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What makes silage special

October 2016

Maize silage is an important source of forage feed for the dairy and beef finishing industry. Several characteristics of maize silage make it attractive to many livestock farmers. It is palatable forage with relatively consistent quality and higher yields and energy content than most other forages.

Silage is the end product of fermenting a high moisture crop (40% - 80% water) and storing the product is called ensiling. Ensiling fodder has been around a long time and now contributes over 50% of the nutrients for beef and dairy cattle production.

Silage has several advantages over hay as a mechanically harvested product. Silage has more nutrients preserved per hectare because there is less field loss. Silage is also less affected by weather damage because the forage does not lie in the field drying.

The main objective of silage is to conserve the digestible fibre, protein and energy in the forage, and to maintain the protein in a form that can be utilised efficiently by the ruminant animal. As with haymaking, choosing when to harvest the forage plants directly influences the quality of the forage to be preserved as silage. Higher yields can be reaped when plants mature, but quality decreases, so balance the desired yield with harvesting the forage when the plants contain excellent feed value.

Good ensiling creates an oxygen-free environment to stimulate lactic acid bacteria growth and prevent growth of moulds and many types of yeast. Fermentation is bacterial growth without oxygen. Lactic acid bacteria use sugars within the plant to produce organic acids which lower the pH from 6,0 to 3,8 - 5,0, depending on the forage species. Lower pH measurements result in restricted cell growth and enzyme activity. So ensiling involves preserving forage, excluding oxygen, and reducing the pH quickly through bacterial fermentation. There are three main players at work within the silo. Plants are undergoing changes, microbes are busy at work, and chemicals are reacting. Plant processes include respiration for a while, cell breakdown (lysis), protein breakdown (proteolysis), and carbohydrate degradation to sugars. Microbial players include yeasts, moulds, acetic acid bacteria and bacilli with lactic acid bacteria and clostridia as the principal anaerobes.

Chemically, there are browning reactions and acid hydrolysis of hemicellulose. Forage crops are best preserved within an oxygen-free (anaerobic) environment with a low pH (5,5 - 6,0). The oxygen-free environment stops the growth of moulds and yeasts, prevents the respiration of plant material, and promotes the growth of lactic acid bacteria. The lactic acid is needed to ferment sugars in the crop which produce lactic, acetic and other acids. The acids lower the pH which prevents the growth of undesirable anaerobic bacteria and inhibits plant enzymes that break down proteins which is good for maintaining feed value.

The forage inside the silo goes through four stages: Pre-seal, active fermentation, stable phase and feedout. Pre-seal describes the filling of the silo when oxygen is available and actively affecting the plants. This is a period of heavy losses in nutritive value of the forage crop. So filling should be done quickly and air should be pushed out with compaction. Once the silo is filled, the remaining oxygen is quickly used up and active fermentation begins. This 1 - 4 week period sees growth of lactic acid bacteria and the lowering of pH. When the lactic acid bacteria use all the plant sugars, the stable (infiltration) phase begins. During this phase little biological activity occurs but some oxygen can creep into the silo and cause some yeast, mould, and anaerobic bacteria growth. When the silo is opened, at feedout, the silage at the opening is exposed to unlimited oxygen resulting in rapid growth of aerobic micro-organisms and a build-up of heat. Caution should be taken in opening a silo.

Many additives are available to promote fermentation and preservation or inhibit detrimental processes within the silo. Additives that promote lactic acid bacteria are stimulants. Inhibitors slow down undesirable activity.

Silage additives are added during wilting or storage. At first molasses were added, then urea to promote lactic acid bacteria growth. More recently bacterial inoculants are used to help natural lactic acid bacteria and lower pH. These additives work best when natural lactic acid bacteria is low as in grass and legume silage. Enzymes are also available to break down forage fibre. Ammonia and acid products are commonly used as well. Dry matter loss can be reduced with the addition of additives but many involve some danger to human health. A firm understanding of the ensiling process is needed for a producer to wisely decide on using additives.

Recognition is given to the National Forage and Grassland Curriculum, Oregon State University, 2008.

Article submitted by Grant Pringle, Agronomist, Pannar Seed.
For more information, send an email to Grant.pringle@pannar.co.za.


Publication: October 2016

Section: Pula/Imvula