Maintaining the quality of total mixed rations (TMR) during feed out remains a key challenge in modern ruminant production systems. While ration formulation and mixing aim to ensure consistent nutrient delivery, exposure to oxygen at the feed bunk initiates microbial activity that can rapidly compromise feed quality.

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Aerobic deterioration of TMR is associated with nutrient losses, reduced palatability, and increased variation in dry matter intake. In this context, organic acids, particularly formic and propionic acid, are widely used to improve feed hygiene and stability. Their combined use offers a practical and well-established approach to managing spoilage under a range of farm conditions.

Why Does TMR Deteriorate?

Once TMR is exposed to air, yeasts are typically the first microorganisms to initiate spoilage by metabolising sugars and fermentation products such as lactic acid. This process generates heat and increases the pH of the ration, creating favourable conditions for the growth of moulds and other aerobic microorganisms.

Typical consequences of aerobic deterioration include:

• Increased feed temperature at the bunk
• Reduced palatability and increased sorting behaviour
• Higher feed refusals and visible mould growth
• Progressive loss of sugars and other readily available nutrients

Even when average dry matter intake is maintained, increased variation in intake between feed outs or cow groups can negatively affect feeding consistency and rumen stability.

The rate of deterioration is influenced by several factors, including ambient temperature, moisture content, ration composition, and feeding management. High moisture rations and longer feed out periods are particularly susceptible to aerobic spoilage.

Mode of Action of Organic Acids

Organic acids inhibit microbial growth by disrupting cellular metabolism and intracellular pH. At low external pH, organic acids are able to enter microbial cells in their undissociated form. Once inside the cell, they dissociate, lowering internal pH and interfering with essential metabolic processes. This ultimately reduces the growth and activity of spoilage microorganisms.

Formic acid is primarily associated with rapid pH reduction, inhibition of undesirable fermentation, and preservation of nutrients. Its fast mode of action makes it particularly effective during the initial stages of preservation. Propionic acid is especially effective against yeasts and moulds and therefore plays a key role in controlling aerobic spoilage processes during feed out.

The combined use of formic and propionic acid provides complementary effects, offering rapid acidification together with effective control of aerobic spoilage microorganisms. As different organic acids are most active at different pH levels, such combinations help maintain antimicrobial effectiveness across the variable pH conditions typically encountered in TMR during feed out. This principle explains the widespread use of acid blends in modern feed preservation strategies.

Effects of Organic Acids on TMR Stability

The use of organic acids in TMR is associated with several practical benefits:

• Temperature control: Organic acids can delay the rise in TMR temperature during feed out, reflecting reduced microbial activity.
• Improved feed hygiene: Limiting the growth of spoilage microorganisms helps maintain hygienic feed conditions and reduce the risk of secondary spoilage.
• Reduced nutrient losses: Slowing aerobic deterioration supports preservation of sugars and other fermentation products in the ration.
• Impact on intake and performance: Under conditions where spoilage pressure is high, improved TMR stability may support more consistent dry matter intake, reduced sorting behaviour, and more stable rumen conditions. Responses in animal performance are typically situation-dependent and most evident where aerobic deterioration would otherwise be pronounced.

From Antimicrobial Efficacy to Practical Application

While organic acids are well recognised for their antimicrobial effectiveness, successful use in TMR applications depends not only on their biological mode of action but also on how reliably they can be applied under practical farm conditions.

In practice, the use of pure organic acids may be associated with challenges related to corrosiveness, volatility, and strong odour. These factors can affect handling safety, application equipment, and consistency of dosage. In some feeding systems, such constraints limit how regularly or uniformly acids are applied, which may reduce their overall impact on aerobic stability during feed out.

As a result, increasing attention is being paid to the importance of application characteristics alongside antimicrobial efficacy. Formulated acid systems have been developed to maintain the functional benefits of organic acids while addressing practical aspects such as handling, safety, and equipment compatibility. By improving the consistency of use and distribution within the ration, such approaches aim to translate proven chemical efficacy into more predictable on-farm results.

One example is a unique technology that combines formic and propionic acid with lignosulfonate-based components. The objective of this formulation approach is to reduce corrosiveness and volatility while maintaining antimicrobial effectiveness, thereby supporting more consistent application in everyday feeding systems.

When Does TMR Stabilisation Deserve Particular Attention?

• High moisture rations or wet by-product inclusion
• Warm ambient temperatures or summer feeding conditions
• Long feed out periods or infrequent feeding
• Observable heating of TMR at the feed bunk
• Increased sorting, refusals, or intake variability

In these situations, proactive management of aerobic stability becomes increasingly relevant.

Conclusion

Formic and propionic acids provide complementary functions in the control of aerobic spoilage in total mixed rations, combining rapid acidification with effective inhibition of yeasts and moulds. Their use can help limit heating, preserve nutrients, and support hygienic feed conditions during feed out.

However, achieving consistent results in practice depends not only on antimicrobial efficacy but also on how well acidification strategies integrate into everyday feeding routines. Formulated organic acid blends that combine proven efficacy with improved handling and application characteristics represent an important development in managing TMR stability under modern farm conditions.