Tuesday, 11 December 2012

The value, quality and preservation of silage

(c) ukagriculture.com
Silage means forage, which is made of grass or other organic material, and has been preserved with methods based on acidity. Preservation, ensiling or silaging is the process where the spoilage of the forage is prevented by lowering it's pH either with acid or by adding anaerobic, acid-producing fermentative bacteria.

Silage, like any animal feed, has two values: a feed value and a dietary value. Feed value means the energy content and the nutrients of the feed for the animal. Feed value includes the chemical  composition, digestability and the rate of metabolisation. Dietary value is a wider concept, which includes the factors directly or indirectly affecting the metabolisation rate of the silage. These factors are the hygienic quality and the feed value of the silage, plus the animal's need for nutrients and the amount of feed it eats.

Microbial quality of silage concerns the amount of volatile fatty acids (VFA) and the pH of the feed. pH clearly over 4.2 is a sign of failed preservation, and increases the risk of the feed spoiling. pH can rise if unwanted bacteria proliferate in the silage, fermenting proteins into ammonia. VFAs and a high amount of ammonium nitrogen from the total amount of nitrogen also indicate the existence of unwanted microbes. Sometimes microbes (spores of Clostridium, yeasts, molds) can be determined from the feed directly. It is not, however, a routine analysis.

No animal eats just one type of food, but the values of the feeds no dot add up. Different feeds in a diet affect one another. For example, fats decrease the digestability of roughage, but proteins may increase it. These combined effects must be considered when planning for the indoor feeding season. The aim of feed planning is to maximise the "output response", the amount of the feed the animal can use for it's maintenance and production. Maximising the output response must be made cost-efficiently, and without endangering animal welfare and health.

There are several reasons why knowing the quality and value of the forage is important, be it silage, hay or any other feed:
  • planning a cost-effective and well balanced diet requires knowledge on the feeds
  • giving the right amount of added vitamins etc requires information on the content of the silage
  • weather conditions and preservation methods can change the quality of the silage during storage
  • dairies must be assured that the feed does not endanger the quality of the milk or any milk products made 
  • improving the silage making process is based on knowing the weaknesses and strength of the current process and products


Aspergillus-mold on a maize silage.
(c) North Dakota State University

Preservation of silage

Silage is preserved mainly by three different ways: with acid, with acid-producing bacteria or without any additives. Preservation with acid is usually the most effective method, since it lowers the pH faster than the other methods.When the pH decreases, the plants stop breathing and unwanted bacteria dies. Anaerobic preservation is vital for keeping mold and yeast away from the feed, and for promoting the growth of useful lactic acid bacteria. Low amount of moisture also helps to prevent enterobacteria and Clostridium. Silage contaminated with unwanted microbes can endanger the health of the animals and/or the people handling the feed, and affect the quality of products derived from the animal.

There are roughly five classes of preservatives used in silaging, and they vary by function. Some types prevent others: using strong acid to quickly decrease the pH leads to minimal production of lactic acid.
  1. Promotes fermentation
    - Lactic acid bacteria, enzymes, sugar (molasses)
  2. Prevents fermentation
    - Formic, lactic and mineral acids
    - Nitrite and sulfite salts
    - NaCl
  3. Prevents spoiling in aerobic conditions
    - Propionic acid
    - Benzoe and sorbic acid
    - Lactic acid bacteria
  4. Nutrients
    - Urea, ammonia, minerals
  5. Sorbents (used to absorb the effluent)
    - Straw and beet pulp
 Preservation method may change for silage of different types due to the different buffer capacities of feeds. Grass+legume-silage can resist the decreasing of the pH much better than barley, so different amounts of preservatives are needed. Fermentation factor describes the easiness of preservation for a type of silage. The formula is
dry matter content (%) + 8 * (sugar content/buffer capacity)
A fermentation factor over 45 predicts only a small probability for wrong types of fermentation, i.e. a well-preservable silage.


There are many forms of fermentation, based on which microbes are involved. For silage, fermentation is the process where anaerobic microbes convert sugars (glucose, fructose and saccharose) into lactic acid. The silage must also have enough sugar to maintain fermentation. If the preservation fails, unwanted microbes proliferate, causing faulty fermentation. These microbes produce VFAs, ethanol and especially ammonia, which increases the pH, spoiling the silage even further. Factors affecting the quality of fermentation and thus the whole process of ensiling are
  • chemical composition of the feed
  • microbiological content
  • time of harvest
  • weather conditions during harvest
  • technique of harvest
  • type and amount of preservatives used
  • dry matter content of the feed (wet feed needs more preservatives than pre-dried feed)
Fermentation does not only produce something, it also uses nutrients of the feed. The products of fermentation are always less useful for the animal than the original nutrients used. The level of fermentation should thus be kept optimal for ensuring the preservation of the silage, but also losing as little nutrients as possible. The fermentation in the silage ends naturally when there's enough lactic acid to keep the silage acidic. If the silage doesn't have enough sugar or it's pH is too high, the lactic acid fermentation stops and unwanted fermentation begins. The amount of sugar left in the silage can be used when estimating the level and types of fermentation.

The fermentation process.
(c) Engormix

Types of fermentation
The basic rule is that the drier the forage is, the less there's fermentation. The difference between the types of fermentation are also less apparent in dry forage.
Limited fermentation may result from using acid or other fermentation preventing preservative. pH is low, sugar content is high and there's only little fermentation end products in the silage. Proteins are scarcely dissolved, so there's little ammonia and soluble nitrogen.
Strong lactic acid fermentation occurs in well-pressed silage or when using preservatives promoting fermentation. pH is low due to high amount of lactic acid, there's only little sugar left and proteins are more dissolved than in limited fermentation. Because the fermentation is caused by lactic acid bacteria, there's not much VFAs (propionate acid, butyric acid and acetic acid).
Faulty fermentation is caused by unwanted microbes. The silage has high pH and lots of VFAs. Proteins are strongly dissolved and mostly transformed into ammonia.

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