Broschüre 2004 zum Download (pdf | 1994,28 KB) - H. Wilhelm ...

additives, bacterial cultures in particular, are the most
popular type of additive in almost all countries, including
The Netherlands and Germany.
Maize is the second most popular silage crop in
The Netherlands. About 3 million tonnes of maize is
conserved as whole crop maize silage annually. Germany
and France are responsible for more than 50 %
of the maize silage production in Europe (Figure 1).
The silage fermentation process
The main principles of preservation by ensilage
are a rapid achievement of a low pH by lactic acid
fermentation and the maintenance of anaerobic conditions.
The ensiling process can be divided into four
phases (3). The first phase, the initial aerobic phase,
usually takes only a few hours as atmospheric oxygen
trapped in the ensiled mass is rapidly reduced due to
the respiratory activity of plant material and (facultative)
aerobic micro-organisms. The second phase, the
fermentation or acidification phase, starts when the
ensiled mass has become anaerobic, and continues
for several days to several months, depending on
crop properties and ensiling conditions. During this
phase different groups of micro-organisms capable of
anaerobic growth (lactic acid bacteria, Enterobacteriaceae,
clostridia, and yeasts) compete for available
nutrients. In well-preserved silage, lactic acid bacteria
rapidly dominate the fermentation, the result being a
decrease in pH due to the accumulation of lactic acid
and, to a lesser extent, acetic acid formed from sugars.
The third phase, the storage phase, usually lasts
several weeks to a year or longer. As long as the pH
is sufficiently low and penetration of air into the silage
is excluded, relatively little occurs during this phase.
Some acid-tolerant micro-organisms (e.g. some yeast
species) survive this period in an almost inactive state,
others, such as clostridia and bacilli, survive as spores.
The fourth phase, the feed-out or aerobic deterioration
phase, starts when the silage becomes exposed to air.
Silage quality problems
Silage quality problems can be divided in two types,
anaerobic instability and aerobic instability (3). Anaerobic
instability results from insufficient acidification
and generally relates to crop composition (e.g.
concentrations of dry matter, fermentable sugars and
nitrate and buffering capacity). Lactic acid-consuming
clostridia, Clostridium tyrobutyricum in particular, are
the main spoilage causing micro-organisms associated
with this type of quality problem. A typical »clostridial
silage« is characterized by a high pH and high
levels of butyric acid, ammonia and amines. Aerobic
instability results from exposure of the silage to O 2 ,
causing proliferation of different groups of undesirable
micro-organisms. Acid-tolerant yeasts, capable of
oxidizing the preservative acids in silage, are recognized
as the most important group responsible for the
onset of aerobic instability. As the oxidation reactions
proceeds, the pH rises and other undesirable microorganisms,
such as moulds and bacilli, start to proliferate
too. Exposure to air is inevitable after the silo
is opened, but aerobic deterioration processes often
start already during the storage phase, for instance
due to damage to the silage plastic and because silage
plastics are not completely airtight. The extent of O 2
penetration depends mainly on the porosity and density
of the material and the rate of silage removal.
Clostridia spores in silage:
the current situation in The Netherlands
For many years, the risk of growth of clostridia in
silage, C. tyrobutyricum in particular, was the main driver
for research efforts and technological innovations
in the field of silage making. Clostridial fermentation
Figure 2: Example of late-blowing of Gouda cheese (right).
77