Fermentation analyses are now available for evaluating silage
quality. An analysis
report will include pH, lactic, acetic, propionic and butyric
acids, ammonia and ethanol. The purpose of this information is to
understand and to be able to interpret correctly the fermentation of
silage. The most commonly asked
question may be what do these reports tell us about the fermentation
process and silage quality? Did the data tell us whether an excellent,
average or poor fermentation occurs? Next, lets direct our attention to
the various crop factors that will affect the rate and degree of
fermentation. They are moisture content, buffering capacity, and sugar
content. Management practices also play an important role; these include
silo packing speed, silage pack density, chop length, silo management
during storage, and silo management during feed out.
These reports cannot be used to balance the dietary needs of an animal,
however they can be used to determine whether silage has poor nutritive
value or low feed intake. Always use this report in conjunction with a
nutritive analysis of the silage. How should
samples be taken and shipped?
If your goal is to evaluate what animals are being fed, then samples
should be taken at time of feeding.
If your goal is to see what type of fermentation the silage went though
then collect the sample as fresh as possible, making sure that it arrives
at our laboratory as quickly as possible.
Please collect about a pound of material to insure a good representative
sample has been harvested. pH
The pH of an ensiled sample is the measure of its acidity, but is also
affected by its buffering capacity. Two samples may have the same pH,
however have different concentrations of acids. In general, legume
(alfalfa) have a higher pH than grass and take longer to ensile because of
their high buffering capacity.
Corn silage has a mean pH of 4.2. Corn silage samples that have a higher
pH of 4.6 can be associated with extremely dry conditions (>42% dry
matter). Some common reasons for legume silages having a pH higher than
4.6 - 4.8 include ensiling at <30% dry matter which causes clostridial
fermentation, and ensiling at> 45 -50% DM, which restricts
fermentation. In the first example, a high pH is a definite indicator of
an undesirable fermentation that has led to poor quality silage. The
second example however restricts the fermentation but does not necessarily
point to poor quality silage, instead points to a more unstable
fermentation. Once the silage is exposed to the air spoilage can occur due
to the insufficient amount of acids being produced. Lactic Acid
Lactic acid should the primary acid produced in good silage.
This acid is stronger than other acids in silage and is primarily
responsible for the drop in silage pH. Also, it is recognized that
fermentation's that produce lactic acid result in lower losses in DM and
energy from the crop during storage. Low acetic acid
concentration may be the result of the following:
1) restricted fermentation due to high DM content
2) cold weather 3) silages
high in butyric acid (Clostridial silages) are usually low in acetic acid
High lactic acid High lactic acid concentration may
be the result of the following:
1) wet silage(<25% DM) 2)
prolonged fermentation (due to high buffering capacity)
3) slow silo filling can result in silages with high concentrations of
acetic acid 4) silage
treated with ammonia, because the fermentation is prolonged by the ammonia
raising the pH. Propionic Acid
Most silage contains very low concentrations of propionic acid (<0.2 to
0.3%) unless the silage is very wet (<25% DM). In silages with more
typical concentrations of DM (35 to 45% DM), concentrations of propionic
acid is undetectable. Butyric Acid
Appropriate levels of butyric acid (<0.5% of DM) indicates again that
the silage has gone through a normal fermentation. However, butyric acid
(>0.5% DM) indicates that the silage has undergone a clostridial
fermentation, this is one of the poorest fermentations. These silages
usually hold low nutritive value and have higher ADF and NDF levels
because many of the soluble nutrients have degraded. Ammonia
High concentrations of ammonia (>12 to 15% of CP) are a result of
excessive protein breakdown in the silo due to a slower than normal drop
in pH or clostridial action. In general, wetter silages have higher
concentrations of ammonia than ones do that are not. Extremely wet silage
(<30% DM) have even higher levels of ammonia concentrations and pose
potential problems. Also please note that silage packed to loosely and or
filled too slowly will tend to have higher than normal ammonia levels.
In theory, high amounts of ammonia by itself should not have negative
impact on animals if the total nitrogen dietary fractions are balanced. If
they are not negative impact may be felt causing loss in milk production
and an animals reproductive performance.
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