Assessing Forage Quality

Lessons learned from analysis can help improve future feed-making work.

Published on: Oct 30, 2009

A month or more after the silage or baleage is made we should examine it to determine the quality of the silage to let us know what we could have done better and to know what we will be feeding.

The silage color should be light to medium brown for alfalfa, grass and corn silage. Red Clover silage will tend to be darker.

A good silage (either legume, grass or corn silage) should have a pleasant smell. It tends to be more vinegary in wet silages (more than 65% moisture) and sweeter in dry silages (less than 55% moisture). The latter (often with darker color) indicates excessive heating and loss of TDN and digestible protein. A strong vinegar smell comes from a high acetic acid content, which indicates less desirable fermentation and, possibly, reduced intake that is often associated with wet silage. Butyric acid smell (often in silage made too wet) or the smell of ammonia can indicate reduced quality and reduced animal intake.

The silo, bunker or silo tube should be checked for leakage. In addition to being a pollution problem, the effluent represents a loss of nutrients because the silage was made too wet.

When you see bright orange color around the silo or silage tube, be careful – this is due to nitrogen dioxide (NO2) which is often associated with nitrous oxide (NO), both of which are toxic to humans and animals.

You can get a better idea of silage fermentation by doing a fermentation analysis, available at many forage testing laboratories. These analyses should always be used in conjunction with standard chemical analyses (i.e. NDF, CP, RDP/RUP, NDF digestion, etc.) to balance animal rations.

The common fermentation analyses are pH, lactic, acetic, propionic and butyric acids, ammonia, and ethanol. The normal ranges for these analyses are shown in the table.

Silage acidity is measured as pH with lower values indicating more acid and better fermentation. Corn silages usually have a pH less than 4.2. Legumes have more buffering capacity and generally have slightly higher silage pH. Some common reasons for high silage pH are:

  • overly mature or drought stricken dry silage (>50% DM)
  • sample taken for analysis before silage is fully fermented
  • cold weather during harvest, and slow or poor packing legume silages with high ash ( > 15%of DM) and/or protein content ( > 23-24% CP)
  • silage with excess ammonia or urea
  • clostridial silages/spoiled or moldy silages
  • silages containing manure

High pH due to restricted fermentation (in high dry matter silage) may be good forage but more rapid heating and mold growth will occur when the silage is exposed to air at feeding because less acid is available to inhibit microbial growth.

Lactic acid is the primary acid in good silage; it is stronger than the other acids in silage, causes most of the drop in silage pH and results from fermentation with the least losses of DM and energy. Lactic acid should be at least 65% of the total silage acids. Some common reasons for low lactic acid content are:

  • restricted fermentation due to high DM content (especially legumes and grasses with > 50% DM.
  • restricted fermentation due to cold weather.
  • Clostridial fermentations with high butyric acid production.
  • Silage sample for analysis taken after considerable exposure to air so that lactic acid has degraded.

High acetic acid (greater than 3% to 4% of DM) usually occurs from wet forage (more than 25% DM), prolonged fermentations (due to high buffering capacity), loose packing, or slow silo filling. High acetic acid indicates that energy and DM recovery were lower than desired.

High butyric acid (more than 0.5% of DM) indicates that clostridial fermentation occurred, which is one of the poorest fermentations. Silages high in butyric acid are usually low in palatability and nutritive value having higher ADF and NDF levels because many of the soluble nutrients have been degraded.

High ammonia (more than 12% to 15% of CP) results from excessive protein breakdown caused by a slow drop in pH or clostridial action. In general, wetter silages have higher concentrations of ammonia.

High concentrations of ethanol are usually an indicator of excessive metabolism by yeasts.

Assessing silage quality by smell, feel, and fermentation analysis can tell us how to make better silage next year and how to make the best of the silage we have this year.

Normal Ranges of Silage Fermentation Analyses

 

Legume Silage

Grass Silage

Corn Silage

moisture: 

65% +

50% - 65%

< 50%

< 70%

60% - 65%

pH

4.0 - 4.3

4.3 - 4.7

4.7 - 5.0

4.3 - 4.7

3.8 - 4.2

Lactic Acid (% of DM)

6 - 8

4 - 6

2 - 4

6 - 10

5 - 10

Acetic Acid (% of DM)

1.0 - 3.0

0.5 - 2.5

0.5 - 2.0

1.0 - 3.0

1.0 - 3.0

Propionic Acid (% of DM)

<0.5

<0.25

<0.1

<0.1

<0.1

Butyric Acid (% of DM)

<0.5

<0.25

<0.1

<0.1

<0.1

Ethanol (% of DM)

<1.0

<1.0

<0.5

<1.0

<3.0

Ammonia-CP (% of CP)

<15

<12

<10

<12

<8

Lactic acid (% of total acids)

>60

>70

>70

>60

>70

Undersander is a University of Wisconsin forage research and Extension agronomist.