value of silage

The contribution silage makes to an animals diet is dependent on two factors:

  • Feeding value of the silage
  • The amount which that animal can eat.

The nutritive value of the silage depends on the quality of the crop ensiled and the changes that occur during the ensiling process. Of prime importance is the metabolisable energy (ME) and protein.

Factors controlling energy

Maturity. As the plant matures digestibility falls and ME (Mj/Kg DM) value falls

Respiration. After cutting, respiration continues resulting in a loss of sugar and a proportionate increase in the less digestible component of the plant, consequently lowering the digestibility and ME value. Wilting for too long a period can lead to a reduction of up to 0,5 Mj of ME/Kg DM.

Respiration continues even after the crop has been put into the pit when a further loss of sugars results from the growth of aerobic micro – organisms. These flourish in the presence of oxygen, especially chopped material and lead to wasteful production of heat.

Less heat is generated when the crop is ensiled quickly due to more rapid exclusion of oxygen and the resulting loss in energy is small. When the temperature rises appreciably, digestibility can be severely depressed and NE value reduced by as much as 0.3 – 1.0 Mj/Kg DM.

Primary fermentation

Once the oxygen has been exhausted, the crop is subject to anaerobic micro – organisms. These ferment carbohydrates to organic acids (mainly acetic and lactic), to alcohols and carbon dioxide. The alcohols and acids make a significant contribution to the energy of the silage because their high energy value relative to the sugars from which they were formed.

Secondary fermentation

When primary fermentation fails to produce sufficient acid to stabilize the crop, a secondary fermentation takes place which results in the loss of lactic acid and production of butyric acid resulting in the reduction of digestibility offsetting the increase in energy content resulting from the primary fermentation.

The ME value of silage may be higher or lower than the original crop. This depends on the balance between the losses between DM and nutrients arising from respiration and fermentation, and the enhanced energy values of the alcohols and acids compared with the sugars from which they were formed.

Chemical composition

Silage’s of similar ME value can have widely differing chemical compositions, depending on the quality of the original crop and the type of fermentation that took place. A mature crop quickly cut and ensiled with good lactic acid fermentation may have the same value as young leafy crops poorly preserved.

Factors controlling protein value of silage

Protein in grass

The crude protein (CP) in herbage is very variable and depends on species, fertilizer treatment and stage of growth. Most grass silage crops vary between 10 – 12% CP in the DM and declines with maturity of the crop. When freshly cut, about 80% of this is true protein the rest is non protein nitrogen (NPN) components such as amino acids and nitrate. At cutting there is no ammonia or ammonium compounds.

Protein breakdown

During wilting, the CP remains fairly constant but the NPN portion increases due to action of plant enzymes with very little ammonia production at this stage. Once the crop is put into the pit/bag fermentation leads to a rapid and extensive breakdown of protein to amino acids and ammonia. Protein breakdown continues until stable acid conditions are reached.

NPN content

Even in a well fermented silage, no more than 40% of the original CP is present as true protein. Of the 60% changed to NPN, some 5 – 10% is present as ammonia.

Protein content

The CP of silage can be higher than the original crop because the loss of carbohydrate is greater then that of protein in the ensiling process.


The value of silage to an animal depends on the amount it can eat. Factors affecting intake and quality have been discussed allowing for the best quality grass silage to be made and maximize on intake.

Date published: 2007-07-10