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Silage: What causes a bad fermentation?

Was your last year’s silage on the nose? If it was, then your silage probably underwent a poor or "bad" fermentation. Any one of a number of silage making practices, if done incorrectly, can result in an undesirable fermentation.

By Department of Primary Industries - 6th October 2005 - Back to News

Understanding what causes a "bad" fermentation can alert you to ways to avoid this in future. Often only small management changes are needed to produce a good fermentation compared to a bad fermentation. The benefits are well worth the effort.

To achieve a desirable lactic acid fermentation, we need anaerobic (no air) conditions in the stack or bale, plenty of plant sugars in the freshly mown crop, the right numbers of the right types of bacteria (lactic acid-producing bacteria) and correct dry matter (DM) content for ensiling.

So what can go wrong?

Too much oxygen
Immediately after cutting plant respiration continues to convert plant sugars to carbon dioxide, water and heat. This results in dry matter and quality (energy and protein) losses. Once the forage is in the stack or bale, the available oxygen allows this to continue, resulting in losses of plant sugars. These plant sugars are needed by the desirable lactic acid-producing bacteria to produce lactic acid, which causes the drop in pH, ie increased acidity in the ensiled crop thereby "pickling" or preserving the mown material. Less sugars, less acid, less desirable fermentation.

Prolonged respiration due to excessive oxygen not only depletes plant sugars, but the heat generated can limit the activity of the lactic acid bacteria. If too hot (35 – 40°C and above), the silage may undergo browning or caramelisation where the plant protein binds to the carbohydrates fraction of the forage. This silage will smell sweet and animals love it but its nutritive value is substantially reduced depending on the silage DM and final temperature reached.

The ideal temperature for activity of the desirable lactic acid bacteria is about 27 - 38°C but excessive air in the compacted forage will result in the temperature rising well above 38°C. In addition, this excessive heat encourages the unwanted growth of undesirable fermentation bacteria, yeasts and moulds.

To worsen matters, this excessive heat increases the rate that plant enzymes (proteases) convert crude protein to soluble protein such as ammonia, nitrites, nitrates, free amino acids amines, etc. All the above result in a poorer fermentation and lost quality and quantity.

Low plant sugar levels
If pH is not lowered rapidly in the early stages of the fermentation, undesirable bacteria and yeast will compete with the lactic acid bacteria, and reduce the likelihood of quickly reaching a "pickled" or stable state.

Bacteria need available plant sugars to produce lactic acid. If these "run out" during fermentation, lactic acid production stops. This may result in a final pH that is too high to restrict the growth of spoilage organisms such as yeasts. Factors such as crop DM, buffering capacity and the amount of plant sugar available affect the quality of fermentation.

Low plant sugar levels in forage in practice are mainly caused by an excessive wilting period, being too wet at ensiling, have also undergone a long fermentation period or be a leguminous crop.

Crops which have been mown but experienced a prolonged wilting period due to rain, mechanical breakdown, etc will have continued respiration and other microbial activity resulting in low levels of plant sugars. In a wet forage, a lower pH than normal is needed to restrict the undesirable bacteria - so even more sugar must be available for conversion to acid. Legumes such as lucerne or balansa clover, have a higher natural buffering capacity due to a higher concentration of organic acids and is associated with a slower acidification rate than say, maize or pasture. The combination of a naturally low sugar content and high buffering capacity means that legumes are particularly prone to a poor fermentation. However, if well wilted, most legumes can produce a high quality and palatable silage.

Butyric acid silage
Forage with low sugar levels and ensiled below 28 – 30% DM in stack silage or below about 35% DM in bales, are prone to a very poor fermentation caused mainly by Clostridia species of bacteria.

Clostridia bacteria will produce butyric acid and other undesirable products from protein breakdown such as ammonia and amines. These bacteria live in manure and soil and grow in silage when oxygen is absent. They typically multiply in silage after most of the acetic and lactic acid bacteria stop growing. These bacteria consume plant proteins and any remaining plant sugars, as well as the lactic and other organic acids produced so far in the previous fermentation stages.

Crops that have been cut and rain affected or experienced a prolonged wilting period may be susceptible to this type of fermentation. A clostridial fermented silage will have a pH above 5.0, high ammonia-nitrogen levels, more butyric acid than lactic acid and a strong unpleasant odour. Some clostridia may produce toxins. Cattle typically eat less or go off feed completely and produce less.

Fortunately, this type of silage is not very common in Australian conditions.

Practical tips

1. Wilt, harvest and seal quickly (within 24 – 48 hours).
2. Ensure wilting of pastures in the range 30% - 35% DM for loader wagon length silage, 30% - 40% DM for precision chopped stack silage or 40% - 55% DM in baled silage.
3. Wilt legume crops to higher end of recommended ranges.
4. Ensure short chop length in drier forages.
5. Ensure dense compaction, greater than 225kg DM per cubic metre in the stack.
6. Use an inoculant if ensiled below the desirable DM levels and not on ground past 2 –4 days.
7. If wilting is difficult or rain is threatening, consider harvesting early, but include an inoculant or another suitable additive from a reputable company.
8. Seal airtight and weight down plastic on stacks.
9. Maintain airtight seal.

Source: http://gippsland.com/

Published by: news@gippsland.com

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