What happens when horse feed contains yeast?

Regulator Completes's choice – and the research behind it

In a previous post "Yeast in horse feed - when does it make sense?" we review what yeast means in practice for horses - and when it can be an advantage or a disadvantage.

In this post, we go a step further and look at what the latest research says about yeast in horse feed. The results are not always clear-cut – and it provides important insight into why RC has chosen not to add yeast to our products.

Introduction

Many feed companies add yeast (Saccharomyces cerevisiae) to their products with the argument that it can support digestion and gut bacteria. RC does not do this – and here is the documented reason.

A recent review ( The Role of Yeast Saccharomyces cerevisiae in Supporting Gut Health in Horses: An Updated Review , Perricone et al. 2022) concludes that although yeast shows beneficial effects in some studies, the results are highly variable , depending on the composition of the diet, dose, strain and the individual horse.
Source: PMC9774806

Facts: Yeast in feed – advantages and disadvantages

What does the research say about yeast for horses?

Digestibility and intestinal flora

• Yeast can in some cases increase apparent digestibility (DM, NDF, ADF), especially in diets with a high concentrate content, because it counteracts pH drops and can support fibrolytic bacteria.
• With diets based on roughage, the effect is often weak or absent .
• The studies are heterogeneous and the results vary widely.

Immunomodulation

• Yeast fermentation products (SCFP) can stimulate the immune system without necessarily changing the gut microbiota.
• In horse studies, altered immune responses are seen after vaccination with yeast supplementation (Lucassen et al. 2022).
• In production animals, effects on immunoglobulins and cytokines have been shown (Liu et al. 2024).

Potential consequences

• Constant stimulation of the immune system can cause low-grade inflammation .
• According to research, this can help influence recovery ability and, in the long term, have an impact on joints and muscles .
• Therefore, yeast addition should be considered critically in horse feed where the goal is long-term shelf life.

Why is yeast used in fattening slaughter animals?

In the production of cattle, pigs and poultry, yeast is consistently used because the goal is rapid and cost-effective growth :

1. Increased feed utilization – more weight per kg of feed.
2. Support for intestinal fermentation and stability during intensive feeding.
3. Immune stimulation can help keep animals healthy in environments with high infection burden.

In these systems, the animals' lifespan is short, so any immune activation is less problematic than in riding and sport horses.

What does it mean for horses – and why RC opts out of yeast

• In riding and sport horses, the goal is long-term health, stable joints and durable musculature – not rapid fattening.
• Constant immune activation can lead to increased inflammation, which research links to impaired recovery and wear and tear in the long term.
• Therefore, RC chooses to focus on what has strong and consistent evidence :
  • Abundant and quality roughage
  • Salt as needed
  • Balanced protein and amino acids
  • Omega-3 fatty acids
  • Low sugar/starch and smooth feed changes

Therefore, Regulator Complete does not add yeast as standard in our products.

Sources

• Perricone V. et al. (2022). The Role of Yeast Saccharomyces cerevisiae in Supporting Gut Health in Horses: An Updated Review… Animals . PMC9774806
• Yi S. et al. (2025). Effects of Yeast Cultures on Growth Performance, Fiber Digestibility, Health of Beef Cattle . Animals .
• Gunun N. et al. (2022). Effect of Dietary Supplementation of Hydrolyzed Yeast on Growing Beef Cattle . Animals .
• Pang Y. et al. (2022). Yeast Probiotic and Yeast Products in Enhancing Livestock . J. Fungi .
• Agazzi A. et al. (2011). Evaluation of live yeast supplementation on digestibility in horses . J. Equine Vet. Sci.
• Liu X. et al. (2024). Yeast peptides in calves: immune and anti-inflammatory effects . Front. Know. Sci.
• Lucassen A. et al. (2022). Saccharomyces cerevisiae fermentation product and the equine gut microbiome . Animals .