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How does acetic acid get into the silo?

As a by-product of various groups of microorganisms, acetic acid can be produced in silage by heterofermentative bacteria, Acetobacter, enterobacteria, and clostridia. These are microorganisms from the natural microflora of plants, whose development and persistence depend on climatic conditions and cultivation methods.
1,2-propanediol (PD), also known as propylene glycol, is in turn produced in larger quantities by heterofermentative Lentilactobacillus, such as Lentilactobacillus buchneri. Don’t be surprised: recently, heterofermenters of the genus Lactobacillus were reclassified as Lentilactobacillus, referring to their slow (“lenti”) growth.

L. buchneri is desirable

All Lentilactobacillus buchneri strains that produce acetic acid are definitely desirable in silage. However, a rapid drop in pH caused by homofermentative bacteria that produce lactic acid is what makes the acetic acid effective in the first place. Acetic acid is known to suppress populations of harmful bacteria, yeasts, and molds in the silo, and to prevent reheating during feed-out (target level: > 1.5% of dry matter).
In the absence of oxygen, L. buchneri is also able to convert lactic acid into acetic acid and 1,2-propanediol.

High acetic acid content = faulty fermentation?

According to the DLG evaluation of fermentation quality, this would be the case. In the DLG key (2/2006), levels above 3% of dry matter lead to point deductions—something that is no longer appropriate given today’s widespread use of heterofermentative silage additives.
For example, in grass with high sugar content and high moisture, acetic acid values above 4% of dry matter are quite common in treated silages. Nevertheless, these silages can have high nutrient and hygienic quality, are readily consumed, and often even increase milk fat content. Looking at acetic acid alone is therefore not sufficient to assess fermentation quality.

Something smells off here!

High levels of NH₃-N and ethanol, together with a “rich” butyric acid content
(> 1% in dry matter) from the activity of clostridia indicate a faulty fermentation in which the acetic acid originates from undesirable bacteria and not from silage additives. In general, such silages have a poor to unbearable smell and should under no circumstances be fed.

Buchneri produce 1,2-PD

The presence of levels above 1% 1,2-PD together with solid acetic acid levels (> 1%) is a clear indicator of the activity of specialized buchneri strains from silage additives. Since this is a slow process, these values are usually only found in analyses after a fermentation period of 60 days.

However, the development of high 1,2-PD levels in the final product is by no means consistent and can vary greatly. Studies show levels ranging from 0.25–1.25% of dry matter up to concentrations exceeding 3%.

Propylene glycol from silage?

Admittedly, the 1,2-PD in silage can be used by the cow as an energy source. However, a reliable effect in preventing ketosis cannot be expected.
An example: a cow consuming 25 kg of dry matter per day, with 50% coming from silage containing 1% 1,2-PD, would ingest about 125 g of 1,2-PD daily. This is below the recommended dosage of 250–400 g. In addition, cows suffering from ketosis eat less, further reducing their intake of 1,2-PD.

Targeted use of propylene glycol

In general, this additional energy from 1,2-PD is only useful during the first 90 days of lactation (the period of highest energy demand). Toward the end of lactation, supplying propylene glycol can lead to excessive fat accumulation in the animal, which makes calving and the start of the next lactation more difficult.
Moreover, 1,2-PD belongs to the group of polyhydric alkanols, a subgroup of alcohols, and is therefore volatile in air. This means that the levels measured in silage analyses only partially reach the animal.

Conclusion: propylene glycol

1,2-PD has no preserving properties. It is merely a by-product of silage bacteria that convert available sugars into acids during fermentation, always consuming energy in the process.

Attacking from both sides

The combination of specialized strains of homo- and heterofermentative bacteria is ideal for silage fermentation. Homofermentative bacteria efficiently convert sugar into lactic acid. This lowers the pH, resulting in minimal dry matter losses while preserving nutrients and energy as much as possible.
With a slight delay, heterofermentative bacteria follow, producing acetic acid—an acid that inhibits the growth of spoilage-causing microorganisms and ensures high microbiological stability during feed-out.
In this way, a single product can both secure and accelerate fermentation while also improving feed hygiene from the moment of removal from the silo right through to the feed trough.