Silage Fermentation Analysis
by Dr. Clay Zimmerman
There have been a number of articles written recently concerning the evaluation of silages for fermentation quality and I have also received a few questions about this topic. A few of the labs in our geography are running these tests and among those labs are Cumberland Valley Analytical Services in Hagerstown, MD and Dairy One in Ithaca, NY. Costs for this test alone range from $18-25 per sample. These analyses tell us in general whether excellent, average or poor silage fermentation has occurred. What specifically are they testing for?
- pH
- Lactic acid
- Acetic acid
- Propionic acid
- Butyric acid
- Ethanol
- Ammonia-N, % of CP
- Lactic:Acetic Ratio
Table 1 lists the common levels of these fermentation end products that are found in various types of silages. Our goals for these silages would be to have pH's below the maximums listed in the table, lactic acid levels should be higher than the listed minimums, acetic acid levels should be below the maximum values in the table, propionic levels should be below the levels in the chart unless a propionic-based preservative is used on the silage, butyric acid levels should be below the minimums listed, ethanol levels should be towards the lower side of the listed values, and ammonia-N levels should be towards the lower end of these values.
| Item | Alfalfa Silage 35% DM | Alfalfa Silage 55% DM | Grass Silage 25-35% DM | Corn Silage 35-40% DM | Hi-Moist Corn 75% DM |
|---|---|---|---|---|---|
| pH | 4.3-4.5 | 4.7-5.0 | 4.3-4.7 | 3.7-4.2 | 4.0-4.5 |
| Lactic acid, % | 7-8 | 2-4 | 6-10 | 4-7 | 0.5-2.0 |
| Acetic acid, % | 2-3 | 0.5-2.0 | 1-3 | 1-3 | <0.5 |
| Propionic acid, % | <0.5 | <0.1 | <0.1 | <0.1 | <0.1 |
| Butyric acid, % | <0.5 | 0 | <0.5 | 0 | 0 |
| Ethanol, % | 0.5-1.0 | 0.5 | 0.5-1.0 | 1-3 | 0.2-2.0 |
| Ammonia-N, % of CP | 10-15 | <12 | 8-12 | 5-7 | <10 |
What do these tests tell us?
1) pH - Has limited diagnostic value by itself. pH tells us if the final pH was adequate, but doesn't indicate the total amount of acids produced, if the acids were produced quickly enough, or if we have desirable acids present. Corn silages, in particular, will almost always reach a low pH, but how long did it take to get there? The key to high quality silage is getting a rapid pH drop. If corn silage has a high pH (>4.2), it is usually due to high dry matter silage that is overly mature and drought stricken. Legume silages may have high pH (>4.6) if low in dry matter (<30% DM) which allows for Clostridial fermentation, or if high in dry matter (>45% DM) which leads to a restricted amount of fermentation. pH may also be low if sampled too early before fermentation is complete, silages are ensiled during cold weather, or if high in ash or manure contents. Some of the labs will test for titratable acidity as well as pH. Titratable acidity is a rough measure of the total acid levels in corn silages and high moisture corn. It is not as accurate for haylages.
2) Lactic acid - The single most important indicator of a good fermentation. This acid will dominate in a good fermentation. Usually responsible for most of the drop in silage pH. Low lactic acid may be due to:
- Restricted fermentation due to dry silage
- Ensiling during cold weather
- Sampling after air exposure has degraded lactic acid
- Silages high in butyric acid
At least 65-70% of total silage acids in good silage should be lactic. In contrast to the volatile fatty acids (VFA's) which follow in this list, lactic acid has a bland odor and does not volatilize when exposed to air.
3) Acetic acid - Often produced if lactic acid production is not rapid enough to inhibit acetic acid production by bacteria. High levels (>3%) suggest inefficient silage fermentation. Very high levels (>5%) may decrease dry matter intake in dairy cattle. High acetic acid silage can be due to:
- Wet silages (<25% DM)
- Prolonged fermentations
- Loose packing
- Slow silo filling
- Silages treated with ammonia (fermentation is slowed by ammonia)
Silages treated with L. buchneri inoculants have increased acetic acid content, but these high acetic levels don't hurt intake. How do you deal with silage that is high in acetic acid? You can try feeding less of the silage to see if it helps dry matter intake. You can also try pulling silage off of the face a day ahead of time and letting it aerate to see if it reduces the acetic acid content.
