Discussion of Silage Fermentation and Quality of Preservation
Instructions: Please answer the following questions as thoughtfully, accurately and completely as you can.
1. Using your understanding of the processes involved in silage fermentation, explain the change the composition of the three forages as illustrated in Table 1 above
When “good fermentation occurs in a silo, organic acids, primarily in the form of lactic acid, is being formed during the fermentation phase at the expense of the NFC (primarily starch). This acid production reduces the pH of the silo. When pH is sufficiently low the stabilization phase has been reached, bacterial activity stops and no further degradation of the feed occurs.
Given the information provided in the table, the three main criteria one can use to evaluate quality of fermentation include:
a. Large drop in pH (the lower the end-point pH, the better);
b. The higher the proportion of lactic acid in total organic acid, the better the quality of fermentation;
c. The lower the disappearance of NFC the better the quality of preservation.
Note that the formation of ammonia in the silo and change in NDF are secondary criteria (see more below).
2. How would you rank the quality of preservation of each of the three silages presented in Table 1?
a) Corn silage:The fact that corn silage shows the highest drop in pH with the least amount of fermentation of NFC into lactic acid is an indicator of how much more easy it is ensile corn compared with alfalfa or grasses. Notice also, that percent of lactic acid 4.6% in total organic acid (6.7%), that is 68% is the highest value of the silage presented here. The higher this percent the better is the quality of preservation of a silage.
b) Alfalfa silage: Overall the quality of preservation of this silage is lower than for corn silage (but higher than for the grass, legume presented below) because:a. Lower drop in pH;
b. Larger disappearance of NFC (26.4 - 14.8) = 11.6 versus 38.3 - 32.0 = 6.3 for corn silage;
c. Lower percentage of lactic acid in total organic acid (63% vs 68% for corn silage).
d. Additionally note that after ensiling, organic acids level is much higher in alfalfa than in corn because of higher buffering capacity of alfalfa. In spite of a sharper drop in NFC and higher level of lactic acid appearance, the change in pH is much lower in alfalfa silage compared with corn silage. c) grass / legume mix:
This silage has a very poor quality of preservation because:
a. Although the magnitude of drop in pH is unknown, the final pH is the highest of the three silages;
b. Almost all of the initial NFC has been fermented;
c. The proportion of lactic acid in total organic acid is the lowest of the three silages (59%)
3. How do you explain the increase in CP and NDF? in the grass /legume in the silage compared to the fresh material entering the silo?
The amount of NDF and CP must have remained the same. Silage fermentation does not “make NDF or CP. Thus the increase in the percentage of NDF and the percentage of CP reflect a loss of the total DM (used as the denominator in calculating the percentage). This loss of DM is in the form of fermentation losses of fermentable carbohydrate in the form of heat, water and volatile compounds)
4. If butyric acid is a volatile fatty acid (VFA) commonly found in the rumen of a cow, than why is it bad to find butyric acid in a silage?
Because butyric acid in a silo is an indication of poor and undesirable type of fermentation (clostridial instead of lactic type fermentation) that may contribute to ketosis because of excess level of butyric acid in the blood of the cow. On the other hand, butyric acid is a normal end-product of carbohydrate fermentation in the rumen coming primarily from the fermentation of fibrous feeds.
5. What are the main drawbacks of clostridial fermentation?
Clostridial fermentation is associated with high protein degradation (which may not be for the best!) which may results in the production of amines, which in turn may have a negative effect on dry matter intake.
6. What are similarities and differences between silage fermentation and rumen fermentation? Think of as many as you can.
|
Similarities/Differences |
Rumen |
Silo |
|
Both are based on
fermentation of carbohydrates such as starch |
Desired end-product are
VFAs, primarily acetic and propionic acid (with some butyric acid). Little to
no ethanol production |
Desired end-products is
lactic acid. This strong acid will reduce pH rapidly. Ethanol may be produced
in large amount |
|
|
Butyric acid is an
expected product of fermentation |
Butyric acid is an
undesired end product of fermentation. It is a sign of poor quality of
fermentation. |
|
|
No worry about yeast |
Yeast may cause loss of
nutritive value |
|
|
NDF is fermented into VFA |
NDF is NOT fermented |
|
Fermentation lowers the pH |
Saliva and other
mechanisms (absorption of VFAs) help keep pH close to neutrality (6.0 – 6.5). |
There are no mechanisms
other than the normal “buffering” capacity of the fermenting material, to
resist drop in pH. Final pH about 4.0 -4.5 is desired. |
|
Both takes place in
anaerobic conditions |
Time of fermentation is in
the order of hours (a day or two) |
Time of fermentation goes
on for weeks until the stable phase is reached. |
|
Major bacteria differ |
Mix of fiber fermentors
and lactic acid bacteria. |
Lactic acid bacteria only. |
|
Protein degradation differ |
Extensive protein
degradation in the rumen |
Extensive protein
degradation is a sign of “poor quality” of fermentation |
|
|
High degree of mixing |
No mixing. |
|
Particle size may
influence fermentation outcomes |
Short particle size
increase nutrient availability, may too short is not desirable and it may
cause acidosis |
Short particle size
increase nutrient availability and likelihood of good fermentation as it also
improve packing of the feed in the silo. |