Pasture Management
for
Horses

Silage Making

The Process Involved

The preservation of grass material by acidification. The principal is the same as pickling onions in vinegar except that in this case the acid is produced by bacteria. The cut material is compressed to squeeze out as much air as possible then sealed. As the plant material is still alive, respiration is continuing and the oxygen present is used creating the required anearobic conditions. Fermentation results in the production of lactic acid from sugars by anaerobic bacteria - Lactobaccilli, Streptococci, Pediococci, to produce the required pH of around 4.0 when the DM is 20%. A higher pH is acceptable for higher dry matter.

It is interesting to note that the majority of bacteria on plants are aerobes - they are normally in aerobic conditions of course. Very few lactoccilli are found on crops, and those will be low down on decaying leaves. Natural inoculation comes from equipment.

Important Factors

The Crop

• Sugar content - sugar is required for the fermenting bacteria.
• Growth stage
• Dry matter - if more water is present, any acid produced will be diluted and the pH will not fall as low. Where there is more water present, a lower pH is infact required to preserve the plant material.
• Buffering capacity of the plant material. Some material, i.e. that from legumes has a high buffering capacity and will therefore require a greater acid production to reduce the pH to a safe level.
• Cutting height - cutting low will increase the yield, but increaces the risk of soil contamination. EXTREMELY IMPORTANT WHEN THE PRODUCT IS TO FED TO HORSES

Wilting - Partial drying of the crop in the field

• Reduces water content resulting in reduced effluent.
• Concentration of sugars promotes lactic acid fermentation.
• It must be remembered that respiration is taking place reducing sugar content and proteolysis (the breakdown of proteins) is taking place

It is necessary to produce anaerobic conditions as quickly as possible by-

• Compaction
• Plastic membrane
• Respiration

Chopping (cutting the material into short lengths)-

• Aids consolidation
• Restricts air movement
• Increases the release of sugars

THE DANGERS

Clostridia - anaerobic gram +ve baccilli (rod shaped bacteria), spore forming, usually motile. This group of bacteria includes those that cause botulism. Though classed as anaerobes (live in an atmosphere devoid of oxygen, some will tollerate low lovels of oxygen.
These are harmful to cattle and especially harmful to horses.

Clostridia are not naturally present on the grass crop but are common in the soil and in animal faeces. Therefore control of cutting height and hygiene are vital. Clostridia will not be active if the pH is sufficiently low (sufficiently acid). If clostridia become active, they will break down lactic acid to form butyric acid. Butyric acid is a weaker acid than lactic acid, therefore this will cause the pH to rise, resulting in more clostridia becoming active and the problem accelerates.
Clostridia will also produce ammonia, and possibly acetic acid, resulting in a foul smell (acetic acid being the acid present in vinegar). Therefore beware of any silage that does not have the normal attractive smell.

Aerobic deterioration

Air leaks will allow aerobic organisms (including moulds) to become active, causing a breakdown of nutrients and the production of mould spores that may cause respiratory problems.

Additives - helpful in difficult conditions

• Sugars - extra sugar in the form of molasses may added to provide a greater food source for the fermentation bacteria.
• Acids - can be added to help reduce the pH level.
• Inoculants - fermentation bacteria may be added.
• Enzymes - may be used to break down polysaccharides to provide more soluble sugar for the fermentation bacteria.

Effluent

The liquid that may seep from a silage clamp, especially if the moisture content of the plant material is high, has a very high BOD (biological oxygen demand) - 200 times that of raw sewerage.

Big Bale Silage

• Longer wilting - 30-40%DM.
• Baled, bagged or wrapped.
• Similar fermentation process.
• pH of 5.0 sufficient due to higher DM.
• More susceptible to aerobic conditions as it is likely to be less compacted than clamp silage.
• Slow ingress of air can lead to development of Listeria.