4) Propionic acid - Most silages contain very low levels (<0.2 to 0.3%) unless the silage is very wet (<25% DM). Propionic acid has a sharp, sweet smell and taste. Propionibacteria inoculants can be added to silages to increase propionic acid concentrations and increase bunk life of silages. Some people also add propionic acid to silages to reduce molds and increase bunk life. Addition of propionic acid to silages should bring the final propionic acid level in silage to 0.15 to 0.30%.
5) Butyric acid - High concentrations are more common in haylages than corn silage. High butyric can result from Clostridial fermentation when low energy forages are ensiled too wet. High butyric acid silages are associated with decreased dry matter intake in dairy cattle and can be related to increased ketosis in herds since butyric acid is a precursor of one of the ketone bodies, beta-hydroxybutyrate. Butyric acid has a very strong, noticeable odor (see Table 2) and levels above 0.5% indicate Clostridial fermentation.
Avoid feeding high butyric acid silages if at all possible. If they must be fed, let them air out for a day or so before feeding. These silages are very stable, so heating and molding will not be an issue.
6) Ethanol - Produced by yeast fermentation and results in an alcohol smell. This is a very inefficient fermentation. Yeast will proliferate when exposed to air, therefore inadequate packing can increase ethanol production in silages. Bunk life of silages high in ethanol is often decreased. Extremely high ethanol (>3-4%) may cause off-flavors in milk. Most common in corn silage or high moisture corn.
7) Ammonia N as % of CP - High levels (>12-15% of CP) indicate that extensive protein degradation has occurred. Can occur in silages stored too wet. Silages packed too loosely or filled too slowly also have higher ammonia. Silages with high ammonia-N levels may indicate high levels of soluble protein. Often silages with high ammonia and high butyric acid also are high in amines, which are very undesirable to cows.
8) Lactic:Acetic ratio - An indicator of fermentation efficiency. A low ratio indicates that lactic-acid-producing bacteria did not dominate the fermentation and that lactic acid production may not have been rapid enough. This can result in increased loss of dry matter during ensiling. Consider the use of an inoculant containing lactic-acid-producing bacteria such as our Silage Supreme.
How Do You Use the Values?
If used, fermentation analyses should always be interpreted in conjunction with a normal chemical analysis of a silage. Fermentation analysis is a diagnostic tool, used primarily for troubleshooting to help elucidate one or more of the following points:
- May explain poor dry matter intakes
- May explain poor performance
- Can be a good tool to sell silage inoculant. Be careful using it for this purpose. There are many management factors that contribute to silage quality, so even properly inoculated silages can have poor fermentation profiles. Use of a high quality inoculant will increase your rate of fermentation success, but it does not guarantee it.
- Can demonstrate poor silage harvesting or management
Remember, problem silages can be hard to deal with. You generally have to feed the silages and there is usually a large amount of poor quality silage with which to deal.
Also, don't forget the importance of being able to recognize problem silage odors as listed in Table 2.
| Silage smell | Management Fix |
|---|---|
| a) Vinegar - acetic acid | Use a microbial inoculant next season. |
| b) Rancid, fishy - butyric acid | Check silage DM. Wilt to >30% DM next cutting. Use an inoculant. |
| c) Alcohol - ethanol | Use microbial inoculant or prop acid-based preservative next crop. |
| d) Moldy, musty | Increase your feed-out rate. Use inoculant next time. |
| e) Tobacco, burnt smell | Check silage DM. Lower your silage DM. Check for silo leaks or poor packing. |
I am certainly not recommending that fermentation analyses be run on a routine basis because they are costly and often unnecessary, but there are occasions where this information can be helpful. When sampling these silages, obtain a fresh sample by digging into the face 8 to 10 inches. Do not just sample off of the face since it has been exposed to air. Cool the sample immediately and ship to the lab as soon as possible.
Keep in mind that moisture content of the various silages is critical to high quality silage and can have a dramatic impact on the fermentation analysis. The number one thing that we need to stress to our dairies is that they harvest silages at correct moisture levels. A moisture error in either direction (either too wet or too dry) can lead to a poor fermentation. Corn silage should be harvested at 60-72% moisture and haylages should be harvested at 50-70% moisture depending upon the type of storage structure. Remember the 4 keys to good quality silage: 1) harvest at the correct moisture; 2) use a good quality inoculant; 3) fill rapidly and pack well; and 4) cover the silage as soon as possible after filling. If these 4 keys are followed, we have a much greater likelihood of having a favorable fermentation analysis.
© Blue Seal Feeds, Inc. – February, 2002